Past exam of the chemistry B course of the University of Cambridge / 2025 / Paper 2 / B7 / a / ii / Solution

The usual qualitative order of increasing ligand field isA chloride ligand is a sigma-donor ligand and pi-donor ligand; its occupied pi orbitals raise the metal $t_{2g}$-derived levels and reduce the splitting. A carbonyl ligand is a particularly effective pi-acceptor ligand, lowering those levels and producing a large splitting. Both phosphine ligands are sigma donors with acceptor orbitals of mainly P–C antibonding molecular orbital character. Replacing phenyl groups by electron-releasing tert-butyl groups increases sigma basicity but generally weakens pi acceptance, so the alkyl-rich phosphine is placed below triphenylphosphine in this qualitative pi-bonding comparison. It would be wrong to equate a stronger sigma donor automatically with a stronger-field ligand, or to invoke empty phosphorus $3d$ orbitals as the essential acceptors.^[ref]

This is a qualitative spectrochemical series prediction, not a universal numerical ranking for all metals and geometries. The opposing sigma and pi trends mean that precise comparisons require matched complexes and measured splittings; a Tolman electronic parameter measures net effects on CO backbonding and is not itself a ligand-field splitting.