Stabilization of Classical [B2 H5 ]- : Structure and Bonding of [(Cp*Ta)2 (B2 H5 )(μ-H)L2 ] (Cp*=η5 -C5 Me5 ; L=SCH2 S).

The room-temperature reaction of [Cp*TaCl4 ] with LiBH4 ⋅THF followed by addition of S2 CPPh3 results in pentahydridodiborate species [(Cp*Ta)2 (μ,η2 :η2 -B2 H5 )(μ-H)(κ2 ,μ-S2 CH2 )2 ] (1), a classical [B2 H5 ]- ion stabilized by the binuclear tantalum template. Theoretical studies and bonding analysis established that the unusual stability of [B2 H5 ]- 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)(CH2 S2 )}2 (B2 H5 )(H){M(CO)3 }] (6: M=Mo and 7: M=W), that retained the diborane(5) unit.