UV irradiation of [Cr(CO)(3)(eta-C(6)Me(6))] with Me(3)SiC=CSiMe(3) in THF gives the orange vinylidene complex [Cr(CO)(2){eta-C=C(SiMe(3))(2)}(eta-C(6)Me(6))] (V) which undergoes oxidative isomerization to the alkyne cation [Cr(CO)(2)(eta-Me(3)SiC=CSiMe(3))(eta-C(6)Me(6))](+) (A(+)), isolable as the orange [PF6](-) salt. The temperature dependence (from 228 to 295 K) of the rate of isomerization of [Cr(CO)(2){eta-C=C(SiMe(3))(2)}(eta-C(6)Me(6))](+) (V+) to A(+), determined by cyclic voltammetry, gives Delta S-double dagger = -6 +/- 13 J mol(-1) K-1 and Delta H-double dagger = 61 +/- 3 kJ mol(-1). The vinylidene cation (V+) has been generated at low temperature, by reacting V with [FeCp(2)](+), and characterized by ESR spectroscopy. Paramagnetic cation A(+) undergoes reductive isomerization to V; the rate of isomerization A to V, determined by cyclic voltammetry (from 279 to 293 K) and spectroelectrochemistry (at 242 K), gives Delta S-double dagger = 84 +/- 4 J mol(-1) K-1 and Delta H-double dagger = 97 +/- 1 kJ mol(-1). At temperatures above 233 K, the isomerization of A to V is catalyzed by V+, via the cross reaction V+ + A reversible arrow V + A(+). The thermodynamic and kinetic parameters are in accord with a mechanism for the isomerization of A to V in which the alkyne ligand slips to eta(1)-coordination prior to the transition state. Accordingly, the isomerization of V+ to A(+) resembles the 1,2-sigmatropic shift of a SiMe(3) group in a purely organic system.