2-(Diphenylphosphinoyl)propanoic acid (Hdpop) has been prepared by a simple one-step procedure. The anion behaves as a versatile bidentate ligand, which co-ordinates to cations of V^III, Cr^III and Fe^III to give octahedral tris(chelate) complexes of the type M(dpop)₃, to Sn^IV to give SnR₃(dpop) and SnR₂(dpop)₂(R = Me or Ph), and to Mo^VI to give cis-[MoO₂(dpop)₂]. The derivative Tl(dpop), on the other hand, appears to be coordinated through the carboxylate group only. The α-hydrogen of the dpop ligands of the molybdenum and thallium complexes undergo a slow and reversible hydrogen–deuterium exchange reaction with deuteriomethanol, and the mechanism of this process is discussed. The structure of V(dpop)₃·H₂O·CH₂Cl₂, a rare example of a tris(dioxygen) chelate complex of vanadium(III), has been determined by single-crystal X-ray methods. There are two pairs of optical antipodes, Λ(R,R,R), and Δ(S,S,S), in the unit cell. The arrangement of donors about the metal is facial, with all the six-membered chelate rings adopting boat conformations. All the O–V–O bond angles are close to 90°, and the V–O(P) bond lengths averaging 2.023(2)Å are slightly longer than those of V–O(C) at 1.979(2)Å.