The kinetics and mechanism of substitution reactions of novel monofunctional Pt(tpdm)Cl and Pd(tpdm)Cl complexes (where tpdm = tripyridinedimethane) and their aqua analogues with thiourea (tu), L-methionine (L-met), glutathione (GSH), and guanosine-5'-monophosphate (5'-GMP) were studied in 0.1 M NaClO4 at pH = 2.5 (in the presence of 10 mM NaCl for reactions of the chlorido complexes). The reactivity of the investigated nucleophiles follows the order tu > L-met > GSH > 5'-GMP. The reported rate constants showed the higher reactivity of the Pd(II) complexes as well as the higher. reactivity of the aqua complex than the corresponding chlorido complex. The negative values reported for the activation entropy as well as the activation volume confirmed an associative substitution mode. In addition, the molecular and crystal structure of [Pt(tpdm)Cl]Cl was determined by X-ray crystallography. The compound crystallizes in a monoclinic space group C2/c with two independent molecules of the complex and unit cell dimensions of a = 38.303(2) angstrom, b = 9.2555(5) angstrom, c = 27.586(2) angstrom, beta = 133.573(1)degrees, and V=7058.3(8) angstrom(3). The cationic complex Pt(tpdm)Cl exhibits square-planar coordination around the Pt(II) center. The lability of the Pt(tpdm)Cl complex is orders of magnitude lower than that of Pt(terpyridine)Cl. Quantum chemical calculations were performed on the Pt(tpdm)Cl and Pt(terpyridine)Cl complexes and their reactions with thiourea. Theoretical computations for the corresponding Ni(II) complexes clearly demonstrated that pi-back-bonding properties of the terpyridine chelate can account for acceleration of the nucleophilic substitution process as compared to the tpdm chelate, where introduction of two methylene groups prevents such an effective pi-back bonding.