SELENIUM-BRIDGED CLUSTERS - SYNTHESIS AND CHARACTERIZATION OF SELENIUM-BRIDGED FE-MO CLUSTERS CP2MO2FE2SE3(CO)6, CP2MO2FE2SE2(CO)7, AND CP2MO2FESE(CO)7

Reflux of a benzene solution containing Fe3(CO)9Se2 and Cp2Mo2(CO)6 yielded the known cluster Cp2Mo2Fe2Se3(CO)6 (1) and the new clusters Cp2Mo2Fe2Se2(CO)7 (2) and Cp2Mo2-FeSe(CO)7 (3). Compound 2 was also isolated from the room temperature reaction of Fe2(CO)6Se2 with Cp2Mo2(CO)4. In solution, 2 was found to slowly convert to 1; on thermolysis, the conversion occurs rapidly. Both 2 and 3 have been characterized by spectroscopic and crystallographic methods. 2 crystallizes in the orthorhombic space group Pna2(1) with a = 13.289(3) angstrom, b = 12.059(2) angstrom, c = 13.045(3) angstrom, V = 2090.5(7) angstrom 3, and Z = 4. The structure refined to R = 3.56% and R(w) = 3.65% for 2109 unique reflections (F(o) greater-than-or-equal-to 4sigma-(F(o))). The structure of 2 consists of a Mo2Fe2 tetrahedron with the Fe-Fe edge bridged by a CO group. Triply bridging Se atoms cap the two Mo2Fe faces, and there is a semitriply bridging CO group above one of the MoFe2 faces. The remaining CO groups are terminally bonded. 3 crystallizes in the monoclinic space group C2/c with a = 30.756(7) angstrom, b = 8.590-(1) angstrom, c = 15.115(3) angstrom, beta = 102.86(3)-degrees, V = 3893.4(13) angstrom 3, and Z = 8. The structure refined to R = 4.81% and R(w) = 6.00% for 3103 unique reflections (F(o) greater-than-or-equal-to 4sigma(F(o))). The structure of 3 consists of a Mo2FeSe tetrahedron. Each Mo atom has one Cp and two terminally bonded CO groups, while the Fe atom has three CO groups bonded to it.