Coordination Compounds: Nomenclature, Isomerism & CFT

Coordination compounds consist of a central metal atom or ion bonded to surrounding molecules or ions called ligands through coordinate (dative) bonds. The central metal with its ligands forms the coordination entity, enclosed in square brackets. The total number of ligand donor atoms bonded to the metal is the coordination number (CN).

Werner's Theory distinguishes between primary valence (oxidation state, satisfied by anions) and secondary valence (coordination number, satisfied by ligands in a fixed spatial arrangement). For example, in [Co(NH₃)₆]Cl₃, the primary valence is 3 (satisfied by Cl-) and the secondary valence is 6 (satisfied by NH₃).

IUPAC Nomenclature Rules

1. Cation is named before anion regardless of which is the complex ion.
2. Ligands are named alphabetically before the metal; anionic ligands end in -o (chlorido, cyano, hydroxido), neutral ligands retain their name (exceptions: aqua for H₂O, ammine for NH₃, carbonyl for CO, nitrosyl for NO).
3. Prefixes di-, tri-, tetra- for simple ligands; bis-, tris-, tetrakis- for complex ligand names.
4. Metal name ends in -ate with Latin roots for anionic complexes (ferrate, cuprate, argentate, plumbate, stannate, aurate).
5. Oxidation state of metal in Roman numerals in parentheses.

Isomerism in Coordination Compounds

Structural Isomerism:

• Ionisation isomerism: Exchange of ions between coordination sphere and ionisation sphere. [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br.
• Linkage isomerism: Ambidentate ligands bind through different atoms. [Co(NH₃)₅ONO]₂+ (nitrito-O) vs [Co(NH₃)₅NO₂]₂+ (nitrito-N).
• Coordination isomerism: Exchange of ligands between two metal centres. [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆].
• Solvate/Hydrate isomerism: Water as ligand vs lattice solvent. [Cr(H₂O)₆]Cl₃ (violet) vs [Cr(H₂O)₅Cl]Cl₂.H₂O (green).

Stereoisomerism:

• Geometrical (cis-trans): Occurs in square planar (CN=4) and octahedral (CN=6) complexes. cis-[Pt(NH₃)₂Cl₂] (cisplatin, anti-cancer) vs trans isomer.
• Optical isomerism: Non-superimposable mirror images. Common in octahedral complexes with bidentate ligands like [Co(en)₃]₃+. cis-[Co(en)₂Cl₂]+ is optically active; trans is not.
• Facial-Meridional (fac-mer): In octahedral [MA₃B₃], fac has three identical ligands on one triangular face; mer has them on a meridian.

Common Ligand Structures

Some important bidentate ligands used in coordination chemistry:

Ethylenediamine (en) — bidentate, donates through both N atoms:

Oxalate (ox) — bidentate dianionic ligand:

Acetylacetonate (acac) — bidentate, forms stable chelates:

Crystal Field Theory (CFT)

CFT treats metal-ligand interaction as purely electrostatic. Ligands are point charges that split degenerate d-orbitals:

• Octahedral field: d-orbitals split into t2g (lower, dxy, dyz, dxz) and eg (higher, dx2-y2, dz2). Splitting energy = $\Delta_{oct}$.
• Tetrahedral field: Splitting is inverted — e (lower) and t2 (higher). $\Delta_{tet}$ = ($\frac{4}{9}$) $\Delta_{oct}$.
• Square planar field: Largest splitting of dx2-y2, common for d8 metals (Ni₂+, Pd₂+, Pt₂+).

Octahedral Crystal Field Splitting:

Tetrahedral Crystal Field Splitting (inverted):

High Spin vs Low Spin — d6 Octahedral Example:

Spectrochemical Series (increasing field strength): I- < Br- < S₂- < Cl- < N₃- < F- < OH- < ox2- < H₂O < NCS- < CH₃CN < py < NH₃ < en < bipy < phen < NO₂- < PPh₃ < CN- < CO < NO+

• Weak field ligands (I- to F-): High spin complexes, small $\Delta$.
• Strong field ligands (CN-, CO, NO+): Low spin complexes, large $\Delta$.

CFSE (Crystal Field Stabilisation Energy): Calculated as CFSE = (-0.$4n_{t2g}$ + 0.$6n_{eg}$) $\Delta_{oct}$ for octahedral. Determines stability, colour, and magnetic properties. d3 and d6 (low spin) have maximum CFSE — extra stability.

Colour: Arises from d-d transitions. The absorbed wavelength corresponds to $\Delta$; the transmitted/reflected wavelength gives the observed colour. [Ti(H₂O)₆]₃+ absorbs green-yellow, appears purple.

Magnetic Properties: Determined by number of unpaired electrons. $\mu$ = $\sqrt{n(n+2}$) BM. High spin d5 (e.g., [MnF₆]₃-) has 5 unpaired electrons; low spin d6 (e.g., [Co(NH₃)₆]₃+) has 0.

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