The First Transition Series

Chapter 45 The First Transition Series 45.1 Introduction 45.2 General Features of the d-Block Elements from Sc to Zn 45.3 Characteristic Properties of the d-Block Elements and their Compounds

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45.1 Introduction (SB p.164)

The first transition series 2

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45.1 Introduction (SB p.164)

d-Block elements (transition elements): • Lie between s-block and p-block elements • Occur in the fourth and subsequent periods • All contains incomplete d sub-shell (i.e. 1 – 9 electrons) in at least one of their oxidation state Titanium Scandium

Iron 3

Vanadium

Cobalt

Chromium

Manganese

Copper

Nickel New Way Chemistry for Hong Kong A-Level Book 4

Zinc 3

45.1 Introduction (SB p.165)

•

Strictly speaking, scandium (Sc) and zinc (Zn) are not transitions elements ∵ Sc forms Sc3+ ion which has an empty d sub-shell (3d0) Zn forms Zn2+ ion which has a completely filled d subshell (3d10)

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45.1 Introduction (SB p.165)

• Cu shows some intermediate behaviour between transition and non-transition elements because of two oxidation states, Cu(I) & Cu(II) • Cu+ is not a transition metal ion as it has a completely filled d subshell • Cu2+ is a transition metal ion as it has an incompletely filled d sub-shell 5

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.165)

Electronic Configurations

Relative energy levels of orbitals before and after filling with electrons 6

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.166)

• Before filling electrons, the energy of 4s sub-shell is lower than that of 3d sub-shell ⇒ 4s sub-shell is filled before 3d sub-shell • Once the 4s sub-shell is filled, the energy will increase ⇒ The lowest energy sub-shell becomes 3d sub-shell, so the next electron is put into 3d sub-shell

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.166) Electronic configurations of the first series of d-block elements

Element Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc 8

Atomic number 21 22 23 24 25 26 27 28 29 30

Electronic configuration [Ar]3d14s2 [Ar]3d24s2 [Ar]3d34s2 [Ar]3d54s1 [Ar]3d54s2 [Ar]3d64s2 [Ar]3d74s2 [Ar]3d84s2 [Ar]3d104s1 [Ar]3d104s2

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.167)

•

Cr is expected to be [Ar] 3d44s2 but the actual configuration is [Ar] 3d54s1

•

Cu has the electronic configuration of [Ar] 3d104s1 instead of [Ar] 3d94s2

• This can be explained by the fact that a half-filled or fully-filled d sub-shell provides extra stability 9

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.167)

d-Block Elements as Metals • d-block elements are typical metals (1) good conductors of heat and electricity, hard, strong, malleable, ductile and lustrous (2) high melting and boiling points except Hg is a liquid at room temperture • These properties make d-block elements as good construction materials e.g. Fe is used for construction and making machinery Ti is used to make aircraft and space shuttles 10

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.168)

•

Transition elements have similar atomic radii which make them possible for the atom of one element to replace those of another element in the formation of alloy

e.g. Mn is for conferring hardness and wearing resistance to its alloy (duralumin) Cr is for conferring inertness on stainless steel

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.168)

Iron is used to make ships

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Tsing Ma Bridge is constructed of steel

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.168) Tungsten in a light bulb

The statue is made of alloy of copper and zinc

Titanium is used in making aircraft 13

Jewellery made of gold

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.169)

Atomic Radii and Ionic Radii Observations: • d-block metals have smaller atomic radii than s-block metals • The atomic radii of the dblock metals do not show much variation across the series • The atomic radii decrease initially, remain almost constant in the middle and then increase at the end of series 14

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.170)

Variations in atomic and ionic radii of the first series of d-block elements 15

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.167)

• The atomic size reduces at the beginning of the series ∵ increase in effective nuclear charge with atomic numbers ⇒ the electron clouds are pulled closer to the nucleus ⇒ causing a reduction in atomic size • The atomic size decreases slowly in the middle of the series ∵ when more and more electrons enter the inner 3d sub-shell ⇒ the screening and repulsive effects of the electrons in the 3d sub-shell increase ⇒ the effective nuclear charge increases slowly 16

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.167)

•

The atomic size increases at the end of the series ∵ the screening and repulsive effects of the 3d electrons reach a maximum

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•

The reasons for the trend of the ionic radii of the d-block elements are similar to those for the atomic radii.

•

Remember that the electrons have to be removed from the 4s orbital first

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.170)

Comparison of Some Physical and Chemical Properties between d-Block and s-Block Metals Density

Densities (in g cm-3) of the s-block metals and the first series of d-block metals 18

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.171)

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•

d-block metals are generally denser than the s-block because most of the d-block metals have close-packed structures while most of the s-block metals do not.

•

The densities increase generally across the first series of d-block metals. This is in agreement with the general decrease in atomic radius across the series

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.171)

Ionization Enthalpy Element K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn 20

Ionization enthalpy (kJ mol–1) 1st

2nd

3rd

4th

418 590

3 070 1 150

4 600 4 940

5 860 6 480

632 661 648 653 716 762 757 736 745 908

1 240 1 310 1 370 1 590 1 510 1 560 1 640 1 750 1 960 1 730

2 390 2 720 2 870 2 990 3 250 2 960 3 230 3 390 3 550 3 828

7 110 4 170 4 600 4 770 5 190 5 400 5 100 5 400 5 690 5 980

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.172)

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1st I.E. of d-block metals are greater than those of s-block elements in the same row of the Periodic Table. ∵ the d-block metals are smaller in size than the s-block metals, thus they have greater effective nuclear charges

•

For K, the 2nd I.E. is exceptionally higher than its 1st I.E

•

For Ca, the 3rd I.E. is exceptionally higher than its 2nd I.E ∵ the electrons are come form the inner fully-filled electron shells

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.172)

