The preparation and characterization of a novel series of cationic sigma-phenylplatinum(II) complexes of the type trans-[Pt(sigma-C6H5)(L)(2)A]OTf (A = picolinic acid, L = PPh3 (4) and PMePh2 (7); A = nicotinic acid, L = PPh3 (5) and PMePh2 (8); A = isonicotinic acid, L = PPh3 (6), PMePh2 (9), and PEt3 (10)) are described. The pK(a) value for the carboxylic acid functionality in selected complexes was found to follow the order 7 (pK(a) = 5.23 +/- 0.09) > 8 (4.85 +/- 0.10) > 9 (3.51 +/- 0.08) > 6 (3,26 +/- 0.07) approximate to10 (3.21 +/- 0.08) by means of potentiometric titration experiments in 50% (v/v) EtOH/H2O solution at 295 K. The X-ray crystal structures of 9 and 10 were also determined. The asymmetric unit of each of 9 and 10 comprises a univalent complex cation, a triflate anion, and a solvent CH2Cl2 molecule of crystallization. Centrosymmetrically related pairs of complex cations in 9 associate via the familiar carboxylic acid dimer motif, whereas with 10, the carboxylic acid dimer motif is absent. Instead, the carboxylic acid residue forms both donor and acceptor interactions to the triflate anion and CH2Cl2 solvent of crystallization, respectively, to afford a 10-membered ring structure. Possible reasons for the observed differences in the solid-state structures of 9 and 10 are presented.