Coordination Compounds — How to Study for NEET

Werner's Coordination Theory — 3 postulates

• Every metal in a complex exhibits two types of valency: primary (ionisable, satisfied by anions, = oxidation number) and secondary (non-ionisable, satisfied by ligands, = coordination number).
• Primary valencies are satisfied by negative ions outside the coordination sphere.
• Secondary valencies are satisfied by neutral or negative ligands inside the coordination sphere. The number of secondary valencies (coordination number) is characteristic of the metal ion and determines the shape.

Werner's evidence: Adding AgNO₃ to CoCl₃·6NH₃ precipitates 3 Cl⁻. To CoCl₃·5NH₃, only 2 Cl⁻. To CoCl₃·4NH₃, only 1 Cl⁻. Conclusion: the number of chlorides inside the sphere (coordinated) do not precipitate — proving primary vs secondary distinction.

The 8 definitions you MUST own cold

Ligand classification — denticity ladder

Chelate effect: chelating ligands form more stable complexes than monodentate equivalents (entropy-driven — one polydentate replaces multiple monodentates).

The 8 steps

1. Cation first, anion second — same as for simple salts (Na⁺Cl⁻ is sodium chloride).
2. Ligands before metal. Whether cation or anion, ligand names precede the metal name.
3. Alphabetical order of ligands (ignore prefixes di, tri, etc. when alphabetising).
4. Anionic ligands end in -o: Cl⁻ = chlorido, CN⁻ = cyanido, OH⁻ = hydroxido, O²⁻ = oxido, S²⁻ = sulfido, SO₄²⁻ = sulfato.
5. Neutral ligands keep names — with 4 exceptions: H₂O = aqua, NH₃ = ammine (double m), CO = carbonyl, NO = nitrosyl.
6. Number of each ligand = di, tri, tetra, penta, hexa. For complex ligand names (e.g. ethylenediamine), use bis, tris, tetrakis to avoid confusion.
7. Metal name + oxidation state in Roman numerals in parentheses. e.g. iron(III), copper(II).
8. If complex is an anion, metal name ends in -ate. Iron → ferrate, copper → cuprate, tin → stannate, lead → plumbate, silver → argentate, gold → aurate. All others just add -ate (nickel → nickelate).

Nomenclature in action

Branch A — bonds differ

Branch B — bonds same, arrangement differs

• Geometrical (cis-trans) — same connectivity, different spatial positions of ligands. Only shown by:
  • CN=4 square planar [Ma₂b₂] — cis (adjacent) vs trans (opposite). Example: [Pt(NH₃)₂Cl₂] → cis-platin (drug) vs trans-platin (inactive).
  • CN=6 octahedral [Ma₄b₂] — cis (adjacent) vs trans (opposite). Also [Ma₃b₃] → fac (facial, all three on one face) vs mer (meridional, three in a plane).
  • Tetrahedral complexes DO NOT show geometrical isomerism — all four positions are equivalent.
• Optical (d/l) — non-superimposable mirror images. Requires the complex to be chiral (no plane of symmetry). Common with:
  • Octahedral [M(en)₃] with 3 bidentate ligands → propellor-shaped, chiral.
  • Cis-[M(en)₂Cl₂] is chiral; trans is not (plane of symmetry).

Hybridisation vs coordination number

Inner (d²sp³) vs outer (sp³d²) — which happens?

For CN=6 complexes of the same metal ion, the ligand's field strength decides:

• Strong-field ligand (CN⁻, CO, NH₃ for Co³⁺) → forces electron pairing in inner (3d) orbitals → uses (n−1)d²ns np³ = d²sp³ inner-orbital → typically low-spin, less paramagnetic.
• Weak-field ligand (F⁻, H₂O, Cl⁻) → doesn't force pairing → uses outer nd orbitals: ns np³ nd² = sp³d² outer-orbital → typically high-spin, more paramagnetic.

Worked example: [Fe(CN)₆]³⁻ vs [FeF₆]³⁻. Both are Fe³⁺ (d⁵). CN⁻ is strong → pairs the 5 electrons in three d orbitals → 1 unpaired → d²sp³ → μ = 1.73 BM. F⁻ is weak → all 5 remain unpaired → sp³d² → μ = 5.92 BM.

Table of 10 — every NEET-favourite complex, worked out

If she can recite this table from memory, she scores every VBT question NEET has ever asked. Cover the right side and re-derive from just the complex name — that's the drill.

d-orbital splitting in an octahedral field

In a free metal ion, all five d orbitals have equal energy (degenerate). When 6 ligands approach along the ±x, ±y, ±z axes:

• The d_x²-y² and d_z² orbitals point directly at the ligands → repelled → energy rises. These two are labelled e_g (two-fold degenerate).
• The d_xy, d_yz, d_zx orbitals point between the ligand axes → less repelled → energy drops. These three are labelled t₂g (three-fold degenerate).
• The energy gap is called the crystal field splitting energy Δ_o (octahedral).
• By convention: e_g goes up by +0.6 Δ_o, t₂g goes down by −0.4 Δ_o — so the barycentre (weighted average) is preserved.

