The 4-phosphacyclohexanones, 2,2,6,6-tetramethyl-1-phenyl-4-phosphorinanone (L_a), 1,2,6-triphenyl-4-phosphorinanone (^PhL_b), 1-cyclohexyl-2,6-diphenyl-4-phosphorinanone (^CyL_b) and 1-tert-butyl-2,6-diphenyl-4-phosphorinanone (^BuL_b) have been made by modifications of literature methods. Phosphines ^RL_b are each formed as mixtures of meso- and rac-diastereoisomers. Isomerically pure rac-^PhL_b, rac-^CyL_b and meso-^BuL_b can be isolated by recrystallisation from MeCN. Heating mixtures of isomers of ^RL_b with TsOH leads to isomerisations to give predominantly the meso-^RL_b. The complex trans-[PdCl₂(L_a)₂] (1) is readily made from [PdCl₂(NCPh)₂] but the analogous platinum complex 2 has not been detected and instead, cyclometallation at the 3-position (α to the ketone) in the phosphacycle occurs to give trans-[PtCl(L_a)(L_a–3H)] (3) (where L_a–3H = L_a deprotonated at the 3-position) featuring a [3.1.1]metallabicycle as confirmed by X-ray crystallography. The analogous palladabicycle 4 has been detected upon treatment of 1 with Et₃N in refluxing toluene. The type of complex formed by ^RL_b depends on which diastereoisomer (meso or rac) is involved. rac-^PhL_b (a mixture of R,R- and S,S-enantiomers, labelled α and β) forms trans-[MCl₂(rac-^PhL_b)₂], M = Pd (5) or Pt (6), as mixtures of diastereoisomers (αα/ββ and αβ forms). The structure of αα-6 has been determined by X-ray crystallography. Ligand competition experiments monitored by ³¹P NMR showed that Pd(II) and Pt(II) have a significant preference to bind rac-^PhL_b over meso-^PhL_b. meso-^BuL_b reacts with [PtCl₂(NCBu^t)₂] under ambient conditions to give the binuclear complex [Pt₂Cl₂(meso-^BuL_b-2′H)₂] (7) where orthometallation has occurred on one of the exocyclic phenyl substituents as confirmed by X-ray crystallography. rac-^BuL_b reacts with [PtCl₂(NCBu^t)₂] to give a mononuclear cyclometallated species assigned the structure trans-[PtCl(rac-^BuL_b-2′H)(^BuL_b)] (8) on the basis of its ³¹P NMR spectrum. rac-^CyL_b reacts with [PtCl₂(NCBu^t)₂] in refluxing toluene to give trans-[PtCl₂(rac-^CyL_b)₂] (9) and the crystal structure of αβ-9 has been determined.