Chlorination of [Ph₃PCH₂Ph][PtCl₃(EtCN)], obtained from the reaction of [PtCl₂(EtCN)₂] with [Ph₃PCH₂Ph]Cl, formed the platinum(IV) complex [Ph₃PCH₂Ph][PtCl₅(EtCN)] which, at ambient temperature and both in solution and in the solid phase, hydrolyses to the ammonia compound [Ph₃PCH₂Ph][PtCl₅(NH₃)] and undergoes nucleophilic addition by ketoximes or amidoxime HONCR¹R² [R¹R² = Me₂, C₄H₈, C₅H₁₀, C₉H₁₆, C₉H₁₈ or Ph(NH₂)] to give the corresponding iminoacylated product [Ph₃PCH₂Ph][PtCl₅{HNC(Et)ONCR¹R²}]. All compounds were characterized by elemental analyses, FAB mass spectrometry, IR and ¹H, ¹³C-{¹H}, ³¹P-{¹H} and ¹⁹⁵Pt NMR spectroscopies. A crystal structure determination of [Ph₃PCH₂Ph][PtCl₅{NHC(Et)ONC(C₉H₁₆)}] disclosed amidine one-end rather than the N,N-bidentate co-ordination mode of the N-donor ligand. The iminoacylation by oximes was investigated by ab initio methods (at RHF level using quasi-relativistic pseudopotentials for platinum) for [PtCl₅(NCMe)]⁻ which were also applied to the related neutral platinum(IV) [PtCl₄(NCMe)₂] and platinum(II) [PtCl₂(NCMe)₂] complexes. The calculations included geometry optimization of the starting and final complexes, location of possible transition states for the reaction discussed and intrinsic reaction coordinate calculations for one reaction. The results obtained provided an interpretation, on the basis of kinetic (activation energies) and thermodynamic (reaction energies) effects, for the order of reactivity observed [neutral Pt^IV > anionic Pt^IV > neutral Pt^II] and indicated that a mechanism based on nucleophilic addition of the protic nucleophile (undeprotonated oxime), to form a transition state with a four-membered NCOH ring, is energetically favoured relative to the alternative one involving prior deprotonation of the oxime, unless base-catalysed conditions are operating.