Two new metallacarboranes, CO2(CO)(6)B2H4 and CO5(CO)(13)(mu-CO)B2H, have been prepared in low yield by the reaction of BH3 . SMe(2) with Co-2(CO)(8). The former has been fully characterized spectroscopically and is isoelectronic with Fe-2(CO)(6)B2H6, on the one hand, and Co-2(CO)(6)C2H2, on the other. The latter has been characterized both spectroscopy and by a single-crystal X-ray diffraction study (orthorhombic Pna2, a = 24.886(4) Angstrom, b = 9.846(5) Angstrom, c = 9.248(3) Angstrom, V = 2266.3(9) Angstrom(3), d(calc) = 2.076 g/cm(3), Z = 4). The structure exhibited by Co-5(CO)(13)(mu-CO)B2H is unusual, and aspects of its electronic structure have been elucidated by molecular orbital calculations. Mechanistic studies suggest solvent-free (CO)(4)CoBH2 as an intermediate and suggest that the formation of the new compounds constitutes an example of B-H, Co-Co sigma-bond metathesis.