Coordination Compounds.pdf

COORDINATION COMPOUNDS INTRODUCTION :-

A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bonds. • Ligands are Lewis bases - they contain at least one pair of electrons to donate to a metal atom/ion. Ligands are also called complexing agents. • Metal atoms/ions are Lewis acids - they can accept pairs of electrons from Lewis bases. • Within a ligand, the atom that is directly bonded to the metal atom/ion is called the donor atom. • A coordinate covalent bond is a covalent bond in which one atom (i.e., the donor atom) supplies both electrons. This type of bonding is different from a normal covalent bond in which each atom supplies one electron. • If the coordination complex carries a net charge, the complex is called a complex ion.

• Compounds that contain a coordination complex are called coordination compounds. • Coordination compounds and complexes are distinct chemical species - their properties and behavior are different from the metal atom/ion and ligands from which they are composed. • The coordination sphere of a coordination compound or complex consists of the central metal atom/ion plus its attached ligands. The coordination sphere is usually enclosed in brackets when written in a formula.

• The coordination number is the number of donor atoms bonded to the central metal atom/ion.

1. [Co(NH3)6]Cl3

hexaamminecobalt (III) chloride

6

+3

2. K[AuCl4]

Potassium tetrachloroaurate(III)

4

+3

3. Cu2[Fe(CN)6]

copper(II) hexacyanoferrate(II)

6

+2

4. [Pt(NH3)6]Cl4

hexaammineplatinum(IV) chloride

6

+4

5. [Cu(NH3)4(H2O)2]Cl2 tetraamminediaquacopper(II) chloride

6

+2

6. Cr(CO)6

hexacarbonylchromium (0)

6

0

7. K3[CoF6]

potassium hexafluorocobaltate(III)

6

+3

8. [Pt(en)2]CO3

bis(ethylenediamine)platinum(II) carbonate

4

+2

9. [Ni(H2O)6]Cl2

hexaaquanickel(II) chloride

6

+2

10 [Zn(en)F2]

(ethylenediamine)difluorozinc

4

+2

11. [Cr(NH3)5(NO2)]2+

pentaamminenitritochromium(III) ion

6

+3

12. Ba[FeBr4]2

barium tetrabromoferrate(III)

4

+3

13. [Co(en)2Br2]2SO4

dibromobis(ethylenediamine)cobalt(III) sulfate

6

+3

14. [Ag(NH3)2]Cl

diamminesilver chloride

2

+1

15. [Cu(CN)4]3-

tetracyanocuprate(I) ion

4

+1

➢ Why are coordination compounds Formed?

Originally, a complex implied a reversible association of

molecules, atoms, or ions through such weak chemical bonds. As applied to coordination chemistry, this meaning has evolved. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are quite strong.

❖The transition elements and main group elements can form coordination compounds, or complexes in which a central metal atom or ion is bonded to one or more ligands by coordinate covalent bonds. Ligands with more than one donor atom are called polydentate ligands and form chelates.

Color of Transition Metal Complexes The variety of color among transition metal complexes has long fascinated the chemists. For example, aqueous solutions of [Fe(H2O)6]3+ are red, [Co(H2O)6]2+ are pink, [Ni(H2O)6]2+ are green, [Cu(H2O)6]2+ are blue and [Zn(H2O)6]2+ are colorless. Although the octahedral [Co(H2O)6]2+ are pink, those of tetrahedral [CoCl4]2- are blue. The green color of [Ni(H2O)6]2+ turns blue when ammonia is added to give [Ni(NH3)6]2+. Many of these facts can be rationalized from CFT.

Why we see Color?

When a sample absorbs light, what we see is the sum of the remaining colors that strikes our eyes. If a sample absorbs all wavelength of visible light, none reaches our eyes from that sample, and then the sample appears black. If the sample absorbs no visible light, it is white or colorless. When the sample absorbs a photon of visible light, it is its complementary color we actually see. For example, if the sample absorbed orange color, it would appear blue; blue and orange are said to be complementary colors.

