A series of [(R-iso-BIPY)Pt(CH3)L]X-+(-) complexes [R-iso-BIPY = N-(2-pyridyl)-R-pyridine-2-ylidene; (R = 4-H, 1; 4-tert-butyl, 2; 4-dimethylamino, 3; 5-dimethylamino, 4); L = SMe2, b; dimethyl sulfoxide (DMSO), c; carbon monoxide (CO), d; X = OTf- = trifluoromethanesulfonate and/or [BPh4](-)] were synthesized by cyclometalation of the [R-iso-BlPY-H](+)[OTF](-) salts 1a-4a ([R-iso-BIPY-H](+) = N-(2-pyridyl)R-pyridinium) with dimethylplatinum-mu-dimethyl sulfide dimer. X-ray crystal structures for 1b, 2c-4c as well as complexes having bipyridyl and cyclometalated phenylpyridine ligands, [(bipy)Pt(CH3)(DMSO)]+ (5c) and (C11H8N)Pt(CH3)(DMSO) (6c), have been determined. The pyridinium-derived N-heterocyclic carbene complexes display localized C-C and C-N bonds within the pyridinium ligand that are indicative of carbene-pi-acidity. The significantly shortened platinum-carbon distance, for "parent" complex 1b, together with NMR parameters and the v(CO) values for carbonyl cations 1d-4d support a degree of Pt-C10 multiple bonding, increasing in the order 3 < 4 < 2 < 1. Degenerate DMSO exchange kinetics have been determined to establish the nature and magnitude of the trans-labilizing ability of these new N-heterocyclic carbene ligands. Exceptionally large second-order rate constants (k(2) = 6.5 +/- 0.4 M-1 (.)s(-1) (3c) to 2300 +/- 500 M(-1.)s(-1) (1c)) were measured at 25 degreesC using H-1 NMR magnetization transfer kinetics and variable temperature line shape analysis. These rate constants are as much as 4 orders of magnitude greater than those of a series of structurally similar cationic bis(nitrogen)-donor complexes [(N-N)Pt(CH3)(DMSO)](+) reported earlier, and a factor of 32 to 1800 faster than an analogous charge neutral complex derived from cyclometalated 2-phenylpyridine, (C11H8N)Pt(CH3)(DMSO) (k(2) = 0.21 +/-0.02 M(-1.)s(-1) (6c)). The differences in rate constant are discussed in terms of ground state versus transition state energies. Comparison of the platinum-sulfur distances with second order rate constants suggests that differences in the transition-state energy are largely responsible for the range of rate constants measured. The pi-accepting ability and trans-influence of the carbene donor are proposed as the origin of the large acceleration in associative ligand substitution rate.