A combined kinetic and DFT study of the uncatalyzed isomerization of cationic solvent complexes of the type cis-[Pt(R')(S)(PR3)(2)](+) (R' = linear and branched alkyls or aryls and S = solvents) to their trans isomers has shown that the reaction goes through the rate-determining dissociative loss of the weakly bonded molecule of the solvent and the interconversion of two geometrically distinct T-shaped 14-electron three-coordinate intermediates. The Pt-S dissociation energy is strongly dependent on the coordinating properties of S and independent of the nature of R'. The energy barrier for the fluxional motion of [Pt(R')(PR3)(2)](+) is comparatively much lower (approximate to 8-21 kJ mol(-1)). The presence of beta-hydrogens on the alkyl chain (R' = Et, Pr-n, and Bu-n) produces a great acceleration of the reaction rate. This accelerating effect has been defined as the beta-hydrogen kinetic effect, and it is a consequence of the stabilization of the transition state and of the cis-like three-coordinate [Pt(R')(PR3)(2)](+) intermediate through an incipient agostic interaction. The DFT optimization of [Pt(R')(PMe3)(2)](+) (R' = Et, Pr-n, and Bu-n) reproduces a classical dihapto Pt center dot center dot center dot center dot eta(2)-HC agostic mode between the unsaturated metal and a dangling C-H bond. The value of the agostic stabilization energy (in the range of approximate to 21-33 kJ mol(-1)) was estimated by both kinetic and computational data and resulted in being independent of the length of the hydrocarbon chain of the organic moiety. A better understanding of such interactions in elusive reaction intermediates is of primary importance in the control of reaction pathways, especially for alkane activation by metal complexes.