Osmaborane Clusters with B<sub>4</sub> and B<sub>5</sub> Rings Stabilized in the Coordination Sphere of {OsL<sub>n</sub>}\n(L<sub>n</sub> = η<sup>5</sup>‑C<sub>5</sub>Me<sub&g

New\nsynthetic routes have been developed to synthesize osmaborane\nclusters featuring B<sub>4</sub> and B<sub>5</sub> rings in the coordination\nspheres of osmium. Thermolysis of [Os(PPh<sub>3</sub>)<sub>3</sub>Cl<sub>2</sub>], <b>1</b> in the presence of excess of [BH<sub>3</sub>·THF] led to the formation of [Os(PPh<sub>3</sub>)<sub>2</sub>H<sub>2</sub>(η<sup>4</sup>-B<sub>4</sub>H<sub>8</sub>)], <b>2</b> along with [HOs(PPh<sub>3</sub>)<sub>2</sub>B<sub>5</sub>H<sub>10</sub>], <b>3</b>. Cluster <b>2</b> features\na planar tetraborane ring coordinated to an osmium center in an η<sup>4</sup> fashion. Cluster <b>3</b> can be considered an osmium\nanalogue of hexaborane(10), in which the osmium center is situated\nat the base of the pentagonal pyramid geometry. In a different synthetic\nprotocol, we have carried out the metathesis reaction of [Cp*OsBr<sub>2</sub>]<sub>2</sub> (Cp* = η<sup>5</sup>-pentamethylcyclopentadienyl), <b>5</b> with [LiBH<sub>4</sub>·THF] followed by thermolysis\nin the presence of [BH<sub>3</sub>·THF] that generated [Cp*Os(η<sup>5</sup>-B<sub>5</sub>H<sub>10</sub>)], <b>6</b>. Cluster <b>6</b> has a planar pentaborane ring that is stabilized in the\ncoordination sphere of osmium, making it a boron analogue of osmocene.\nClusters <b>2</b> and <b>6</b> are the first examples\nof structurally characterized planar B<sub>4</sub> and B<sub>5</sub> rings, respectively, that are stabilized in the coordination sphere\nof osmium. All the synthesized molecules were characterized using\nmultinuclear NMR and IR spectroscopy, mass spectrometry, and single\ncrystal X-ray diffraction analyses. Theoretical calculations were\ncarried out to visualize the electronic structures and bonding scenarios\nin <b>2</b> and <b>6</b>.