•

•

The first few successive I.E. for d-block elements do not show dramatic changes ∵ removal of electrons does not involve the disruption of inner electron shells The 1st I.E. of the d-block metals increase slightly and irregularly across the series ∵ Going across the first transition series, the nuclear charge of the elements increases, and additional electrons are found in the inner 3d sub-shell ⇒ The additional screening effect of the additional 3d electrons is so significant that the effective nuclear charge of the elements increases only very slowly across the series

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.172)

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•

Successive ionization enthalpies exhibit a similar gradual increase across the first transition series

•

The increases in the 3rd and 4th ionization enthalpies across the series are progressively more rapid

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.172)

• Some abnormal high ionization enthalpy, e.g. 1st I.E. of Zn, 2nd I.E. of Cr & Cu and the 3rd I.E. of Mn ∵The removal of an electron from a fullyfilled or half-filled sub-shell requires a relatively large amount of energy Variation of successive ionization enthalpies of the first series of the d-block elements 24

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.173)

Check Point 45-1 Explain the following variation in terms of electronic configurations. (a) The second ionization enthalpies of both Cr and Cu are higher (a) than their next elements respectively. Thethose secondof ionization enthalpies of both Cr and Cu are higher than those of their next elements respectively. In the case of Cr,Answer the second ionization enthalpy involves the removal of an electron from a half-filled 3d sub-shell, which has extra stability. Therefore, this second ionization enthalpy is relatively high. The case is similar for copper where its second ionization enthalpy involves the removal of an electron from a fully-filled 3d sub-shell which also has extra stability. Thus, its second ionization enthalpy is also relatively high.

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.173)

Check Point 45-1 (cont’d) Explain the following variation in terms of electronic configurations. (b) The third ionization enthalpy of Mn is higher than that of its next element. Answer (b) The third ionization enthalpy of Mn is higher than that of its next element. It is because its third ionization enthalpy involves the removal of an electron from a halffilled 3d sub-shell which has extra stability. Therefore, its third ionization enthalpy is relatively high.

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.173)

Electronegativity

Electronegativity values of the s-block metals and the first series of the d-block metals 27

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.173)

•

The electronegativity of d-block metals are generally higher than those of the s-block metals ∵ Generally, d-block metals have smaller atomic radii than s-block metals ⇒ the nuclei of d-block metals can attract the electrons in a bond more tightly towards themselves

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.173)

• The electronegativity shows a slight increase generally with increasing atomic numbers across the series ∵ Gradual increase in effective nuclear charge and decrease in atomic radius across the series ⇒ The closer the electron shell to the nucleus, the more strongly the additional electron in a bond is attracted ⇒ Higher electronegativity

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.174)

Melting Point and Hardness

Melting points (°C) of the s-block metals and the first series of the d-block metals 30

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.174)

• The melting points of the d-block metals are much higher than those of the s-block metals Reasons: 1. d-block metal atoms are small in size and closely packed in the metallic lattice. All Group I metals and some Group II metals do not have close-packed structures 2. Both 3d and 4s electrons of d-block metals participate in metallic bonding by delocalizing into the electron sea, and thus the metallic bond strength is very strong s-Block metals have only 1 to 2 valence electrons per atom delocalizing into the electron sea 31

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.174)

•

The hardness of a metal depends on the strength of the metallic bonds ∵

The metallic bond of d-block metals is stronger

than that of s-block metals ⇒

d-block metals are much harder than the s-block

metals

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.174)

Check Point 45-2 What are the differences between the structures and bonding of the d-block and s-block metals? How do these differences affect their melting points? The d-block metals are comparatively small, and the metallic atoms are closely packed in the metallic lattice. Besides, both the 3d and 4s electrons of the d-block metals participate in metallic bonding Answer by delocalizing into the electron sea. The strength of metallic bond in these metals is thus very strong. In the case of s-block metals, the metallic radius is larger and most of them do not have close-packed structures. Also , as they have only one or two valence electrons per atom delocalizing into the electron sea, the metallic bond formed is weaker. Therefore, the d-block metals have a much higher melting point than the s-block metals. 33

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45.2 General Features of the d-Block Elements from Sc to Zn (SB p.175)

Reaction with Water

•

Generally, s-block metals (e.g. K, Na & Ca) react with H2O vigorously to form metal hydroxides and H2

•

d-block metals react only very slowly with cold water. Zn and Fe are relatively more reactive ⇒ Zn and Fe react with steam to give metal oxides and H2 Zn(s) + H2O(g) → ZnO(s) + H2(g) 3Fe(s) + 4H2O(g)

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Fe3O4(s) + 4H2(g)

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.175)

Variable Oxidation States •

d-block elements has ability to show variable oxidation states ∵ 3d & 4s electrons are of similar energy levels, the electrons in both of them are available for bonding ⇒ When the first transition elements react to form compounds, they can form ions of roughly the same stability by losing different numbers of electrons ⇒ Form compounds with a wide variety of oxidation states

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.175) Oxidation states of the elements of the first transition series in their oxides and chlorides Oxidation state

Oxide/Chloride Cu2O

+1

Cu2Cl2 TiO TiCl2

+2 +3 +4 +5 +6 +7 36

VO CrO VCl2 CrCl2

MnO MnCl2

Sc2O3 Ti2O3 V2O3

Cr2O3 Mn2O3

ScCl3

CrCl3

TiCl3 VCl3 TiO2

VO2

TiCl4

VCl4

MnCl3

FeO CoO NiO FeCl2 CoCl2 NiCl2 Fe2O3

CuO CuCl2

ZnO ZnCl2

Ni2O3·xH2O

FeCl3

MnO2 CrCl4

V2O5 CrO3 Mn2O7 New Way Chemistry for Hong Kong A-Level Book 4

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.176) Oxidation states of the elements of the first transition series in their compounds Element Sc Ti V Cr Mn Fe Co Ni Cu Zn 37

Possible oxidation state +1 +1 +1 +1 +1 +1 +1 +1

+2 +2 +2 +2 +2 +2 +2 +2 +2

+3 +3 +3 +3 +3 +3 +3 +3 +3

+4 +4 +4 +4 +4 +4 +4

+5 +5 +5 +5 +5 +5

+6 +6 +6

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+7

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.176)

