Investigation of the Stability of the M‐H‐B Bond in Borane σ Complexes [M(CO)5(η1‐BH2R⋅L)] and [CpMn(CO)2(η1‐BH2R⋅L)] (M=Cr, W; L=Tertiary Amine or Phosphine): Substituent and Lewis Base Effects

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Abstract
We investigated the influence of a substituent and a Lewis base on boron upon the thermodynamic stability of metal complexes of borane–Lewis base adducts, [M(CO)5(η1‐BH2R⋅L)] (M=Cr, W) and [CpMn(CO)2(η1‐BH2R⋅L)], where R=Cl, I, m‐C6H4F, Ph, H, Me, Et; L=PMe3, PPh3, NMe3, quinuclidine. In these compounds, the stability of the metal–borane linkage was enhanced by increasing the electron‐releasing ability of the substituent on boron. A stronger base L additionally stabilized the complexes. The strength of the borane–metal interaction is thus mainly ascribed to the electron donation from the BH σ orbital to metal rather than the back‐donation into the BH σ* orbital. This result supports the bonding model for the B‐H‐M linkage in the borane complexes suggested by MO calculations, where the borane‐to‐metal electron donation is predominant while the metal back‐donation into the BH σ* orbital is negligible. Such a stability trend of the borane complexes makes a sharp contrast to that of many silane and dihydrogen complexes.

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