Synthesis, Structure and Reactivity of the Molybdenum Cycloheptatrienyl Tetrahydroborate Complex {(η7-C7H7)Mo(η2-BH4)[P(cyclo-C6H11)3]}

The reaction of the cycloheptatrienyl-toluene sandwich complex [(η7-C7H7)Mo(η6- C6H5Me)]BF4 with tricyclohexylphosphine in acetonitrile furnishes the cationic half-sandwich cycloheptatrienyl complex {(η7-C7H7)Mo[P(C6H11)3](CH3CN)2}BF4 (1). Treatment of 1 with NaBH4 in ethanol results in the formation of the tetrahydroborate complex {(η7-C7H7)Mo(η2- BH4)[P(cyclo-C6H11)3]} (2), in which the borohydride ligand is coordinated to the molybdenum atom through two three-center, two-electron bonds. The complex is stable in ethanol and water. The expected formation of a metal trihydride of the type {(η7-C7H7)MoH3[P(cyclo-C6H11)3]} as a hydrolysis product could not be observed. Since this behaviour differs from the reactivity reported for related cyclopentadienyl-ruthenium complexes, a comparative computational study on the model complexes [(η5-C5Me5)RuH3(PMe3)] (4) and [(η7-C7H7)MoH3(PMe3)] (5) was performed revealing that the classical trihydride form [MH3] represents the global minimum for the ruthenium complex 4, whereas the dihydrogen-hydride form [MH(η2-H2)] is more stable for the molybdenum counterpart.