Observations:

1. Sc and Zn do not exhibit variable oxidation states. Sc3+ has electronic configuration of argon (i.e. 1s22s22p63s23p6). Zn2+ has the electronic configuration of [Ar] 3d10. Other oxidation states are not possible. 2. Except Sc, all elements have +2 oxidation state. Except Zn, all elements have +3 oxidation state 3. The highest oxidation state is +7 at Mn. This corresponds to removal of all 3d & 4s electrons. (Note: max.oxidation state is NEVER greater than the total number of 3d & 4s electrons) 38

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.176)

4. There is a reduction in the number of oxidation states after Mn. ∵ decrease in the number of unpaired electrons and increase in nuclear charge which holds the 3d electrons more firmly 5. The relative stability of various oxidation states can be correlated -with the stability of empty, half-filled and fullyfilled configuration e.g. Ti4+ is more stable than Ti3+ (∵ [Ar]3d0 configuration) Mn2+ is more stable than Mn3+ (∵ [Ar]3d5 configuration) Zn2+ is more stable than Zn+ (∵ [Ar]3d10 configuration) 39

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.177)

Variable Oxidation States of Vanadium and their Interconversions

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•

Vanadium shows oxidation states from +2 to +5 in its compounds

•

In these oxidation state, vanadium forms ions which have distinctive colours in aqueous solutions Ion

Oxidation state

Colour

V2+ V3+ VO2+ VO2+

+2 +3 +4 +5

Violet Green Blue Yellow

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.177)

•

In acidic medium, vanadium(V) state occurs as VO2+(aq); vanadium(IV) state occurs as VO2+(aq)

•

In alkaline medium, vanadium(V) state occurs as VO3–(aq)

•

Most compounds with vanadium(V) are good oxidizing agents while those with vanadium(II) are good reducing agents

• The starting material for the interconversions of common oxidation states of vanadium is ammonium vanadate(V) (NH4VO3) • When NH4VO3 is acidified, vanadium exists in the form of VO2+(aq) which the oxidation state of +5 41

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.177)

• Vanadium(V) ions can be reduced sequentially to vanadium(II) ions by the action of Zn powder and acid • The sequence of colour changes forms a characteristic test for vanadium Zn

Zn

Zn

2+ VO2 (aq) → VO (aq) → V (aq) → V (aq) conc. HCl conc. HCl conc. HCl +

yellow

42

2+

blue

3+

green

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violet

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.178)

•

The feasibility of the changes in oxidation number of vanadium can be predicted by using electrode potentials easily Half reaction Zn2+(aq) + 2e– Zn(s) VO2+(aq) + 2H+(aq) + e–

VO2+(aq) + H2O(l)

VO2+(aq) + 2H+(aq) + e–

V3+(aq) + H2O(l)

V3+(aq) + e–

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E (V)

V2+(aq)

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–0.76 +1.00 +0.34 –0.26

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.178)

•

Under standard conditions, Zn can reduce vanadium(V) to vanadium(IV) as the Ecell value is +ve

2 × (VO2+(aq) + 2H+(aq) + e– –) Zn2+(aq) + 2e–

VO2+(aq) + H2O(l)) E = +1.00 V

Zn(s)

E = –0.76 V

2VO2+(aq) + Zn(s) + 4H+(aq) 2VO2+(aq) + Zn2+(aq) + 2H2O(l)

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Ecell = +1.76 V 44

45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.178)

•

Further reduction of vanadium(IV) to vanadium(III) by Zn is feasible as the Ecell value is +ve

2 × (VO2+(aq) + 2H+(aq) + e– –) Zn2+(aq) + 2e–

V3+(aq) + H2O(l)) E = +0.34 V

Zn(s)

E = –0.76 V

2VO2+(aq) + Zn(s) + 4H+(aq) 2V3+(aq) + Zn2+(aq)+ 2H2O(l) 45

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Ecell = +1.10 V 45

45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.179)

•

Further reduction of vanadium(III) to vanadium(II) by Zn is also feasible 2 × (V3+(aq) + e– –) Zn2+(aq) + 2e– 2V3+(aq) + Zn(s)

V2+(aq))

E = +0.34 V

Zn(s)

E = –0.76 V

2V2+(aq) + Zn2+(aq)

Ecell = +0.50 V

Conclusion: Zn acts as a strong reducing agent which reduces vanadium(V) through vanadium(IV), vanadium(III) and finally to vanadium(II) in an acidic medium 46

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.179)

Variable Oxidation States of Manganese and their Interconversions

•

Mn shows oxidation states from +2 to +7 in its compounds

•

The most common oxidation states of Mn include +2, +4, +7

•

Mn also forms coloured compounds or ions in these oxidation states Ion/compound Oxidation state Mn2+ +2 Mn(OH)3 +3 MnO2 +4 MnO42– +6 +7 MnO4–

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Colour Very pale pink Dark brown Black Green Purple

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.179)

•

Mn is most stable in +2 oxidation state

•

The most common Mn compound in +4 oxidation state is MnO2 which is a strong oxidizing agent. It reacts with reducing agents and is reduced to Mn2+ +4 + – MnO (s) + 4H (aq) + 2e 2 black

+2

Mnpale(aq) very pink 2+

•

+ 2H2O(l)

E = +1.23 V

MnO2 is used in the laboratory production of chlorine MnO2(s) + 4HCl(aq) → MnCl2(aq) + 2H2O(l) + Cl2(g)

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.180)

•

The most common Mn compound in +7 oxidation state is KMnO4 which is an extremely powerful oxidizing agent. Its oxidizing power depends on pH

•

In acidic medium, MnO4– ions are reduced to Mn2+ ions +7

+2

– + – MnO (aq) + 8H (aq) + 5e 4 purple

2+ Mn (aq) very pale pink + 4H2O(l)

E = +1.23 V • In alkaline medium, MnO4– ions are reduced to MnO2 +7

+4

– – MnO (aq) + 2H O(l) + 3e 4 2 purple

– MnO (s) + 4OH (aq) 2 black

E = +0.59 V 49

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.180) Mn(II)

