Bridging cyclobutadiene ligands with agostic hydrogen atoms in binuclear chromium carbonyl derivatives

Abstract The binuclear cyclobutadiene chromium carbonyls (C4H4)2Cr2(CO)n (n = 7, 6, 5, 4, 3), potentially accessible from the experimentally known (η4-C4H4)Cr(CO)4, have been examined by density functional theory. The low-energy structures for the heptacarbonyl (C4H4)2Cr2(CO)7 have terminal (η4-C4H4)2Cr2(CO)7 rings with a central Cr–Cr single bond bridged by one or three CO groups and the remaining CO groups as terminal ligands evenly distributed among the chromium atoms. The lowest energy structure for the hexacarbonyl (C4H4)2Cr2(CO)6 has a (η4-C4H4)2Cr(CO)2 complex functioning as a bidentate chelating ligand to a Cr(CO)4 fragment through the formation of two three-center C–H–Cr bonds with agostic hydrogen atoms exhibiting short Cr–H distances of ∼1.85 A. The pentacarbonyl (η4-C4H4)2Cr2(CO)5 resembles the analogous experimentally known (η5-C5H5)2V2(CO)5 by having terminal η4-C4H4 rings, two bridging CO groups, and a central Cr ≡Cr triple bond. The three lowest energy structures for the tetracarbonyl (η4-C4H4)2Cr2(CO)4 are singlet structures with two four-electron donor bridging η2-μ-CO groups, a terminal CO group on each chromium atom, and a central Cr =Cr double bond. The lowest energy structure for the more highly unsaturated tricarbonyl (η4-C4H4)2Cr2(CO)3 by a margin of more than 12 kcal/mol is a triplet spin state structure with a terminal η4-C4H4 ring, a bridging η4,η1-C4H4 ring through an agostic C–H–Cr interaction, and a central Cr ≡Cr triple bond. Thermochemistry suggests that the lowest energy structures for all of the (C4H4)2Cr2(CO)n (n = 7, 6, 5, 4) systems, especially the pentacarbonyl, are viable towards carbonyl dissociation.