Stabilization of Classical [B<inf>2</inf>H<inf>5</inf>]^−: Structure and Bonding of [(Cp*Ta)<inf>2</inf>(B<inf>2</inf>H<inf>5</inf>)(μ-H)L<inf>2</inf>] (Cp*=η⁵-C<inf>5</inf>Me<inf>5</inf>; L=SCH<inf>2</inf>S)

The room-temperature reaction of [Cp*TaCl4] with LiBH4⋅THF followed by addition of S2CPPh3 results in pentahydridodiborate species [(Cp*Ta)2(μ,η2:η2-B2H5)(μ-H)(κ2,μ-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 sp2-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.