Combining a weak-field rigid chelating bidentate dicarbene ligand with a strong-field carbonyl ligand in binuclear cyclopentadienyliron carbonyl derivatives

Abstract

Binuclear cyclopentadienyliron carbonyl derivatives of the weak-field rigid chelating bidentate dicarbene vegiMe ligand with a tricyclic backbone are shown by theoretical studies on the Cp2Fe2(vegiMe)(CO)n (n = 3, 2, 1) systems to have higher spin triplet and quintet structures of comparable energies to isomeric singlet structures satisfying the 18-electron rule. Thus triplet structures of the tricarbonyl Cp2Fe2(vegiMe)(CO)3 with the vegiMe ligand as a bidentate chelate to a single iron atom are of comparable energies to singlet isomers with no iron-iron bond and the vegiMe ligand bridging the two iron atoms. Similarly, triplet and quintet structures for the dicarbonyl Cp2Fe2(vegiMe)(CO)2 are of comparable energies to singlet structures with Fe–Fe single bonds that are substitution products of the well-known Cp2Fe2(CO)2(µ-CO)2. The only low-energy singlet structure for the monocarbonyl Cp2Fe2(vegiMe)(CO) has an unusual six-electron donor bridging vegiMe ligand forming not only the usual two C(carbene)→Fe dative bonds but also a N→Fe dative bond. The other low-energy Cp2Fe2(vegiMe)(CO) structures are various types of triplet and quintet spin state structures.