The reaction of the platinum(U) methyl cation [(N-N)Pt(CH3)(solv)](+) (N-N = ArN=C(Me)C(Me)=NAr, Ar = 2,6-(CH3)(2)C6H3, solv = H2O (1a) or TFE = CF3CH2OH (1b)) with benzene in TFE/H2O solutions cleanly affords the platinum(11) phenyl cation [(N-N)Pt(C6H5)(solv)](+) (2). High-pressure kinetic studies were performed to resolve the mechanism for the entrance of benzene into the coordination sphere. The pressure dependence of the overall second-order rate constant for the reaction resulted in DeltaV(double dagger) = -(14.3 +/- 0.6) cm(3) mol(-1). Since the overall second order rate constant k = K(eq)k(2), DeltaV(double dagger) = DeltaVdegrees(K-eq) + DeltaV(double dagger)(k(2)). The thermodynamic parameters for the equilibrium constant between la and 1b, K-eq = [1b][H2O]/[1a][TFE] = 8.4 x 10(-4) at 25 degreesC, were found to be DeltaHdegrees = 13.6 +/- 0.5 kJ mol(-1), DeltaSdegrees = -10.4 +/- 1.4 J K-1 mol(-1), and DeltaVdegrees = -4.8 +/- 0.7 cm(3) mol(-1). Thus DeltaVdouble dagger(k(2)) for the activation of benzene by the TFE solvento complex equals -9.5 +/- 1.3 cm(3) mol(-1). This significantly negative activation volume, along with the negative activation entropy for the coordination of benzene, clearly supports the operation of an associative mechanism.