Coordination Compounds

Ligands are the neutral or negatively charged entities surrounding the central metal atom of the coordination complex which possesses at least one unshared pair of electrons.

Based on the number of donor sites of these ligands, Ligands are classified as:

Unidentate ligands: These Ligands which have only one donor site are called unidentate ligands.

Example: F^-, Cl ^– etc.

Didentate ligands: These Ligands which have only two donor site are called didentate ligands.

Example: Ethane-1,2-diamine, Oxalate ion etc.

Ambidentate ligands: These ligands which can attach them with the central metal atom by two different atoms are called as ambidentate ligands.

(1) Coordination Entity: Coordination entity is a charged entity having positive or negative charge in which the central atom is surrounded by molecules which may be neutral/negatively charged called Ligands

Examples:

i. Cationic Complexes: [Cu (H₂O)₆]²⁺, [Al (H₂O)₆]³⁺

ii. Anionic Complexes: [CuCl₄]^2-, [Al (H₂O)₂ (OH)₄]^-

iii. Neutral Complexes: [Co (NH₃)₄ Cl₂], [Ni (CO)₄]

(2) Ligands: Ligands are the neutral or negatively charged entities surrounding the central metal atom of the coordination complex which possesses at least one unshared pair of electrons

Example: F^-, Cl^-, Br^-, I^-, H₂0, and NH₃

(3) Coordination Number: Coordination Number which is also called as Ligancy is the total number of ligands that are attached to the central metal atom of the coordination

Example: [Cr (NH₃)₂Cl₂Br₂]^? has Cr³⁺ as its central cation, and has a coordination number of 6

Al³⁺ has coordination number 4 in [AlCl₄]^- but 6 in [AlF₆]^3-.

(4) Coordination polyhedron: Coordination polyhedron is defined as the spatial arrangement of the ligands around the central atom of a coordination

Example:

In the figure: The pink Sphere depicts the central atom of the coordination entity.

(5) Homoleptic complexes: Complexes in which the central metal atom is surrounded only by the same kind of donor groups which are the ligands.

Example: [Ag (CN)₂]^-, [Fe (CN₆)]⁴⁺ etc.

(6) Heteroleptic complexes: Complexes in which the central metal atom is surrounded by more than one kind of donor groups which are the ligands.

Example: [Co (NH₃)₄ Cl₂] ⁺, [Co (NH₃)₅ Cl]²⁺

Bonding in coordination compounds in terms of Werner's postulates is explained as:

(a) Metals can show two types of valencies which are Primary valency and Secondary

1. Primary Valency: Primary Valency shows Oxidation Primary valencies are ionizable.

2. Secondary Valency: Secondary Valency shows coordination These are non-ionizable.

(b) Both Primary and secondary valency of the metal are to be satisfied which is done by negative ions in case of primary valency and negative or neutral species in case of secondary

(c) Metals have a fixed number of secondary valencies/ Coordination number around the central atom, these secondary valencies are placed in such a way which leads to a specific geometry of the coordination

In [Fe (H₂O)₆]³⁺

Electronic configuration of Fe is: [Ar]3d⁶4s²

[Ar] = 1s²2s²2p⁶3s²3p⁶

Electronic configuration of Fe⁺³ = [Ar]3d⁵

Outer electronic configuration of Fe⁺³ = 3d⁵

Overall charge balance:

X + 6 (0) = 3

X = + 3

In [Fe (CN)₆]^3-

Overall charge balance:

X + 6 (-1) = -3

X = + 3

In both the compounds Fe is in + 3 oxidation state.

In case of [Fe (H₂O)₆]³⁺

H₂O is weak field ligand so it does not pair the unpaired electron. Total no. of the unpaired electron, n =5, Spin only magnetic moment is given by:

μ = [n (n + 2)]^1/2

μ = [5*7]^1/2

μ = 5.916BM

In case of [Fe (CN)₆]^3-

CN^- is a strong field ligand so it pairs up the electron.

Total no. of unpaired electrons = 1

Spin only magnetic moment is given by:

μ = [n (n + 2)]^1/2

μ = [1*3]^1/2

μ = 1.732BM

as we can see spin only magnetic moment of [Fe (H₂O)₆]³⁺ is more than [Fe (CN)₆]^3- .

so, [Fe (H₂O)₆]³⁺ is strongly paramagnetic whereas [Fe (CN)₆]³ weakly paramagnetic.

Fe in ground state

Fe in + 3 oxidation state

Electrons from 5 CN^- ligands

In [Ni (Cl)₄]^2- ion, Cl^- is a weak field ligand so it will not pair the unpaired electrons of Ni⁺² ion. Electronic configuration of Ni is: [Ar]3d⁸4s² where [Ar] = 1s²2s²2p⁶3s²3p⁶

Electronic configuration of Ni⁺² = [Ar]3d⁸ Outer electronic configuration of Ni⁺² = 3d⁸ Overall charge balance:

X + 4 (-1) = -2 X = + 2.

Therefore it undergoes sp³ hybridization. So it will have tetrahedral geometry.

Since there are 2 unpaired electrons in the d orbital so it is a paramagnetic compound. In [Ni (Co)₄]:

Overall charge is neutral and oxidation state of Ni can be calculated as:

X + 4 (0) = 0

x = 0

Ni is in zero oxidation state.

Co is a strong field ligand it causes pairing of the 4 unpaired electrons in d orbital. Also, it causes the 4s electrons to shift to the 3d orbital, thereby undergoes sp³ hybridisation forming tetrahedral geometry. Since it has no unpaired electron, therefore it is diamagnetic compound.

According to the valence band theory, the central metal atom or ion under the influence of ligands can use its (n-1)d, ns, np (inner orbital complex) or ns, np, and (outer orbital complex)orbitals for hybridisation to form equivalent set of orbitals of definite geometry.

In [Ni (CN)₄]^2-, oxidation state of Ni can be calculated as :

Using overall charge balance as the whole ion has overall -2 charge:

x + 4 (-1) = -2 (? CN^- has -1 negative charge) x = + 2

Ni is in + 2 oxidation state.

Electronic configuration of Ni is: [Ar]3d⁸4s² Where, [Ar] = 1s²2s²2p⁶3s²3p⁶

Electronic configuration of Ni⁺² = [Ar]3d⁸ Outer electronic configuration of Ni⁺² = 3d⁸

Since there are 4 CN ions so they can either form tetrahedral or square planar geometry. And CN^- is a strong field ligand (according to experimental data of spectro-chemical series) it causes pairing of the 2 unpaired electrons.

It undergoes dsp² (one d orbital, one s and two p orbitals used by the ligands) hybridization and forms square planar structure.

Since all the electrons are paired so it is diamagnetic.

Paramagnetic compounds- those compounds which have one or more no. of unpaired electrons in their atomic orbitals.

Diamagnetic compounds-those compounds in which all the electrons in their atomic orbitals are paired.

In case of [Ni (Cl)₄]^2- ion, Cl^- is a weak field ligand so it will not pair the unpaired electrons of Ni⁺² ion. Therefore it undergoes sp³ hybridization.

Overall charge balance: X + 4 (-1) = -2

X = + 2.

Since there are 2 unpaired electrons in the d orbital so it is a paramagnetic compound.