Mn(IV)

Mn(III)

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Mn(VII)

Mn(VI)

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.180)

Check Point 45-3 (a) The oxidation numbers of copper in its compounds are +1 and +2. (i) Give the names, formulae and colours of compounds formed between copper and oxygen. (ii) Is copper more stable in the oxidation state of +1 or +2? Answer (a) (i) Copper(I) oxide Cu2O – reddish brown Copper(II) oxide CuO – black (ii) +2

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.180)

Check Point 45-3 (cont’d) (b) Explain the following: (i) When iron(II) sulphate(VI) (FeSO4) is required, it has to be freshly prepared. (ii) When aluminium reacts with chlorine and hydrogen chloride respectively, aluminium chloride (AlCl3) is (b) (i) Iron(II) sulphate(VI) solution cannot be stored for a long time. It formed in both cases. However, two different products will be oxidized by air to form iron(III) sulphate(VI). are produced when iron reacts with these two (ii) Aluminium has only one oxidation state (+3) in its compounds, chemicals respectively. whereas iron has two (+2 & +3). Iron reacts with the oxidizing Answer agent Cl2 to form FeCl3 but with the non-oxidizing agent HCl to give FeCl2.

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.181)

Formation of Complexes A complex is formed when a central metal atom or ion is surrounded by other molecules or ions which form dative covalent bonds with the central metal atom or ion. • The molecules or ions that form the dative covalent bonds are called ligands • In a ligand, there is at least one atom having a lone pair of electrons which can be donated to the central metal atom or ion to form a dative covalent bond 53

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.181)

Examples of ligands:

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.181)

• Depending on the overall charge of the complex formed, complexes are classified into 3 main types: cationic, neutral and anionic complex

Cationic complex ions 55

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.181)

Neutral complex

Anionic complex ions 56

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.182)

•

The coordination number of the central metal atom or ion in a complex is the number of ligands bonded to this metal atom or ion e.g. in [Cu(NH3)4]2+(aq), there are 4 ligands are bonded to the central Cu2+ ion, so the coordination number is 4

•

57

The most common coordination numbers are 4 and 6

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.182)

•

For the first series of d-block metals, complexes are formed using the 3d, 4s, 4p and 4d orbitals present in the metal atoms or ions

•

Due to the presence of vacant, low energy orbitals, d-block metals can interact with the orbitals of the surrounding ligands

•

Due to the the relatively small sizes and high charge of d-block metal ions, they introduce strong polarization on the ligands. This favours the formation of bonds of high covalent character

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.182)

Diagrammatic representation of the formation of a complex 59

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.183)

Nomenclature of Complexes

•

Complexes are named according to the rules recommended by IUPAC The rules of naming a complex are as follow: 1. (a) For any ionic compound, the cation is named before the anion. (b) If the compound is neutral, then the name of the complex is name of the compound (c) In naming a complex, the ligands are named before the central metal atom or ion, negative ones first and then neutral ones

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.183)

(d) The number of each type of ligands are specified by the Greek prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, etc. (e) The oxidation number of the metal ion in the complex is named immediately after it by Roman numerals Therefore, K3[Fe(CN)6]

potassium hexacyanoferrate(III)

[CrCl2(H2O)4]Cl

dichlorotetraaquachromium(III) chloride

[CoCl3(NH3)]

trichlorotriamminecobalt(III)

Note: in the formulae, the complexes are always enclosed in [ ] 61

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.183)

2. (a) The root names of anionic ligands always end in -o. e.g. CN– cyano Cl– chloro (b) The names of neutral ligands are the names of the molecules, except NH3, H2O, CO and NO e.g. NH3 ammine H2O aqua

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Name of Anionic ligand Neutral ligand ligand Bromide (Br–) Bromo Ammonia (NH3) Chloride (Cl–) Chloro Water (H2O) Cyanide (CN–) Cyano Carbon monoxide Fluoride (F–) Fluoro (CO) Hydroxide (OH–) Hydroxo Nitric oxide (NO) Sulphate(VI) Sulphato (SO42–) New WayAmido Chemistry for Hong Kong A-Level Book 4 Amide (NH2–)

Name of ligand Ammine Aqua Cabonyl Nitrosyl 62

45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.184)

3. (a) If the complex is anionic, then the suffix -ate is attached to the name of the metal, followed by the oxidation state of that metal e.g. K2CoCl4

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potassium tetrachlorocobaltate(II)

K3Fe(CN)6

potassium hexacyanoferrate(III)

[CuCl4]2–

tetrachlorocuprate(II) ion

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.184) Metal Name in anionic complex Titanium Chromium Manganese Iron Cobalt Nickel Copper Zinc Platinum

Titanate Chromate Manganate Ferrate Cobaltate Nickelate Cuprate Zincate Platinate

(b) If the complex is cationic or neutral, then the name of the metal is unchanged. e.g. [CrCl2(H2O)4]+ dichlorotetraaquachromium(III) ion [CoCl3(NH3)3] 64

trichlorotriamminecobalt(III)

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.184)

Examples: 1. Ionic complexes

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.185)

2. Neutral complex

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.185)

Check Point 45-4 (a) Name the following compounds. (i) [Fe(H2O)6]Cl2 (ii) [Cu(NH3)4]Cl2 (iii) [PtCl4(NH3)2] (iv) K2[CoCl ] Hexaaquairon(II) chloride (a) 4(i)

Answer

(ii) Tetraamminecopper(II) chloride (iii) Tetrachlorodiammineplatinum(IV) (iv) Potassium tetrachlorocobaltate(II)

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.185)

Check Point 45-4 (cont’d) (b) Write the formulae of the following compounds. (i) chloropentaamminecobalt(III) chloride (ii) ammonium hexachlorotitanate(IV) (iii) dihydroxotetraaquairon(II) Answer (b) (i) [CoCl(NH3)5]Cl2 (ii) (NH4)2[TiCl6] (iii) [Fe(H2O)4(OH)2]

