Binuclear Acetoniminato Derivatives of Iron and Chromium Carbonyls: A Theoretical Study

The structures and energetics of binuclear acetoniminato metal carbonyl derivatives of the types (Me₂CN)₂Fe₂(CO) _n and (Me₂CN)₂Cr₂(CO) _n have been investigated by density functional theory. The (Me₂CN)₂Fe₂(CO)₆ structure, with bridging Me₂CN ligands using only their nitrogen atoms to bridge an FeFe bond related to experimentally known species, as well as a related (Me₂CN)₂Cr₂(CO)₈ structure, appear to be energetically preferred. Among carbonyl-richer systems, the iron system energetically prefers a {Me₂CNC-(O)}₂Fe₂(CO)₆ structure with bridging Me₂CNC-(O) ligands, whereas the chromium system energetically prefers a structure with a 2,3-diazabutadiene ligand having each nitrogen atom bonded to a Cr-(CO)₅ moiety. Preferred structures for systems with fewer CO groups include the tetracarbonyl (Me₂CN)₂Fe₂(CO)₄ in which one of the bridging Me₂CN uses its CN double bond to bond to an iron atom, as well as (Me₂CN)₂Cr₂(CO) _n (n = 7, 6) structures with chromium-chromium multiple bonds bridged by Me₂CN groups.