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>i</i>Pr₃)₂ (<b>1</b>) loses a hydrogen molecule and the resulting unsaturated OsH₄(P^<i>i</i>Pr₃)₂ species coordinates 9-borabicycle[3.3.1]­nonane (HBbn) and pinacolborane (HBpin) to give the dihydrideborate derivatives OsH₃{κ²-<i>H</i>,<i>H</i>-(H₂BR₂)}­(P^<i>i</i>Pr₃)₂ (BR₂ = Bbn (<b>2</b>), Bpin (<b>3</b>)). The bonding situation in these compounds and in the related osmium­(II) derivative Os­(Bcat)­{κ²-<i>H</i>,<i>H</i>-(H₂Bcat)}­(CO)­(P^<i>i</i>Pr₃)₂ (<b>4</b>) (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 B₂H₆. The tetrahydride OsH₄(P^<i>i</i>Pr₃)₂ also coordinates catecholborane, which initially affords OsH₃{κ²-<i>H</i>,<i>H</i>-(H₂Bcat)}­(P^<i>i</i>Pr₃)₂ (<b>5</b>). In contrast to <b>2</b> and <b>3</b>, complex <b>5</b> reacts with a second molecule of HBcat to give the elongated σ-borane-{bis­(elongated σ)<i>-</i>dihydrideborate}-osmium­(II) derivative OsH­(η³-H₂Bcat)­(η²-HBcat)­(P^<i>i</i>Pr₃)₂ (<b>6</b>). Complexes <b>5</b> and <b>6</b> have been also analyzed via the AIM method. Complex <b>5</b> displays the same topology as complexes <b>2</b>–<b>4</b>. However, the OsH₂B unit of <b>6</b> 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 <b>1</b> with dimethylamine–borane (H₃B·NHMe₂) and <i>tert</i>-butylamine–borane (H₃B·NH₂^<i>t</i>Bu) give OsH₂(η²:η²-H₂BNR₂)­(P^<i>i</i>Pr₃)₂ (NR₂ = NMe₂ (<b>7</b>), NH^<i>t</i>Bu (<b>8</b>)). The AIM analyses of <b>7</b> and <b>8</b> 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