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.185)

Displacement of Ligands and Relative Stability of Complex Ions

• The tendency to donate unshared electrons to form dative covalent bonds varies with different ligands • Different ligands form dative covalent bonds of different strength with the metal atom or ion • The ligand within a complex can be replaced by another ligand if the incoming ligand can form a stronger bond with the metal atom or ion • When different ligands are present, they compete for a metal ion 69

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.185)

• A stronger ligand (e.g. CN–, Cl–) can displace a weaker ligand (e.g. H2O) from a complex, and a new complex is formed e.g. [Fe(H2O)6]2+(aq) + 6CN–(aq) hexaaquairon(II) ion

[Ni(H2O)6]2+(aq) + 6NH3(aq) hexaaquanickel(II) ion

•

[Fe(CN)6]4–(aq) + 6H2O(l) hexacyanoferrate(II) ion

[Ni(NH3)6]2+(aq) + 6H2O(l) hexaamminenickel(II) ion

Complex ions are usually coloured and the colours are related to the types of ligands present ⇒ Displacement of ligands usually associated with colour changes which can be followed during experiments easily

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.186)

Example: •

0.5 M CuSO4 solution is put into a test tube. The complex ion present is [Cu(H2O)6]2+ which is pale blue

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•

Conc. HCl is added dropwise to the CuSO4 solution

•

The solution turns from pale blue to green and finally to yellow

•

This is due to the stepwise replacement of H2O ligands by Cl– ligands

•

Each stage is charaterized by an equilibrium constant called the stepwise stability constant New Way Chemistry for Hong Kong A-Level Book 4

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.186)

[Cu(H2O)4]2+(aq) + Cl–(aq)

[Cu(H2O)3Cl]+(aq) + H2O(l) K1 = 6.3 × 102 dm3 mol–1

[Cu(H2O)3Cl]+(aq) + Cl–(aq)

[Cu(H2O)2Cl2](aq) + H2O(l) K2 = 4.0 × 101 dm3 mol–1

[Cu(H2O)2Cl2](aq) + Cl–(aq)

[Cu(H2O)Cl3]–(aq) + H2O(l) K3 = 5.4 dm3 mol–1

[Cu(H2O)Cl3]–(aq) + Cl–(aq)

[CuCl4]2–(aq) + H2O(l) K4 = 3.1 dm3 mol–1

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.186)

Overall equation: [Cu(H2O)4]2+(aq) + 4Cl–(aq)

[CuCl4]2–(aq) + 4H2O(l)

Overall stability constant of [CuCl4]2–(aq) is: K st =

[[CuCl 4 ]2− (aq )]eqm [[Cu(H 2O) 4 ]2+ (aq )]eqm [Cl − (aq)]4 eqm

which is given by: Kst = K1 × K2 × K3 × K4 = 4.2 × 105 dm12 mol–4 73

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.186)

•

The larger the overall stability constant, the more stable is the complex

•

In this example, the overall equilibrium lies mainly on the right and [CuCl4]2–(aq) is predominant over [Cu(H2O)4]2+(aq) ⇒ Cl– ligands can replace H2O ligands to form a more stable complex with Cu2+ ion

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.186)

• The stepwise stability constant decreases from K1 to K4 Reasons: 1. When the central Cu2+ ion is surrounded by an increasing number of Cl– ligands, the chance for an addition Cl– ligand to replace a remaining bonded H2O decreases 2. There is a progressive change from a cationic complex to a neutral complex, and then anionic complex. Due to the electrostatic repulsion between anionic complex and Cl– ions, the approach of Cl– ligands becomes more difficult 75

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.187)

•

NH3 forms a more stable complex with Cu2+ ion than Cl– and H2O ligands do

•

NH3 can displace both H2O ligands from [Cu(H2O)4]2+(aq) and Cl– ligands from [CuCl4]2–(aq), forming the deep blue [Cu(NH3)4]2+(aq) ion [Cu(H2O)4]2+(aq) + 4NH3(aq) [Cu(NH3)4]2+(aq) + 4H2O(l)

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•

The displacement also occurs in stepwise reaction [Cu(H2O)4]2+(aq) + NH3(aq) [Cu (NH3)(H2O)3]2+(aq) + H2O(l) K1 = 1.9 × 104 dm3 mol–1 [Cu(NH3)(H2O)3]2+(aq) + NH3(aq)

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[Cu (NH3)2(H2O)2]2+(aq) + H2O(l) K2 = 3.9 × 103 dm3 mol–1

[Cu(NH3)2(H2O)2]2+(aq) + NH3(aq)

[Cu (NH3)3(H2O)]2+(aq) + H2O(l) K3 = 1.0 × 103 dm3 mol–1

[Cu(NH3)3(H2O)]2+(aq) + NH3(aq)

[Cu (NH3)4]2+(aq) + H2O(l) K4 = 1.5 × 102 dm3 mol–1

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.187)

• By adding the above 4 equations, overall equation is obtained. [Cu(H2O)4]2+(aq) + 4NH3(aq)

[Cu(NH3)4]2+(aq) + 4H2O(l)

• Overall stability constant of [Cu(NH3)4]2+(aq) is: K st =

[[Cu(NH3 ) 4 ]2+ (aq)]eqm [[Cu(H 2O) 4 ]2+ (aq)]eqm [ NH 3 (aq)]4 eqm

which is given by Kst = K1 × K2 × K3 × K4 = 1.1 × 1013 dm12 mol–4 • The overall stability constant for [Cu(NH3)4]2+(aq) is larger than that for [CuCl4]2–(aq)

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⇒ NH3 is a stronger ligand compared with Cl– or H2O ⇒ [Cu(NH3)4]2+(aq) is more stable than [CuCl4]2–(aq) New Way Chemistry for Hong Kong A-Level Book 4

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.187)

• The displacement of the H2O ligands in [M(H2O)m] by another ligand L can be represented as: [M(H2O)m] + mL

