Coordination Compounds
On this page
Direct answer
A coordination compound carries a central metal atom or ion bonded to surrounding ligands through coordinate bonds — as in [Co(NH3)6]Cl3, where [Co(NH3)6]^3+ is the complex ion in square brackets. JEE analysis uses three tools: IUPAC naming, isomerism (ionisation, hydrate, linkage, geometrical, optical), and bonding theories — valence bond theory with inner (d2sp3) and outer (sp3d2) orbital complexes, and crystal field theory, which splits d orbitals into t2g and eg sets by delta_o to explain colour, magnetism and stability.
What you must remember
- Naming: ligands alphabetically with di/tri/tetra, then the metal with its oxidation state in Roman numerals; anionic complexes end in -ate — [Fe(CN)6]^3- is hexacyanoferrate(III).
- Charge bookkeeping: metal oxidation state = complex charge minus the sum of ligand charges; coordination number counts donor atoms, so bidentate en counts 2.
- Valence bond theory: strong-field ligands pair electrons giving inner-orbital d2sp3 — [Co(NH3)6]^3+ and [Fe(CN)6]^4- are diamagnetic; weak-field ligands give outer-orbital sp3d2 — [CoF6]^3- and [FeF6]^3- are paramagnetic (four and five unpaired electrons).
- Crystal field theory: octahedral fields give lower t2g (three) and higher eg (two); CFSE = (-0.4 × t2g count + 0.6 × eg count) × delta_o; pairing energy P decides high-spin versus low-spin for d4 to d7.
- Tetrahedral splitting is reversed (e below t2) with delta_t = (4/9) delta_o, so tetrahedral complexes are practically always high spin.
- Spectrochemical series, weak to strong: I- < Br- < Cl- < F- < OH- < H2O < NH3 < en < CN- < CO.
- Spin-only moment mu = sqrt(n(n+2)) BM — five unpaired electrons give 5.92 BM.
- Colour comes from d-d transitions across delta_o: [Ti(H2O)6]^3+ (d1) is purple; d0 and d10 complexes are colourless.
- Isomer pairs: ionisation — [Co(NH3)5Br]SO4 versus [Co(NH3)5(SO4)]Br; geometrical — cis/trans [Co(NH3)4Cl2]+; optical — cis-[Co(en)2Cl2]+ is chiral, trans is not.
Common confusion
The recurring muddle is the hybridisation label: d2sp3 means inner d orbitals (pairing occurred), sp3d2 means outer (no pairing) — same shape, different magnetic signature. Students then equate "low spin" with "diamagnetic", true for d6 but not d5 (low-spin d5 keeps one unpaired electron). Counting en as one site instead of two halves the coordination number of complexes like [Co(en)3]^3+, which is 6.
Exam-focused takeaway
JEE Main tests nomenclature, oxidation-state arithmetic, unpaired-electron counts and CFSE as quick single-concept items. JEE Advanced prefers comparison — high spin versus low spin from ligand identity, magnetic moments across a series, isomers distinguished by a chemical test (BaCl2 or AgNO3 precipitation), and the spectrochemical series in matching lists. Count the d electrons first; every question starts from that number.
Frequently asked questions
How is the metal's oxidation state found?
Add the ligand charges (CN- is -1; NH3 and en are 0), then solve metal charge plus ligand total equal to the complex charge.
What does the spectrochemical series order?
Ligands by field strength: weak members like I- give high spin; strong members like CN- pair electrons, giving low-spin, intensely coloured complexes.
Why are tetrahedral complexes always high spin?
delta_t is only 4/9 of delta_o, always below the pairing energy, so electrons stay unpaired.
What is the spin-only moment formula?
mu = sqrt(n(n+2)) BM with n unpaired electrons — for instance 5.92 BM for five.
Which isomerism separates [Co(NH3)5Br]SO4 from [Co(NH3)5(SO4)]Br?
Ionisation isomerism: the first precipitates BaSO4 with BaCl2, the second AgBr with AgNO3 — same composition, different free ions.
Why is [Ti(H2O)6]^3+ coloured?
Its single d electron jumps t2g to eg by absorbing visible light of energy delta_o; d0 and d10 ions have no such transition.