Overview of Coordination Compounds

A compound in which a central metal atom or ion is bonded to a fixed number of ions or molecules through coordinate bonds is called coordination compound.

The central metal atom/ion together with the ligands attached to it enclosed in square brackets is called coordination entity.

The atom or ion to which a fixed number of ligands are bonded in a definite geometrical arrangement is called central atom or ion.

An ion or molecule which donates one or more pairs of electrons to the central metal atom/ion is called ligand.

The number of donor atoms directly bonded to the central metal atom/ion in a complex is called coordination number.

The central metal atom/ion along with the ligands attached to it and enclosed in square brackets is called coordination sphere.

The spatial arrangement of ligands directly bonded to the central metal atom defines a geometry which is called coordination polyhedron.

A complex in which the central metal atom is bonded to only one kind of ligand is called homoleptic complex.

A complex in which the central metal atom is bonded to more than one kind of ligand is called heteroleptic complex.

The isomerism arising due to different spatial arrangement of ligands around the central metal atom is called geometrical isomerism.

The isomerism in which complexes are non-superimposable mirror images of each other is called optical isomerism.

The isomerism arising due to different modes of attachment of an ambidentate ligand is called linkage isomerism.

The isomerism arising due to exchange between a ligand inside the coordination sphere and an ion outside it is called ionisation isomerism.

The isomerism arising due to difference in the number of solvent molecules inside and outside the coordination sphere is called solvate isomerism.

Statement:
Werner proposed that metals in coordination compounds exhibit two types of valencies — primary and secondary valencies.

Explanation:

• Primary valency corresponds to oxidation state.
• Secondary valency corresponds to coordination number.
• Primary valencies are ionisable.
• Secondary valencies are non-ionisable and have definite geometry.
• Secondary valencies are directed in space.

Statement:
Coordination compounds are named by following additive nomenclature rules recommended by IUPAC.

Important Rules:

• Cation named first.
• Ligands named alphabetically before metal.
• Prefixes: di, tri, tetra, etc.
• Oxidation state written in Roman numerals.
• Metal name ends with “-ate” in anionic complexes.

Statement:
Coordination compounds exhibit isomerism due to different arrangement of ligands either in space or within the coordination sphere.

Types:

• Structural isomerism
• Stereoisomerism

Explanation:
Structural isomers differ in bonding, while stereoisomers differ in spatial arrangement.

Statement:
According to Valence Bond Theory, metal ions undergo hybridisation of atomic orbitals to form equivalent hybrid orbitals which overlap with ligand orbitals to form coordinate bonds.

Explanation:

• Explains geometry (tetrahedral, square planar, octahedral)

• Predicts magnetic behaviour

• Inner orbital and outer orbital complexes possible

Statement:
In an octahedral field, the five degenerate d-orbitals split into two sets of different energies due to electrostatic repulsion between ligand electrons and metal d-electrons.

Explanation:

• d_x²−y²,d_z²​ form e_g (higher energy)

• d_xy,d_xz,d_yz​ form t_2g​ (lower energy)

• Energy difference = Δ₀

• Splitting depends on ligand strength

A complex in which inner (n−1)d orbitals participate in hybridisation is called inner orbital complex.

A complex in which outer nd orbitals participate in hybridisation is called outer orbital complex.

The splitting of degenerate d-orbitals in the presence of ligands due to electrostatic interactions is called crystal field splitting.

The arrangement of ligands in order of increasing field strength is called spectrochemical series.

Ligands that produce small crystal field splitting and form high spin complexes are called weak field ligands.

Ligands that produce large crystal field splitting and form low spin complexes are called strong field ligands.

A complex in which electrons occupy higher energy orbitals before pairing due to small Δ₀ is called high spin complex.

A complex in which electrons pair in lower energy orbitals due to large Δ₀ is called low spin complex.

A compound containing carbon monoxide ligand bonded to a transition metal is called metal carbonyl.

The mutual strengthening of σ-donation and π-back bonding between metal and ligand is called synergic bonding.

Statement:
The electronic configuration of a complex depends on the relative magnitude of crystal field splitting energy (Δ₀) and pairing energy (P).

Explanation:

• If Δ₀ < P → High spin complex

• If Δ₀ > P → Low spin complex

• Strong field ligands → Low spin

• Weak field ligands → High spin

Statement:
The colour of coordination compounds arises due to d–d electronic transitions between split d-orbitals in presence of ligands.

Explanation:

• Absorption of specific wavelength of visible light

• Complementary colour observed

• Greater Δ₀ → higher energy light absorbed

• d⁰ and d¹⁰ complexes are colourless

Statement:
In metal carbonyls, bonding involves both σ-donation from CO to metal and π-back donation from metal to CO, strengthening the bond.

Explanation:

• CO donates lone pair to metal (σ bond)

• Metal donates electron density into π* orbital of CO

• Back bonding strengthens M–C bond

• Weakens C–O bond

Coordination compounds are important due to their roles in:

• Analytical chemistry (EDTA titrations)

• Metallurgy (cyanide extraction of gold)

• Medicine (cis-platin in cancer therapy)

• Biology (haemoglobin, chlorophyll, vitamin B₁₂)

• Catalysis (Wilkinson catalyst)