[MLm] + mH2O

• The stability constant for the complex [MLm] at a given [ML m ] temp.: K st = [M(H 2 O) m ][ L]m Equilibrium Cr(OH)3(aq) + OH–(aq)

[Cr(OH)4]–(aq)

[Fe(H2O)6]2+(aq) + 6CN–(aq)

[Fe(CN)6]4–(aq) + 6H2O(l)

[Fe(H2O)6]3+(aq) + 6CN–(aq)

[Fe(CN)6]3–(aq) + 6H2O(l)

[Fe(H2O)4]3+(aq) + 4Cl–(aq)

[FeCl4]–(aq) + 4H2O(l)

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Kst ((mol dm–3)– n ) 1 × 10–2 ≈ 1024 ≈ 1031 8 × 10–2 79

45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.188) Equilibrium [Co(H2O)6]2+(aq) + 6NH3(aq)

[Co(NH3)6]2+(aq) + 6H2O(l)

[Co(H2O)6]3+(aq) + 6NH3(aq)

[Co(NH3)6]3+(aq) + 6H2O(l)

[Ni(H2O)6]2+(aq) + 6NH3(aq)

[Ni(NH3)6]2+(aq) + 6H2O(l)

[Cu(H2O)4]2+(aq) + 4Cl– [Cu(H2O)4]2+(aq) + NH3(aq)

[CuCl4]2–(aq) + 4H2O(l) [ Cu(NH3)(H2O)3]2+(aq) + H2O(l)

Kst ((mol dm–3)–n) 7.7 × 104 4.5 × 1033 4.8 × 107 4.8 × 105 1.9 × 104 (K1)

[Cu(NH3)(H2O)3]2+(aq) + NH3(aq) [ Cu(NH3)2(H2O)2]2+(aq) + H2O(l)

3.9 × 103 (K2)

[Cu(NH3)2(H2O)2]2+(aq) + NH3(aq) [ Cu(NH3)3(H2O)]2+(aq) + H2O(l)

1.0 × 103 (K3)

[Cu(NH3)3(H2O)]2+(aq) + NH3(aq) [ Cu(NH3)4]2+(aq) + H2O(l)

1.5 × 102 (K4)

[Cu(H2O)4]2+(aq) + 4NH3(aq) 80

[Cu(NH3)4]2+(aq) + 4H2O(l)

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1.1 × 1013 (Kst = K1×K2×K3×K4) 80

45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.188)

Equilibrium [Zn(H2O)4]2+(aq) + 4CN–(aq)

[Zn(CN)4]2– (aq) + 4H2O(l)

[Zn(H2O)4]2+(aq) + 4NH3(aq)

[Zn(NH3)4]2+(aq) + 4H2O(l)

Zn(OH)2(s) + 2OH–(aq)

•

[Zn(OH)4]2– (aq)

Kst ((mol dm–3)–n) 5 × 1016 3.8 × 109 10

As shown in the table, the values of stability constants are very large ⇒ The complex ions of the d-block metals are generally very stable

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.189)

Check Point 45-5 Answer the following questions by considering the stability constants of the silver complexes. Ag+(aq) + 2Cl–(aq)

[AgCl2]–(aq)

Kst = 1.1 × 105 mol–2 dm6

Ag+(aq) + 2NH3(aq)

[Ag(NH3)2]+(aq)

Kst = 1.6 × 107 mol–2 dm6

Ag+(aq) + 2CN–(aq)

[Ag(CN)2]–(aq)

Kst = 1.0 × 1021 mol–2 dm6

(a) Give the most stable and the least stable complexes of Answer silver. (a) The most stable complex of silver is [Ag(CN)2]–(aq), whereas the least stable one is [AgCl2]–(aq) 82

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.189)

Check Point 45-5 (cont’d) Answer the following questions by considering the stability constants of the silver complexes. Ag+(aq) + 2Cl–(aq)

[AgCl2]–(aq)

Kst = 1.1 × 105 mol–2 dm6

Ag+(aq) + 2NH3(aq)

[Ag(NH3)2]+(aq)

Kst = 1.6 × 107 mol–2 dm6

Ag+(aq) + 2CN–(aq)

[Ag(CN)2]–(aq)

Kst = 1.0 × 1021 mol–2 dm6

(b) (i) What will be formed when CN–(aq) is added to a solution of [Ag(NH3)2]+? – when NH (aq) is added to a (ii) What formed (b)will (i) be [Ag(CN) 3 2] (aq) and NH3(aq) – solution(ii) of [Ag(CN) Answer No reaction2] ?

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.189)

Stereostructures of Tetra- and HexaCoordinated Complexes

•

The spatial arrangement of ligands around the central metal atom or ion in a complex is referred to as the stereochemistry of the complex

•

The coordination number of the central metal atom or ion is determined by: 1. The size of the central metal atom or ion; 2. The number and the nature of vacant orbitals of the d-block metal atoms or ions available for the formation of dative covalent bonds

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.189)

Shape 1. Tetra-coordinated complexes (a) Tetrahedral complexes Tetrahedral shape is a common geometry of tetra-coordinated complexes Examples:

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.190)

(b) Square planar complexes Some tetra-coordinated complexes show a square planar structure Examples:

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.190)

2. Hexa-coordinated complexes For complexes with coordination no. of 6, the ligands occupy octahedral position to minimize the repulsion from six electron pairs around the central metal ion Examples:

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.191) Shapes of tetra- and hexa-coordinated complexes Coordination number of the central metal atom or ion

Shape of complex

Example

Tetrahedral [Zn(NH3)4]2+ [CoCl4]2–

4 Square planar [Cu(NH3)4]2+ [CuCl4]2– 88

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.191) Shapes of tetra- and hexa-coordinated complexes (cont’d) Coordination number of the central metal atom or ion

Shape of complex

Example

Octahedral

[Cr(NH3)6]3+

6

89

[Fe(CN)6]3–

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.191)

Isomer Isomers are different compounds that have the same molecular formula • Isomers of complexes are classified into: 1. Structural isomers 2. Geometrical isomers

