A new class of CVD precursors to metal borides: Cr(B3H8)2 and related octahydrotriborate complexes

Treatment of CrCl3 with sodium octahydrotriborate, NaB3H8, affords a thermally unstable purple liquid thought to be a chromium(III) hydride of stoichiometry CrH(B3H8)2. This hydride converts rapidly at room temperature to the chromium(II) complex Cr(B3H8)2, which adopts a square-planar structure in which four hydrogen atoms form the coordination sphere of the chromium atom. This chromium(II) species forms six-coordinate Lewis base adducts Cr(B3H8)2L2, where L is Et2O, thf, or PMe3; the first two of these adopt trans geometries, whereas the latter is cis. Volatile Cr(B3H8)2 is the first homoleptic transition metal complex of the octahydrotriborate anion, and it readily forms CrB2 thin films by CVD at 250 degrees C.