| Sumario: | 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(PiPr3)2 (1) loses a hydrogen molecule and the resulting unsaturated OsH4(PiPr3)2 species coordinates 9-borabicycle[3.3.1]nonane (HBbn) and pinacolborane (HBpin) to give the dihydrideborate derivatives OsH3{κ2-H,H-(H2BR2)}(PiPr3)2 (BR2 = Bbn (2), Bpin (3)). The bonding situation in these compounds and in the related osmium(II) derivative Os(Bcat){κ2-H,H-(H2Bcat)}(CO)(PiPr3)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 B2H6. The tetrahydride OsH4(PiPr3)2 also coordinates catecholborane, which initially affords OsH3{κ2-H,H-(H2Bcat)}(PiPr3)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-H2Bcat)(η2-HBcat)(PiPr3)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 OsH2B 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 (H3B·NHMe2) and tert-butylamine–borane (H3B·NH2tBu) give OsH2(η2:η2-H2BNR2)(PiPr3)2 (NR2 = NMe2 (7), NHtBu (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.
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