Stabilization of Classical [B 2 H 5 ] − : Structure and Bonding of [(Cp*Ta) 2 (B 2 H 5 )(μ‐H)L 2 ] (Cp*=η 5 ‐C 5 Me 5 ; L=SCH 2 S)

The room‐temperature reaction of [Cp*TaCl 4 ] with LiBH 4 ⋅THF followed by addition of S 2 CPPh 3 results in pentahydridodiborate species [(Cp*Ta) 2 (μ,η 2 :η 2 ‐B 2 H 5 )(μ‐H)(κ 2 ,μ‐S 2 CH 2 ) 2 ] ( 1 ), a classical [B 2 H 5 ] − ion stabilized by the binuclear tantalum template. Theoretical studies and bonding analysis established that the unusual stability of [B 2 H 5 ] − 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)(CH 2 S 2 )} 2 (B 2 H 5 )(H){M(CO) 3 }] ( 6 : M=Mo and 7 : M=W), that retained the diborane(5) unit.