What is a Coordination Compound?
A coordination compound contains a central metal atom or ion bonded to a number of molecules or ions called ligands. These are NOT regular ionic compounds — the bonds are called coordinate covalent bonds (the ligand donates both electrons).
Example: [Fe(CN)₆]⁴⁻ — Fe²⁺ centre + 6 CN⁻ ligands
Key Terms (MUST KNOW)
- Ligand: Molecule/ion that donates electron pair to metal. (NH₃, CN⁻, Cl⁻, en)
- Coordination Number (CN): Number of ligand atoms directly bonded to metal. (2, 4, 6)
- Coordination Sphere: The central metal + ligands inside [ ].
- Oxidation State: Charge on the metal after removing all ligands.
- Denticity: Number of donor atoms in one ligand. Mono=1, Bi=2, Poly=many.
The 4 Geometries (Most Important!)
- 🔵 Octahedral (CN=6): 6 ligands, 90° angles. Most common. e.g. [Co(NH₃)₆]³⁺
- 🟡 Square Planar (CN=4): 4 ligands in a flat square, 90° angles. d⁸ metals (Pt²⁺, Pd²⁺, Ni²⁺). e.g. [PtCl₄]²⁻
- 🟢 Tetrahedral (CN=4): 4 ligands, 109.5° angles. e.g. [NiCl₄]²⁻
- ⚪ Linear (CN=2): 2 ligands, 180° angle. e.g. [Ag(NH₃)₂]⁺
IUPAC Naming Rules (Step by Step)
- Step 1: Name the cation first, then anion (like any ionic compound).
- Step 2: Name ligands alphabetically with their prefix (di, tri, tetra…).
- Step 3: Name the metal — use English name if cation, Latin name if in anion (ferrate, cuprate, plumbate…).
- Step 4: Add oxidation state in Roman numerals in brackets: (II), (III).
- Anionic complex: add -ate suffix to metal name.
Example: [Fe(CN)₆]⁴⁻ → hexa (6) + cyano (CN) + ferrate (Fe as anion) + (II) (oxidation state)
Crystal Field Theory (d-Orbital Splitting)
When ligands approach, they repel the d-orbitals differently causing them to split into two energy levels:
- Octahedral: eg (dx²-y², dz²) go UP, t₂g (dxy, dxz, dyz) stay DOWN. Gap = Δo
- Tetrahedral: e (dx²-y², dz²) stay DOWN, t₂ (dxy, dxz, dyz) go UP. Gap = Δt ≈ 4/9 Δo
- Strong field ligands (CN⁻, CO) → large Δ → low spin (paired electrons)
- Weak field ligands (Cl⁻, F⁻) → small Δ → high spin (unpaired electrons)
Isomers of Coordination Compounds
- Geometric (cis/trans): Ligands on same side (cis) or opposite (trans). Only in square planar and octahedral.
- Optical (mer/fac or Δ/Λ): Mirror images that are non-superimposable. Only in octahedral with 3 bidentate ligands.
- Linkage: Ambidentate ligand bonded through different atoms. e.g. NO₂ vs ONO
- Ionisation: Different ions inside/outside coordination sphere. e.g. [CoBr(SO₄)] vs [Co(SO₄)]Br
Spectrochemical Series (Memorise This!)
Weak field ← CO₃²⁻ < F⁻ < Cl⁻ < Br⁻ < I⁻ < OH⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO → Strong field