COMPARATIVE RATES OF LIGAND SUBSTITUTION-REACTIONS OF PT-C-BONDED COMPLEXES IN AQUEOUS-SOLUTION AND THE X-RAY CRYSTAL-STRUCTURE OF [PT(C6H3(CH(2)NME(2))(2)-2,6)(OH2)][OSO2CF3]

The substitution behavior of the organometallic complex cation [Pt{C6H 3(CH(2)NMe(2))(2)-2,6}-(OH2)](+) (1) has been studied with a series of nucleophiles (Cl-, Br-, I-, N-3(-), NCS,(-) thiourea, N,N'-dimethylth iourea, N,N,N'N'-tetramethythiourea) as a function of nucleophile conc entration ([Nu]), pH, temperature, and pressure. Complex 1 affords pse udo-first-order rate constants, k(obs), for the complex-formation reac tion given by k(obs), = k(1)[Nu]+k(-1); the k(-1) term is due to the r everse aquation reaction and is insignificant for the stronger, S-dono r nucleophiles. The mechanism for the substitution of the coordinated water molecule is associative, as demonstrated by the large negative v alues of Delta S-double dagger: and Delta V-double dagger. The results are compared to those obtained earlier for the geometrically related neutral zwitterionic complex [Pt{C6H4(CH(2)NMe(2))-2}(NC5H4SO3-3)(OH2) ] (2). Both cyclometalated complexes 1 and 2 have very high substituti on reactivities that can be ascribed to labilization which arises from the a-bonded carbon atom of the ortho-metalated aryl. ligand in combi nation with back-bonding to the in-plane aryl ligand that increases th e electrophilic character of the Pt(II) center. In complex 1 the sigma -donicity of the second chelating dimethylamine group and/or the incre ased chelation effect of the tridentate ligand lead to a significantly increased lability of the coordinated water molecule, compared to com plex 2. The X-ray structure of the ionic complex [Pt{C6H3(CH(2)NMe(2)) (2)-2,6}(OH2)](+)[OSO2CF3](-) (1a) is described. The crystal structure comprises an aggregate of two complex cations and two triflate anions . The structure contains two fully equivalent square-planar coordinate d Pt(II) centers with an O-ligated neutral H2O molecule (Pt-O = 2.186 (2) A) trans to the Pt-C,1 bond (Pt-C = 1.903(2) Angstrom; the aggrega te results from the involvement of both Il atoms of the two H2O molecu les in asymmetric hydrogen bonding to O atoms of the two triflate anio ns.