Methylation of the monooxide Ph₂PNHP(O)Ph₂(HL) with MeI in tetrahydrofuran afforded [{Ph₂P(Me)NP(O)Ph₂}₂H]I which contains a symmetrical hydrogen bond, whereas benzylation with PhCH₂Br gave [Ph₂P(CH₂Ph)NP(OH)Ph₂]Br. The compound HL can be complexed, as a monodentate P-bound ligand, to various, metal centres (Ru^II, Ru^IV, Rh^III and Ir^III). These compounds are excellent precursors for the construction of new five-membered metallacycles containing either M–P–N(H)–P–O or M–P–N–P–O rings. Treatment of [Ir{Ph₂PNHP(O)Ph₂-P,O}(cod)][BF₄](cod = cycloocta-1,5-diene) with an excess of MeI gave the oxidative addition compound [IrI(Me){Ph₂PNHP(O)Ph₂-P,O}(cod)][BF₄] in which a cis configuration of the iodide and methyl ligands is tentatively assigned. Deprotonation of it with NEt₃ gave the known neutral iridium(III) complex [IrI(Me){Ph₂PNP(O)Ph₂-P,O}(cod)]. All compounds were characterised by a combination of ³¹P-{¹H} and ¹H NMR spectroscopy, IR spectroscopy and microanalysis. The solid-state structures of [{Ph₂P(Me)NP(O)Ph₂}₂H]I, [Ph₂P(CH₂Ph)NP(OH)Ph₂]Br, [Rh{Ph₂PNHP(O)Ph₂-P,O}(cod)]-[BF₄]·0.33CHCl₃ and [RuCl{Ph₂PNP(O)Ph₂-P,O}(η⁶-p-MeC₆H₄Prⁱ)] have been elucidated by X-ray crystallography. The two complexes contain the P,O-chelating ligand Ph₂PNHP(O)Ph₂ and [Ph₂PNP(O)Ph₂]^– respectively. With the exception of the rhodium complex the P–N–P–O fragments all show π-electron delocalisation.