Coordination Compounds

01Chapter map & Werner's theory

Coordination Compounds reliably gives 2–3 NEET questions, and they are unusually predictable: one on IUPAC naming or oxidation state, one on hybridisation / magnetic moment, one on isomerism or CFT colour. Almost none of it is calculation — it is rules applied carefully, which makes it the highest return-per-hour chapter in Inorganic.

Werner's theory (1893)

• Primary valency = oxidation state; satisfied by negative ions; ionisable; shown by dotted lines.
• Secondary valency = coordination number; satisfied by ligands (neutral or negative); non-ionisable; fixed and directional, so it decides the geometry.

02Ligands, denticity & coordination number

• Coordination number = number of sigma bonds the ligands make with the metal — count donor atoms, not ligand molecules. [Co(en)₃]³⁺ has CN = 6, not 3. [Pt(en)Cl₂] has CN = 4.
• Coordination sphere = metal + ligands, inside [ ], written as one ion; the counter ions sit outside.
• Oxidation number of the metal: charge left on the metal when every ligand is removed with its lone pair. Neutral ligands (NH₃, H₂O, CO, NO⁺ is +1) contribute 0.
• Homoleptic = one kind of ligand ([Co(NH₃)₆]³⁺); heteroleptic = more than one ([Co(NH₃)₄Cl₂]⁺).

03IUPAC nomenclature — the six rules

1. Cation first, anion second, exactly as in simple salts.
2. Inside the sphere, ligands are named alphabetically (ignore the multiplying prefix), then the metal.
3. Anionic ligands end in -o: chlorido, cyanido, hydroxido, oxalato, sulphato, nitrito-N (M–NO₂) / nitrito-O (M–ONO). Neutral ligands keep their name except aqua (H₂O), ammine (NH₃), carbonyl (CO), nitrosyl (NO).
4. Prefixes di, tri, tetra; use bis, tris, tetrakis when the ligand name already contains di/tri or is complicated (e.g. bis(ethane-1,2-diamine), tris(ethane-1,2-diamine)).
5. Oxidation state of the metal in Roman numerals in brackets right after its name, no space.
6. If the complex ion is an anion, the metal takes the suffix -ate and often its Latin name: ferrate (Fe), cuprate (Cu), argentate (Ag), aurate (Au), stannate (Sn), plumbate (Pb), but cobaltate, nickelate, chromate, manganate, platinate.

04Isomerism

Structural isomerism

Stereoisomerism

• Geometrical (cis–trans): only for square planar MA₂B₂, MA₂BC, MABCD and octahedral MA₄B₂, MA₃B₃, M(AA)₂B₂. Tetrahedral complexes show no geometrical isomerism — every position is adjacent to every other.
• fac–mer for MA₃B₃: facial = three identical ligands on one triangular face (all 90°); meridional = three around a meridian (two at 180°).
• Optical: no plane of symmetry. Common in octahedral chelates — [Co(en)₃]³⁺ (d and l), cis-[CoCl₂(en)₂]⁺ is optically active while the trans form is not. Square planar complexes are almost never optically active (the molecule is planar ⇒ it has a plane of symmetry).

05Valence bond theory — hybridisation & magnetism

Method, in order: (1) find the oxidation state, (2) write the dⁿ configuration of the ion, (3) decide whether the ligand is strong or weak, (4) pair up if strong, (5) count vacant orbitals and name the hybridisation, (6) count unpaired electrons for μ.

06Crystal field theory

Ligands are treated as point negative charges; the electrostatic field they create splits the degenerate d orbitals. The energy gap is Δ_o (octahedral) or Δ_t (tetrahedral).

• Δ_o > P (pairing energy) → electrons pair in t_2g → low spin, strong field ligand.
• Δ_o < P → electrons spread out by Hund's rule → high spin, weak field ligand.
• Configurations d¹, d², d³, d⁸, d⁹, d¹⁰ give the same arrangement either way — the high/low spin question only bites for d⁴ to d⁷.
• Δ increases with the oxidation state of the metal and down a group (3d < 4d < 5d), which is why 4d and 5d complexes are nearly always low spin.

Spectrochemical series — learn the ends

Memory line: I Browse Some Class Sites For OH Oxford Water, Never Edta… NH₃ < en < CN < CO. The last four are what actually get asked.

07Colour, d–d transitions & magnetic behaviour

Visible light promotes an electron from t_2g to e_g (a d–d transition). The complex absorbs that wavelength and we see the complementary colour.

• Ions with d⁰ (Sc³⁺, Ti⁴⁺, Zn in +2 is d¹⁰) or d¹⁰ (Cu⁺, Zn²⁺, Cd²⁺, Ag⁺) are colourless — no d–d transition is possible.
• Anhydrous CuSO₄ is white; CuSO₄·5H₂O is blue because water ligands create the field that makes the transition possible.
• Order of colour with ligand strength for the same metal: [Cu(H₂O)₄]²⁺ pale blue → [Cu(NH₃)₄]²⁺ deep blue-violet.

08Stability, chelate effect & metal carbonyls

Stability

• Stability rises with the charge density of the metal ion (higher charge, smaller size) and with the basicity of the ligand.
• Chelate effect: a chelating ligand gives a far more stable complex than the same number of comparable monodentate donors, because ring formation releases several free ligand molecules and so raises the entropy (ΔS positive → ΔG more negative). So [Ni(en)₃]²⁺ is much more stable than [Ni(NH₃)₆]²⁺.
• CN⁻ and NH₃ give stable complexes with Cu(II) but not with Cu(I); F⁻ and O-donors prefer hard, highly charged ions.

Metal carbonyls & synergic bonding

• The M–C bond has two components: a σ bond from the carbon lone pair into an empty metal orbital, plus a π bond formed by back-donation from a filled metal d orbital into the vacant antibonding π* orbital of CO.
• Each part strengthens the other — this mutual reinforcement is the synergic effect. Consequence: the M–C bond gets stronger and the C–O bond gets weaker (C–O stretching frequency falls).
• Shapes: [Ni(CO)₄] tetrahedral, [Fe(CO)₅] trigonal bipyramidal, [Cr(CO)₆] octahedral, [Co₂(CO)₈] has bridging CO groups. The metal is in oxidation state zero.
• EAN = Z − (oxidation state) + 2 × (coordination number); a stable carbonyl usually reaches the next noble-gas count. Ni in [Ni(CO)₄]: 28 − 0 + 8 = 36 (krypton).

09Importance & applications

10Last-hour recall sheet & rapid drill