Color of Coordination Complexes The color of coordination complexes arises from electronic transitions between levels whose spacing corresponds to the wavelengths available in the visible light. In complexes, these

transitions are frequently referred to as d-d transitions because they involve the orbitals that are mainly d in character (for examples: t2g and eg for the octahedral complexes and e and t2 for the tetrahedral complexes). Obviously, the colors exhibited are intimately related to the magnitude of the spacing between these levels. Since this spacing depends on factors such as the geometry of the complex, the nature of the ligands, and the oxidation state of the central metal atom, variation on colors can often be explained by looking carefully at the complexes concerned. Below are a few examples to illustrate this

▪ The variation in the color of the Cr(III) complexes can be

explained following similar arguments

▪ [V(H2O)6]3+ [V(H2O)6]2+ V(III) = d2 ion V(II) = d3 ion violet light absorbed yellow light absorbed complex appears yellow complex appears violet

importance of coordination compounds Application Coordination compounds are used as catalysts for many industrial processes and have many applications in qualitative/quantitative chemical analysis within analytical chemistry. The of coordination compounds has importance in biological system, the coordination compounds play a vital role in metallurgy and medicine. Coordination compounds are used as catalysts for many industrial processes and have many applications in qualitative/quantitative chemical analysis within analytical chemistry. The of coordination compounds has importance in

biological system, the coordination compounds play a vital role in metallurgy and medicine. for example it issued to detect cupric ions in the salt, when copper sulphate solution is mixed with aqueous ammonia, a deep blue complex soluble in water is formed. There is a application of complexes in metallurgy as follows: (a) Noble metals like silver and gold are extracted from their ore by the formation of cyanide complexes - dicyanoargentite and dicyanoaurate (b) Metals can be purified by the formation and subsequent decomposition of their coordination compounds.

Nickel can be purified by its reaction with carbon monoxide to form the volatile complex, tetracarbonyl Nickel (0), which is decomposed thermally to yield pure Nickel. The application of complexes in biological systems as follows: (a) The hemoglobin red pigment of blood that acts as oxygen carrier is a coordination compound of iron. (b) There is so many enzymes that regulate biological processes are metal complexes. Carboxypeptidase is a protease enzyme that hydrolytic enzyme important in digestion, contains a zinc ion coordinated to several amino acid residues of the protein. The applications of coordination compounds as catalysts in industrial processes as follows:

(a) In the polymerization of ethene, The Ziegler-Natta catalyst is used which is a combination of titanium tetrachloride and tri ethyl aluminum. (b) In the hydrogenation of alkenes, a complex metal catalyst is used The applications of coordination compounds in analytical chemistry as follows: (a) In the qualitative methods of analysis, complex formation is of immense importance in the identification and separation of most inorganic ions. (When copper sulphate solution is mixed with aqueous ammonia, a deep blue complex soluble in water is formed. This reaction is used to detect cupric ions in the salt).

(b) The water hardness is estimated by titration with the sodium salt of EDTA, the calcium and magnesium ions in hard

water forms the stable complexes, magnesium EDTA and calcium EDTA.

Examples of ambidentate ligands SCN- is an example of an ambidentate ligand. This is because it can bond to a coordination center through nitrogen as well as sulphur. The below image shows how SCN- can act as an ambidentate ligand.

As you can see, SCN- can bond either through the sulphur atom or the nitrogen atom, but not at the same time.

Another example of an ambidentate ligand is NO2-. This ion can bind to a central atom through either the nitrogen or one of the oxygen atoms, but again, not at the same time.

NOMENCLATURE OF COORDINATION COMPOUNDS IUPAC RULE

INTRODUCTION According to the Lewis base theory, ligands are Lewis bases since they can donate electrons to the central metal atom. The metals, in turn, are Lewis acids since they accept electrons. Coordination complexes consist of a ligand and a metal center cation. The overall charge can be positive, negative, or neutral. Coordination compounds are complex or contain complex ions, for example: • • • •

Complex Cation: [CO(NH3)6]3+ Complex Anion: [CoCl4(NH3)2]− Neutral Complex: [CoCl3(NH3)3][CoCl3(NH3)3] Coordination Compound: K4[Fe(CN)6]

A ligand can be an anion or a neutral molecule that donates an electron pair to the complex (NH3, H2O, Cl-). The number of

ligands that attach to a metal depends on whether the ligand is monodentate, bidentate, or polydentate. For more information, see Ligands and Chelation. To begin naming coordination complexes, here are some things to keep in mind. 1. Ligands are named first in alphabetical order. 2. The name of the metal comes next. 3. The oxidation state of the metal follows, noted by a Roman numeral in parentheses (II, IV)

