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

For electron counting, use the ionic method: subtract the metal oxidation state from its group number, then add two electrons for each neutral phosphine ligand or carbonyl ligand and two for each chloride ligand.

$\mathrm{Ni}(\mathrm{PP}{\mathrm{h}}_3)(\mathrm{CO}{)}_3$ is Ni(0), $d^{10}$, 18-electron and tetrahedral. The count is $10+2+3(2)=18$. With only one distinct ligand in an $\mathrm{M}{\mathrm{L}}_3{\mathrm{L}}^{\prime }$ tetrahedral molecular geometry, there is only one geometrical arrangement.

$\mathrm{Rh}(\mathrm{PP}{\mathrm{h}}_3{)}_2(\mathrm{CO})\mathrm{Cl}$ is Rh(I), $d^8$, 16-electron and square planar, with the two phosphines trans. Its count is $8+2(2)+2+2=16$. A square planar molecular geometry is characteristic of this low-spin $4d^8$ configuration. Opposite triphenylphosphine groups avoid their largest mutual steric hindrance; CO and chloride occupy the other trans pair.

$\mathrm{Pd}[\mathrm{P}{(}^{\mathrm{t}}\mathrm{Bu}{)}_2\mathrm{Ph}{]}_2\mathrm{C}{\mathrm{l}}_2$ is Pd(II), $d^8$, 16-electron and square planar, with the bulky phosphines trans. Its count is likewise $8+4+4=16$. The two di-tert-butylphenylphosphine groups are particularly crowded if cis. These predictions concern the favored intact four-coordinate species; ligand dissociation can occur under other conditions.

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.