The oxidation reactions of the unusual d⁸ trigonal-bipyramidal systems [MX(PP₃)]X (M = Pd: X = Cl(1), Br(2), I(3); M = Pt: X = Cl(4), Br(5), I(6); PP₃ = tris[2-(diphenylphosphino)ethyl]phosphine) by dissociation of the bound phosphino groups with H₂O₂ in chloroform and with DMSO-d₆ in the presence of reduced glutathione (GSH) have been explored with emphasis on the factors that influence the number of M–X and PO functionalities of the resulting products. The oxidations with H₂O₂ required a higher concentration for Pd than Pt and occurred with formation of both ionic distorted square-planar compounds [MX(PP₂PO)]X [M = Pd: X = Cl(7); M = Pt: X = Cl(10), Br(11)], that contain one dangling PO group, and the less common neutral species [MX₂(PP(PO)₂] [M = Pd: X = Br(8), M = Pt: X = I(12)] showing MX₂P₂ environments and two PO arms, or with release of all bound phosphorus from 3 to give OP(PO)₃ (9). Although there was no evidence of the formation of neutral species with two Pt–Cl bonds and dissociation of one bound phosphorus by adding H₂O₂ or M′Cl (M′ = Cu, Ag) to 10, the interaction with [AuCl(tdg)] (tdg = thiodiglycol) afforded [PtAuCl₃(PP₂PO)] (10a) containing dangling PO and PAuCl arms and one chiral central phosphorus. Thus, while the halide anions and ligands allowed both ionic (7, 10, 11) and neutral (8, 9, 10a, 12) systems, the larger anions [S(CH₃)₃(CH₂)₃(SO₃)]⁻ or GS⁻ favour only the ionic compounds, namely [PdCl(PP₂PO)][S(CH₃)₃(CH₂)₃(SO₃)] (7a), obtained in coexistence with [Pd(GS)(PP₂PO)]Cl (7a′) from the reaction of 1 + 3 eq. GSH in DMSO-d₆. The X-ray structure of 7a confirmed the distorted ClPdP₃ arrangement and the presence of a dangling PO group predicted by LSI-MS and ³¹P{¹H}NMR data.