Rule 1: Anionic Ligands Ligands that act as anions which end in "-ide" are replaced with an ending "-o" (e.g., Chloride → Chloro). Anions ending with "-ite" and "-ate" are replaced with

endings "-ito" and "-ato" respectively (e.g., Nitrite → Nitrito, Nitrate → Nitrato)

Table 1: Anionic Monodentate Ligands Molecular Formula

Ligand Name

Molecular Formula

Ligand Name

F-

Fluoro

OH-

Hydroxo

Cl-

Chloro

SO42-

Sulfato

Br-

Bromo

S2O32-

Thiosulfato

I-

Iodo

O2-

Oxo

ONO-

Nitrito-O-; Nitrito

CN-

Cyano

SCN-

Thiocyanato-S-; Thiocyanato

NO2-

Nitrito-N-; Nitro

NC-

Isocyano

NCS-

Thiocyanato-N-; Isothiocyanato

Rule 2: Neutral Ligands Most neutral molecules that are ligands carry their normal name. The few exceptions are the first four on the chart: ammine, aqua, carbonyl, and nitrosyl.

Table 2 : Select Neutral Monodentate Ligands.Note: Ammine is spelled with two m's when referring to a ligand. Amines are a class of organic nitrogencontaining compounds.

Molecular Formula of Ligand NH3 H2O

Ligand Name

Ammine Aqua

CO

Carbonyl

NO

Nitrosyl

CH3NH2 C5H5N

Methylamine Pyridine

Polydentate ligands follow the same rules for anions and neutral molecules.

Short name en

Extended name

Ethylenediamine

ox2EDTA4-

Oxalato Ethylenediaminetetraacetato

Table 3: Select Polydentate ligands RULE 3: LIGAND MULTIPLICITY The number of ligands present in the complex is indicated with the prefixes di, tri, etc. The exceptions are polydentates that have a prefix already in their name (en and EDTA4- are the most common). When indicating how many of these are

present in a coordination complex, put the ligand's name in parentheses and use bis, tris, and tetrakis. Table 4: Prefixes for indicating number of ligands in a complex Number of Ligands

1

Monodentate Ligands Polydentate Ligands mono

-

2

di

bis

3

tri

tris

4

tetra

tetrakis

5

penta

-

6

hexa

-

Prefixes always go before the ligand name; they are not taken into account when putting ligands in alphabetical order. Note that "mono" often is not used. For example, [FeCl(CO)2(NH3)3]2+ would be called triamminechlorodicarbonyliron(III) ion. Remember that ligands are always named first, before the metal is EXERCISE 1 What is the name of this complex ion: [CrCl2(H2O)4]?

SOLUTION Let's start by identifying the ligands. The ligands here are Cl and H2O. Therefore, we will use the monodentate ligand names of "chloro" and "aqua". Alphabetically, aqua comes before chloro, so this will be their order in the complex's name. There are 4 aqua's and 2 chloro's, so we will add the number prefixes before the names.

Since both are monodentate "tetra[aqua]di[chloro]".

ligands,

we

will

say

Now that the ligands are named, we will name the metal itself. The metal is Cr, which is chromium. Therefore, this coordination complex is called tetraaquadichlorochromium(III) ion. See the next section for an explanation of the (III). EXERCISE 1 What is the name of this.complex ion: [CoCl2(en)2]+? SOLUTION There are two chloro and ethylenediamine ligands. The metal is Co, cobalt. We follow the same steps, except that en is a polydentate ligand with a prefix in its name (ethylenediamine), so "bis" is used instead of "bi", and parentheses are added. Therefore, this coordination complex is called dichlorobis(ethylenediamine)cobalt(III) ion.

RULE 4: THE METALS When naming the metal center, you must know the formal metal name and the oxidation state. To show the oxidation state, we use

Transition Metal

Latin

Iron

Ferrate

Copper

Cuprate

Tin

Stannate

Silver

Argentate

Lead

Plumbate

Gold

Aurate

Roman numerals inside parenthesis. For example, in the problems above, chromium and cobalt have the oxidation state of +3, so that is why they have (III) after them. Copper, with an oxidation state of +2, is denoted as copper(II). If the overall coordination complex is an anion, the ending "-ate" is attached to the metal center. Some metals also change to their Latin names in this situation. Copper +2 will change into cuprate(II). The following change to their Latin names when part of an anion complex:

Table 5: Latin terms for Select Metal Ion The rest of the metals simply have -ate added to the end (cobaltate, nickelate, zincate, osmate, cadmate, platinate, mercurate, etc. Note that the -ate tends to replace -um or -ium, if present). Finally, when a complex has an overall charge, "ion" is written after it. This is not necessary if it is neutral or part of a coordination compound (Example 3). Here are some examples with determining oxidation states, naming a metal in an anion complex, and naming coordination compounds.

