A COMPARATIVE-STUDY OF THE STRUCTURES AND FLUXIONALITY OF THE CARBENIUM ION COMPLEXES [FVMO(2)(CO)(4)(MU-ETA(2),ETA(3)-RC CCR(2)')](BF4-)-B-- X-RAY CRYSTAL-STRUCTURE OF [FVMO(2)(CO)(4)(MU-ETA(2),ETA(3)-HC CCME(2))](+)BF4-( (R=H, ME, R'=H, ME, PH, FV=FULVALENE) )

Protonation of the fulvalene complexes [FVMo2(CO)(4)(mu-eta(2),eta(3)- RC=CCR'(2)OMe)], 1 (R = Me, R' = H), 2 (R = H, R' = Me), and 3 (R = H, R' = Ph), yields the corresponding carbenium ion complexes [FvMo(2)(C O)(4)(mu-eta(2),eta(3)-RC=CCR'(2))](+), 4-6. The cluster 2)(CO)(4)(mu- eta(2),eta(3)-HC=CCMe(2))](BF4-)-B-+, 5, crystallizes in the monoclini c space group P2(1)/c, with a = 10.509(4) Angstrom, b = 8.109(2) Angst rom, c = 23.137(6) Angstrom, beta = 96.83 degrees, V = 1958 Angstrom(3 ), and Z = 4. The Mo-C+ distance of 2.75 Angstrom is markedly longer t han that found in the CH2+ complex, 4, where Mo-C+ = 2.44 Angstrom. Va riable-temperature NMR measurements on 5 reveal that the barrier to ro tation of the CMe(2)(+) group and the energy for migration of the cati on from one Mo to the other are both approximately 10 kcal mol(-1); th is contrasts with the behavior for 4. Finally, it is shown that in 6 t he CPh(2)(+) group interacts weakly with a molybdenum vertex. The stru ctures and fluxional behavior of primary versus tertiary carbenium com plexes of metal clusters are discussed.