↑ The eₘ level rises 0.6 Δ_o and the t₂g level falls 0.4 Δ_o (animation loops)

Splitting in tetrahedral field — the opposite pattern

• In tetrahedral geometry, ligands point between the axes.
• Opposite splitting: e (d_z², d_x²-y²) goes down; t₂ (d_xy, d_yz, d_zx) goes up.
• Δ_t = (4/9) Δ_o — the tetrahedral gap is roughly half the octahedral. So tetrahedral complexes are always high-spin (Δ_t is too small to force pairing).
• Labels lose their "g" subscript (no centre of inversion in a tetrahedron).

Crystal Field Stabilisation Energy — the formula

CFSE (octahedral) = (−0.4 × n_t2g + 0.6 × n_eg) × Δ_o, where n = number of electrons in each set.

Note: P = pairing energy penalty. Low-spin only forms when Δ_o > P.

Order of ligand field strength (weak → strong)

I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < bipy < phen < NO₂⁻ < CN⁻ < CO

Memory trick (mnemonic): "I Bought Some Chocolate So Fresh On Christmas With NAME Only Not Costly Ordered" — take first letter of each ligand starting with I⁻: I, Br, SCN, Cl, S, F, OH, C₂O₄, H₂O (Water), NCS, edta (EDTA), NH₃ (aMmine), en (ethylenediamine), bipy, phen, NO₂ (Nitro), CN, CO. Or make your own — just say it out loud 10× today.

How to use the series in 3 seconds

1. Find the ligand on the series.
2. If right of H₂O → strong field → causes pairing → low-spin, less paramagnetic, larger Δ_o, higher-energy absorbed → complex may look yellow/colourless (absorbs UV/violet).
3. If left of H₂O → weak field → no pairing → high-spin, more paramagnetic, smaller Δ_o, lower-energy absorbed → complex looks blue/green (absorbs red/orange).
4. H₂O sits near the middle — Δ depends on the metal.

Spin-only formula (automatic in NEET)

μ_spin = √n(n+2) BM, where n = unpaired electrons.

Diamagnetic = no unpaired e⁻ = repelled by magnetic field. Paramagnetic = has unpaired e⁻ = attracted. [Ni(CN)₄]²⁻ is diamagnetic (dsp², all 8 d electrons paired). [NiCl₄]²⁻ is paramagnetic (sp³, 2 unpaired) — classic NEET contrast.

Why coordination compounds are coloured

• Colour arises from d-d transitions: an electron in t₂g absorbs a photon of visible light and jumps to e_g. The absorbed frequency corresponds to ΔE = Δ_o (or Δ_t).
• The complex transmits/reflects the complementary colour to what it absorbs (colour wheel).
• No d-d transition = no colour: d⁰ (Sc³⁺, Ti⁴⁺, Cr(VI)) and d¹⁰ (Zn²⁺, Cd²⁺, Cu⁺) ions are colourless. Except when charge-transfer absorption happens (KMnO₄ = purple even though Mn⁷⁺ is d⁰ — this is a ligand-to-metal charge transfer, LMCT, not d-d).
• Stronger field ligand → larger Δ → higher-energy photon absorbed → shifts colour towards blue/violet region.

• ☐ Can name any complex in ≤ 15 seconds using the 8-step algorithm?
• ☐ Can determine hybridisation of any CN=6 or CN=4 complex in ≤ 20 seconds using the field-strength rule?
• ☐ Spectrochemical series memorised? (Weak → strong, at least the 12 most common ligands.)
• ☐ 5 magnetic moment values (n=1 to 5) recognised on sight?
• ☐ Know that Δ_t = (4/9) Δ_o and therefore tetrahedrals are always high-spin?
• ☐ Cis-platin vs trans-platin story cold?
• ☐ EDTA denticity = 6 (not 4, not 8)?
• ☐ Ni(CO)₄, Fe(CO)₅, Cr(CO)₆ all diamagnetic + metal in 0 oxidation state?
• ☐ Ambidentate pairs (NO₂/ONO, SCN/NCS) with donor atoms identified?
• ☐ Coordination sphere = inside [ ]; anything outside is a counter-ion?

If all 10 boxes are ticked → she's at NEET pace. If any 3+ are unchecked → repair those before Day 5.