Stabilization of Classical [B2H5]-: Structure and Bonding of [(Cp*Ta)2(B2H5)(μ-H)L2] (Cp*=η5-C5Me5; L=SCH2S)

The room-temperature reaction of [Cp*TaCl4] with LiBH4.THF followed by addition of S2CPPh3 results in pentahydridodiborate species [(Cp*Ta)(2)(mu,eta(2):eta(2)-B2H5)(mu-H)(kappa(2),mu-S2CH2)(2)] (1), a classical [B2H5](-) ion stabilized by the binuclear tantalum template. Theoretical studies and bonding analysis established that the unusual stability of [B2H5](-) in 1 is mainly due to the stabilization of sp(2)-B center by electron donation from tantalum. Reactions to replace the hydrogens attached to the diborane moiety in 1 with a 2 e {M(CO)(4)} fragment (M=Mo or W) resulted in simple adducts, [{(Cp*Ta)(CH2S2)}(2)(B2H5)(H){M(CO)(3)}] (6: M=Mo and 7: M=W), that retained the diborane(5) unit.