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.192)

1. Structural isomers Structural isomers are isomers that have different ligands bonded to the central metal atom or ion Example: Cr(H2O)6Cl3 has four structural isomers which have different colours:

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[Cr(H2O)6]Cl3

violet

[Cr(H2O)5Cl]Cl2 • H2O

light green

[Cr(H2O)4Cl2]Cl • 2H2O

dark green

[Cr(H2O)3Cl3] • 3H2O

brown

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.192)

2. Geometrical isomers Geometrical isomers are isomers that have different arrangement of ligands in space • Only square planar and octahedral complexes have geometrical isomers (a)

Square planar complexes (i)

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Square planar complexes of the form [Ma2b2] may exist in cis- or trans- form

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.192)

Example:

Isomers in which two ligands of the same type occupy adjacent corners of the square are called cis-isomer Isomers in which two ligands of the same type occupy opposite corners of the square are called trans-isomer 93

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.193)

(ii) Square planar complexes of the form [Ma2bc] may also exist in cis- or trans- form

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.193)

(b)

Octahedral complexes (i)

Octahedral complexes of the form [Ma4b2] may exist in cis- or trans- form

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.193)

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.193)

Example:

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.193)

(ii)

Octahedral complexes of the form [Ma3b3] may exist in fac- or mer- form

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.194)

Example:

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.195) Shape of Chemical complex formula

Geometrical isomer

[Ma2b2] Square planar

cis

trans

cis

trans

[Ma2bc]

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.195) Shape of complex

Chemical formula

Geometrical isomer cis

trans

fac

mer

[Ma4b2]

Octahedral

[Ma3b3]

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.195)

Check Point 45-6 (a) Are there any geometrical isomers for a complex of the form [Ma2b2]? Explain your answer with suitable drawings. (a) The square complex the form [Mab2bare (M represents the planar central metalofion, a and two 2] may in cis trans forms. differentexist kinds of and ligands.) Answer

There is no geometrical isomer for a tetrahedral complex of the form [Ma2b2] 102

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.195)

Check Point 45-6 (cont’d) (b) The four isomers of chromium(III) (i.e. [Cr(H2O)6]Cl3, (b)[Cr(H Besides using colours, suggest two experimental methods to 2O)5Cl]Cl2 • H2O, [Cr(H2O)4Cl2]Cl • 2H2O and distinguish the four isomers of Cr(H2O) 3: One [Cr(H2O)between Cl6–Cl ions. 3Cl3] • 3H2O) have different numbers of free [Cr(H , [Cr(H Cl]Cl way to distinguish is by2O) the5use of acidified 2O)6]Cl3them 2 • H2O,silver nitrate(V) solution. When excess AgNO is added to one mole of3each of the isomers, 3(aq) [Cr(H O) Cl ]Cl • 2H O, [Cr(H O) Cl ] • 3H O. 2 4 2 2 2 3 2 [Cr(H2O)6]Cl3 gives three moles of AgCl, [Cr(H2O)5Cl]Cl2 • H2O gives two moles of AgCl, [Cr(H2O)4Cl2]Cl • 2H2O gives one mole of AgCl,Answer and [Cr(H2O)3Cl3] • 3H2O does not give AgCl. Another way to distinguish them is by measuring their electrical conductivities. As the electrical conductivity depends on the number of ions formed when dissolved in water, [Cr(H2O)6]Cl3 has the highest electrical conductivity whereas [Cr(H2O)3Cl3] • 3H2O has the least.

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.195)

Coloured Ions • The natural colours of precious gemstones are due to the existence of small quantities of d-block metal ions • Most of the d-block metals form coloured compounds and most of their complexes are coloured too ∵the presence of incompletely filled

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d orbitals in the d-block metal ions New Way Chemistry for Hong Kong A-Level Book 4

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.196)

•

When a substance absorbs visible light of a certain wavelength, light of wavelengths of other regions of the visible light spectrum will be reflected or transmitted. ⇒ the substance will appear coloured

•

105

The absorption of light energy is associated with electronic transition (i.e. electron jumping from a lower energy level to a higher one). The energy required for electronic transition is quantized

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.196)

•

If the energy involved in electronic transition does not fall into visible light region, the substance will not appear coloured

•

s-block and p-block elements are usually colourless because an electronic transition is from one principle energy level to a higher one ⇒ the energy involved is too high in energy and it falls into ultraviolet region

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.196)

•

For the d-block elements, the five 3d orbitals are degenerate in gaseous ions

•

However, under the influence of a ligand, the 3d orbitals will split into 2 groups of orbitals with slightly different energy levels ⇒ due to the interaction of the 3d orbitals with the electron clouds of the ligands

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.196)

•

When a sufficient amount of energy is absorbed, electrons will be promoted from 3d orbitals at lower energy level to those at the higher energy level

•

The energy required for the d-d transition falls within the visible light spectrum. ⇒ This leads to light absorption, and reflects the remainder of the visible light ⇒ d-block metal ions have specific colours

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.197) The colours of some hydrated d-block metal ions Number of unpaired d electrons

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Hydrated ion

Colour

0

Sc3+ Ti4+ Zn2+ Cu+

Colourless

1

Ti3+ V4+ Cu2+

Purple Blue Blue

2

V3+ Ni2+

Green Green

3

V2+ Cr3+ Co2+

Violet Green Pink

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.197) The colours of some hydrated d-block metal ions (cont’d) Number of unpaired Hydrated ion Colour d electrons

Co2+(aq)

110

4

Cr2+ Mn3+ Fe2+

Blue Violet Green

5

Mn2+ Fe3+

Very pale pink Yellow

Zn2+(aq)

Fe3+(aq)

Mn2+(aq)

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Fe2+(aq)

Cu2+(aq)

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.197)

•

For d-d electronic transition and absorption of visible light to occur, there must be unpaired d electrons in the d-block metal atoms or ions ⇒ Sc3+ and Zn2+ are colourless due to the empty 3d sub-shell and the fully-filled 3d sub-shell respectively

