The complexes [MCl(cod)(HL)](M = Rh or Ir), [PdCl(η³-C₃H₅)(HL)], [PdCl(L–L)(HL)], trans-[RhCl(CO)(HL)₂] and [AuCl(HL)] have been synthesised by reaction of either [{M(µ-Cl)(cod)}₂](M = Rh or Ir; cod = cycloocta-1,5-diene), [{Pd(µ-Cl)(η³-C₃H₅)}₂], [{Pd(µ-Cl)(L–L)}₂](L–L = C₁₂H₁₂N, C₁₀H₈N or C₉H₁₂N), [{Rh(µ-Cl)(CO)₂}₂] or [AuCl(tht)](tht = tetrahydrothiophene) with Ph₂PNHP(O)Ph₂(HL). In all these complexes the ligand is monodenate P-bound. Chloride abstraction from [PdCl(C₁₂H₁₂N)(HL)] or [PdCl(η³-C₃H₅)(HL)], using Ag[BF₄], gave the cationic compounds [Pd(C₁₂H₁₂N)(HL)][BF₄] or [Pd(η³-C₃H₅)(HL)][BF₄] in which the ligand HL is P,O-chelated. Removal of the amine proton in metal complexes of HL (monodenate P-bound) with KOBu^t afforded a new class of neutral metallacycles incorporating either an MP₂NO or M₂P₄N₂O₂ framework based on the [Ph₂PNP(O)Ph₂]^–(L^–) ligand. Reaction of [{Pd(µ-Cl)-(C₉H₁₂N)}₂] with 4 equivalents of HL in methanol at ambient temperature yielded the neutral palladium(II) complex [PdCl{Ph₂PNP(O)Ph₂-P,O}(Ph₂POMe)] in which one HL ligand has undergone P–N bond scission. The reaction of the iridacycle [Ir(cod){Ph₂PNP(O)Ph₂-P,O}] with Mel was shown to proceed with cis addition affording the iridium(III) metallacycle [IrI(Me)(cod){Ph₂PNP(O)Ph₂-P,O}]. All compounds described have been characterised by a combination of ³¹P-{¹H} NMR spectroscopy, microanalysis and in some cases by ¹H NMR and IR spectroscopy. The molecular structures of four complexes have been determined by single-crystal X-ray diffraction. The metallacycles shown π-electron delocalisation in the P–N–P–O fragment.