Examples Give the systematic names for the following coordination compounds: 1. [Cr(NH3)3(H2O)3]Cl3

Answer: triamminetriaquachromium(III) Solution: •

•

•

•

chloride

The complex ion is found inside the parentheses. In this case, the complex ion is a cation. The ammine ligands are named first because alphabetically, “ammine” comes before “aqua.” The compound is electrically neutral and thus has an overall charge of zero. Since there are three chlorides associated with one complex ion and each chloride has a –1 charge, the charge on the complex ion must be +3. From the charge on the complex ion and the charge on the ligands, we can calculate the oxidation number of the metal. In this example, all the ligands are neutral molecules.

Therefore, the oxidation number of chromium must be the same as the charge of the complex ion, +3. 2. K4[Fe(CN)6]

Answer:

potassium hexacyanoferrate(II) Solution: • •

•

•

Potassium is the cation, and the complex ion is the anion. Since there are 4 K+ associated with the complex ion (each K+ having a +1 charge), the charge on the complex ion must be - 4. Since each ligand carries –1 charge, the oxidation number of Fe must be +2. The common name of this compound is potassium ferrocyanide.

3. Pt(NH3)2Cl4

Answer: diamminetetrachloroplatinum(IV) Solution: •

•

4.

Answer:

This is a neutral molecule because the charge on Pt+4 equals the negative charges on the four chloro ligands. If the compound is [Pt(NH3)2Cl2]Cl2, even though the number of ions and atoms in the molecule are identical to the example, it should be named: diamminedichloroplatinum(IV) chloride because the platinum in the latter compound is only four coordinated instead of six coordinated.

Fe(CO)5 pentacarbonyliron(0) •

Solution:

Since it is a neutral complex, it is named in the same way as a complex cation. The common name of this compound, iron carbonyl, is used more often.

[Pt(H2NCH2CH2NH2)2Cl2]Cl2 Answer: dichlorobis(ethylenediamine)platinum(IV) Solution: 5.

•

chloride

Since Ethylenediamine is a bidentate ligand, the prefix bisis used instead of the prefix di-.

[Co(H2NCH2CH2NH2)3]2(SO4)3 Answer: tris(ethylenediamine)cobalt(III) Solution: 6.

•

•

•

•

sulfate

The sulfate has a charge of –2 and is the counter anion in this molecule. Since it takes 3 sulfates to bond with two complex cations, the charge on each complex cation must be +3. Since ethylenediamine is a neutral molecule, the oxidation number of cobalt in the complex ion must be +3. Again, remember that you never have to indicate the number of cations and anions in the name of an ionic compound.

The Jahn-Teller Theorem In a nonlinear molecule, if degenerate orbitals are asymmetrically occupied, a distortion will occur to remove the degeneracy. (or) In an electronically degenerate state, a nonlinear molecule undergoes distortion to remove the degeneracy by lowering the symmetry and thus by lowering the energy.

What is electronically degenerate state? An electronically degenerate state represents the availability of more than one degenerate orbitals for an electron. In this condition the degenerate orbitals are asymmetrically occupied. E.g. In octahedral symmetry, the d1 configuration is said to be electronically degenerate since three t2g orbitals with same energy are

available for the electron to occupy. In this condition, the degenerate orbitals are also said to be asymmetrically occupied by electrons.

Whereas the d3 configuration in octahedral geometry is nondegenerate and symmetric. It is not possible to put two electrons in one orbital, which is against of Hund's rule of maximum multiplicity.

important is that if the two orbitals of the eg level have different numbers of electrons, this will lead to J-T distortion. Cu(II) with its d9 configuration is degenerate and has J-T distortion:

JAHN TELLER DISTORTION : EXPLANTION the electronically degenerate state, the orbitals are said to be asymmetrically occupied and get more energy. Therefore the system tries to get rid of this extra energy by lowering the overall symmetry of the molecule i.e., undergoing distortion, which is otherwise known as Jahn Teller distortion. E.g. In case of octahedral d9 configuration, the last electron may occupy either dz2 or dx2-y2 orbitals of eg set. If it occupies dz2 orbital, most of the electron density will be concentrated between the metal and the two ligands on the z axis. Thus, there will be greater electrostatic repulsion associated with these ligands than with the other four on xy plane. This asymmetric distribution of the electron density may increase the overall energy of the system. To get rid of this, the complex suffers elongation of bonds on z-axis and thus lowers the symmetry.

