Bonding of Iron Tricarbonyl Units to Heptafulvene:Trimethylenemethane, Butadiene, and Allylic Coordination Modes.

Complexationof the very unstable free heptafulvene with iron carbonylsis known experimentally to lead to stable complexes, including twoisomers of (η4-C7H6CH2)Fe(CO)3as well as the binuclear (C7H6CH2)2Fe2(CO)6.Density functional theory shows that structures of the mononuclear(C7H6CH2)Fe(CO)3withtrimethylenemethane and butadiene subunits of the heptafulvene ligandbonded to the Fe(CO)3moiety have nearly equal energieswithin ∼3 kcal/mol, consistent with the experimental observationof two isomers depending upon the synthetic method. For (C7H6CH2)Fe2(CO)6a trans-η4:η4structurewith no iron–iron bond and a cis-η3:η3allylic structure with an iron–ironbond are predicted to have energies within ∼3.0 kcal/mol. Comparisonof the predicted ν(CO) frequencies for these two structureswith the experimental ν(CO) frequencies for the structurallyuncharacterized (C7H6CH2)Fe2(CO)6suggests the cis-η3:η3allylic structure for the latter. [ABSTRACT FROM AUTHOR]