Evidence for a Bis(Elongated σ)-Dihydrideborate Coordinated to Osmium.

The formation and Atoms in Molecules (AIM) analysis of osmium(IV) and osmium(II) complexes containing dihydrideborate groups and primary aminoborane ligands are reported. Complex OsH6 (P i Pr3 )2 (1) loses a hydrogen molecule and the resulting unsaturated OsH4 (P i Pr3 )2 species coordinates 9-borabicycle[3.3.1]nonane (HBbn) and pinacolborane (HBpin) to give the dihydrideborate derivatives OsH3 {κ2 - H, H-(H2 BR2 )}(P i Pr3 )2 (BR2 = Bbn (2), Bpin (3)). The bonding situation in these compounds and in the related osmium(II) derivative Os(Bcat){κ2 - H, H-(H2 Bcat)}(CO)(P i Pr3 )2 (4) (HBcat = catecholborane) has been analyzed by the AIM method. The Laplacian distributions in the Os-H-B plane exhibit a four-membered cyclic topology possessing two Os-H and two B-H bond critical points associated with one OsHHB ring critical point, which resembles that found for B2 H6 . The tetrahydride OsH4 (P i Pr3 )2 also coordinates catecholborane, which initially affords OsH3 {κ2 - H, H-(H2 Bcat)}(P i Pr3 )2 (5). In contrast to 2 and 3, complex 5 reacts with a second molecule of HBcat to give the elongated σ-borane-{bis(elongated σ) -dihydrideborate}-osmium(II) derivative OsH(η3 -H2 Bcat)(η2 -HBcat)(P i Pr3 )2 (6). Complexes 5 and 6 have been also analyzed via the AIM method. Complex 5 displays the same topology as complexes 2-4. However, the OsH2 B unit of 6 shows, besides the Os-H and B-H bond critical points, an additional Os-B bond critical point, which is associated with a bond path running between these atoms. This double triangular topology is completed with the respective ring critical points. Reactions of 1 with dimethylamine-borane (H3 B·NHMe2 ) and tert-butylamine-borane (H3 B·NH2 t Bu) give OsH2 (η2 :η2 -H2 BNR2 )(P i Pr3 )2 (NR2 = NMe2 (7), NH t Bu (8)). The AIM analyses of 7 and 8 also reveal the occurrence of an Os-B bond critical point associated with a bond path running between those atoms. However, neither Os-H bond critical points nor bond paths are observed in the latter species.