Conversely, occupation of the dx2-y2 orbital would lead to elongation of bonds along the x and y axes. • The Jahn Teller distortion is mostly observed in octahedral environments. Theoretically the electronic degeneracy in octahedral symmetry is possible in all the configurations except d3, d8, d10, high spin d5 and low spin d6 configurations.

However considerable distortions are usually observed in high spin d4 , low spin d7 and d9 configurations in the octahedral environment. It is because the Jahn Teller distortion is usually

significant for asymmetrically occupied eg orbitals since they are directed towards the ligands and the energy gain is considerably more. • In case of unevenly occupied t2g orbitals, the Jahn Teller distortion is very weak since the t2g set does not point directly at the ligands and therefore the energy gain is much less. E.g. d1; d2; low spin d4 & d5; high spin d7 & d7 configurations. Because of same reason, the tetrahedral complexes also do not exhibit Jahn-Teller distortion. Again, in this case also the ligands are not pointing towards the orbitals directly and hence there is less stabilization to be gained upon distortion.

d4 HS stabilization is driving force for distortion

no net change in energy

tetragonal elongation

Note: The Jahn-Teller distortion in case of low spin d8 octahedral systems to give square planar complexes as explained by some authors is controversial Jahn-Teller Theorem: electron configurations with unequal occupancy of degenerate orbitals are not stable. d4 HS Various types of distortions are possible (tetragonal, trigonal, etc.).

d4 HS

stabilization is driving force for distortion.

no net change in energy

tetragonal

compression

electron configurations with unequal occupancy of degenerate orbitals are not stable. d5 HS Only some complexes can lower their energy by distorting

Square Planar Complexes

Consider a CFT diagram of a tetragonal elongation taken to its extreme:

Z-OUT & Z-IN JAHN TELLER DISTORTIONS The degeneracy of orbitals can be removed by lowering the symmetry of molecule. This can be achieved by either elongation of bonds along the z-axis (Z-out distortion) or by shortening the bonds along the z-axis (Z-in distortion). Thus an octahedrally symmetrical molecule is distorted to tetragonal geometry. *

Z-out distortion: In this case, the energies of d-orbitals with z factor ( i.e., dz2, dxz, dyz ) are lowered since the bonds along the z-axis are elongated. This is the most preferred distortion and occurs in most of the cases, especially when the degeneracy occurs in eg level. E.g. Usually the octahedral d2, d4 high spin, d7 low spin, d8 low spin & d9 configurations show the z-out distortion. Theoretically it is not possible to predict the type of distortion occurs when the degeneracy occurs in eg level. However it is observed that zout distortion is more preferred.

Z-in distortion: In this case the energies of orbitals with z factor are increased since the bonds along the z-axis are shortened. This type of distortion is observed in case of octahedral d1 configuration. The only electron will now occupy the dxy orbital with lower energy. E.g. The octahedral d1 configurations like Ti(III) in [Ti(H2O)6]3+can show z-in distortion (theoretically?). In this case, the z-out distortion do not remove the degeneracy since even after distortion there are still two degenerate orbitals i.e., dxz and dyz available for the electron to occupy. See the following diagram. Also remember that the Jahn-Teller theorem does NOT predict how large a distortion should occur.

CERTIFICATE This is to certified that the project work entitled 'JAHN-TELLER DISTORTION AND IUPAC NOMENCLATURE' is a bonafied work carried out by ASMITA SINGH , ROLL NO. :- 16CHE013 under the guidance of Dr.SASMITA SAMAL in partial fulfilment for the award of BSc in chemistry of GOVERNMENT AUTONOMOUS COLLEGE ROURKELA , PANPOSH during the year 2016 - 2019 .The project have been approved as it certifies the academic requirement in respect of project work prescribed.

Signature of H.O.D

Signature of

Dr. SASMITA SAMAL

Principle