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.197)

•

The colours of hydrated metal ions are determined by the oxidation states of the particular d-block elements e.g. Fe2+(aq) is green while Fe3+(aq) is yellow ⇒ different oxidation states are caused by different numbers of d electrons in the d-block metal ion ⇒ this has direct effects on the wavelength of the radiation absorbed during electronic transition

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.198)

Catalytic Properties of Transition Metals and their Compounds The use of some d-block metals and their compounds as catalysts in industry d-block element Catalyst Reaction catalyzed V Fe Ni

Pt 113

Contact process V2O5 or vanadate(V)(VO3–) 2SO2(g) + O2(g)

2SO3(g)

Fe

Haber process N2(g) + 3H2(g)

Ni

Hardening of vegetable oil (Manufacture of margarine) RCH = CH2 + H2 → RCH2CH3

2NH3(g)

Catalytic oxidation of ammonia Pt (Manufacture of nitric(V) acid) 4NH3(g) + 5O2(g) → 4NO(g) + 2O(l) New Way Chemistry for Hong6H Kong A-Level Book 4

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.198)

•

d-block metals and their compounds exert their catalytic actions in either heterogeneous catalysis or homogeneous catalysis

•

The function of a catalyst is to provide an alternative pathway of lower activation energy ⇒ enabling the reaction to proceed faster than the uncatalyzed one

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.198)

Heterogeneous Catalysis

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•

In heterogeneous catalysis, the catalyst and reactants are in different phases

•

The most common heterogeneous catalysts are finely divided solids for gaseous reactions

•

A heterogenous catalyst provides a suitable reaction surface for the reactants to come close together and react

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.199)

e.g.:

Synthesis of gaseous ammonia from N2 and H2 N2(g) + 3H2(g) 2NH3(g)

•

In the absence of a catalyst, the formation of gaseous ammonia proceeds at an extremely low rate ∵ the probability of collision of four gaseous molecules is very small ⇒ the four reactant molecules have to collide in a proper orientation in order to give products ⇒ the bond enthalpy of N ≡ N is very large ⇒ the reaction has a high activation energy

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•

In the presence of iron catalyst, the reaction proceeds faster as it provides an alternative reaction pathway

•

The catalyst exists in a different phase from that of both reactant and products

•

The catalytic action occurs at the interface between two phases, and the metal provides an active reaction surface for the reaction to occur

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.200)

The catalytic mechanism of the formation of NH3(g) from N2(g) and H2(g) 118

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.200)

Energy profiles of the reaction pathways in the presence and absence of a heterogeneous catalyst 119

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.199)

Summary: • In heterogeneous catalysis, the d-block metals or compounds provide a suitable reaction surface for the reaction to take place ∵ the presence of partly-filled d-orbitals ⇒ this enables the metals to accept electrons from reactant particles on one hand and donate electrons

to

reactant particles on the other 120

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.201)

Homogeneous Catalysis

•

A homogenous catalyst is in the same phase as the reactants and products

•

The catalyst forms an intermediate with the reactants ⇒ it changes the reaction mechanism to a new one with a lower activation energy

•

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The ability of d-block metals to exhibit variable oxidation states enables the formation of the reaction intermediates New Way Chemistry for Hong Kong A-Level Book 4

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.201)

• e.g. reaction between peroxodisulphate(VI) ions and iodide ions S2O82–(aq) + 2I–(aq)

2SO42–(aq) + I2(aq) Ecell = +1.47 V

•

The standard e.m.f. calculated for the reaction is a highly positive value ⇒ there is high tendency for the forward reaction to occur

•

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However the reaction is very slow due to kinetic factors

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.201)

•

In catalytic process, Fe3+(aq) ions oxidizes I–(aq) to I2(aq) with themselves being reduced to Fe2+(aq) 2I–(aq) + 2Fe3+(aq)

I2(aq) + 2Fe2+(aq) Ecell = +0.23 V

• The Fe2+(aq) are subsequently oxidized by S2O82–(aq) and Fe3+(aq) ions are regenerated 2Fe2+(aq) + S2O82–(aq)

2Fe3+(aq) + 2SO42–(aq) Ecell = +1.24 V

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.201)

The overall reaction: 2I–(aq) + 2Fe3+(aq) +) 2Fe2+(aq) + S2O82–(aq) 2I–(aq) + S2O82–(aq) •

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I2(aq) + 2Fe2+(aq) 2Fe3+(aq) + 2SO42–(aq) I2(aq) + 2SO42–(aq)

Fe(III) ions catalyze the reaction by acting as an intermediate for the transfer of electrons between peroxodisulphate(VI) and iodide ions

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.201) Energy profiles for the oxidation of I–(aq) ions by S2O82–(aq) ions in the presence and absence of a homogeneous catalyst

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45.3 Characteristic Properties of the d-Block Elements and their Compounds (SB p.202)

Check Point 45-7 Which of the following redox systems might catalyze the oxidation of iodide ions by peroxodisulphate(VI) ions in an aqueous solution? Cr2O72–(aq) + 14H+(aq) + 6e–

2Cr3+(aq) + 7H2O(l) E +2.01V = +1.33V Systems with E greater than +0.54V and smaller than are

to + catalyze of iodideMn ions2+by peroxodisulphate(VI) ions – + the oxidation – MnOable (aq) 8H (aq) + 5e (aq) + 4H O(l) 4 in an aqueous solution. Hence, the following two redox 2systems are able to E = +1.52V catalyze the reactions. – Cr2O+72–2e (aq) + 14H+(aq) + (aq) 6e– Sn4+(aq) Sn2+ + – MnO4–(aq) 2– + 8H (aq) – + 5e

2Cr3+(aq) + 7H2O(l) E = +0.15V Mn 2–

(Given: S2O8 (aq) + 2e 2SO4 (aq)(aq) + 4H2O(l) E = +2.01V I2(aq) + 2e– 2I–(aq) E = +0.54V) 2+

Answer

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The END

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