From the reaction of HgPh2 with cis-[PtCl2(CO)L], the benzoyl complexes [Pt2(µ-Cl)2(COPh)2L2] can be isolated when L = PEt3, PMe2Ph, PMePh2, PPh3, or P(C6H11)3, but when L = P(C6H4Me-o)3, AsMePh2, or AsPh3 only the [PtCl(Ph)(CO)L] complexes are formed. Metathetical replacements of the chloride ions in [Pt2(µ-Cl)2(COPh)2-(PMePh2)2] lead to the corresponding bromide or iodide complexes. Low-temperature n.m.r. studies show that the reaction between HgPh2 and cis-[PtCl2(CO)L] gives first the carbonyl derivative [PtCl(Ph)(CO)L], from which CO insertion proceeds. The equilibrium positions in solution between [PtX(Ph)(CO)L](each with Ph trans to L) and the binuclear benzoyl complexes are reported. The effect of X on the equilibrium position depends on the bridging ability of that group in the benzoyl dimers, stronger bridges favouring the insertion product. The neutral ligands L exert an effect by both electronic and steric factors. The magnitude of the trans influence of L directly affects the tendency of the trans phenyl group to migrate to CO, unless a critical size of L is exceeded. In these cases, the steric bulk of L favours the mononuclear carbonyl derivative.