Platinum(II) and palladium(II) complexes of the potentially hexadentate P,N-donor ligand family Ar2P-X-PAr2 (X = (CH2)(2) [dmape], cyclic-C5H8 [dmapcp]; Ar = o-NN-dimethylanilinyl) are described. In CH2Cl2, the dmape complexes exist as equilibrium mixtures of MCl2(P,P'-dmape) and [MCl(P, P, N-dmape)]Cl isomers (M = Pd, Pt), governed by Delta H degrees = -19 +/- 4 U mol(-1) and Delta S degrees = -100 +/- 30 J mol(-1) K-1 for M = Pt, and Delta H degrees = -11 +/- 7 U mol-1 and Delta S degrees = -60 +/- 20 J mol-1 K-1 for M = Pd. The water-soluble dmapcp complexes exist solely in the [MCl(P,P',N-dmapcp)]Cl form, but the free and coordinated anilinyl rings in these complexes are in slow diastereoselective exchange, X-ray crystal structures for MCl2(P,P'-dmape) (M = Pd, Pt), and the [PdCl(P,P',N-dmape)](+) and [PtCl-P,P',N-dmapcp)](+) cations, are presented. Some of the complexes show marginal activity in water for the catalyzed hydration of maleic to malic acid, giving about 6-7% conversion in 24 h at 100 degrees C and substrate: catalyst loadings of 100:1. Attempts to synthesize a PdCl(P,P',N-dmapm)(+) species led instead to isolation of [Pd(mu-Cl)(P,P-dmapm)](2)-[PF6](2) (dmapm = Ar2PCH2Ar2).