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(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.