3JJ, ' P N.m.r. spectra show that [PtCIH( PEQ,] reacts with PCI, in CH,CI, at 180 K to give [PtCI,H,(PEt,),],(5), and the previously unknown [PtCI(PEt,),(P'CI,)], (4). On warming to 240 K, complex (5) decomposes and ( 4) is reversibly protonated to give [PtCI( PEt,),( P'CI,H)] +. Above 240 K, ( 4) is converted into the novel [PtCI(PEt,),(P'CI,H,)], (8). This complex is stable in solution up to 260 K, but decomposes irreversibly at higher temperatures. Changes in the n.m.r. spectra on adding HCI or BCI, are interpreted in terms of dissociation of ( 8) into [PtCI(PEtJ,(P'CIH,)]+ and [HCI,]or [BCIJ-. Reaction of [PtBrH(PEtJ,] with PBr,at 160 K gives [ PtBr,H (PEt,),( P'Br,)], which loses H Br at 1 90 K, giving [ PtBr( PEt,),( P'Br,)] and [PtBr,H,( PEtJ,]. At 240 K, [ PtBr( PEt,),( P' Br,H,)]is produced. This persists for short periods at room temperature, and is stable at 170 K; addition of HBr or BBr, leads to the formation of [PtBr( PEtJ2( P'BrH,)] + by loss of bromide ion. All these species were identified by n.m.r.