Polyazolyl Chelate Chemistry. 13. An Osmaboratrane<sup>1</sup>

The reaction of [Os(C<sub>6</sub>H<sub>5</sub>)Cl(CO)(PPh<sub>3</sub>)<sub>2</sub>] with Na[HB(mt)<sub>3</sub>] (mt = methimazolyl) results in benzene elimination and the formation of the first osmaboratrane [B(mt)<sub>3</sub>Os(CO)(PPh<sub>3</sub>)](<i>Os</i>→<i>B</i>), the crystal structure of which reveals a somewhat long transannular dative bond between osmium(0) and boron(III).