Unsaturation in binuclear cyclopentadienyliron carbonyl thiocarbonyls: Four-electron donor bridging thiocarbonyl groups versus metal–metal multiple bonds

Theoretical studies on the binuclear cyclopentadienyliron carbonyl thiocarbonyl derivatives Cp2Fe2(CO)2(μ-CS)(μ-CO) and Cp2Fe2(CO)2(μ-CS)2 indicate that the trans and cis isomers are nearly degenerate in energy, consistent with experiment. Structures with bridging CS groups are of lower energy than corresponding structures with bridging CO groups. The corresponding unbridged Cp2Fe2(CS)(CO)3 and Cp2Fe2(CS)2(CO)2 isomers are predicted to lie 11 and 16 kcal/mol, respectively, above their global minima, indicating increasing activation energies for the cis/trans interconversion as bridging CO groups are replaced by bridging CS groups. The unsaturated species Cp2Fe2(μ-CS)(μ-CO)2 and Cp2Fe2(μ-CS)2(μ-CO) are predicted to have triply bridged triplet spin state structures with FeFe double bonds of lengths 2.26 Å, analogous to the experimentally known triplet (Me5C5)2Fe2(μ-CO)3. However, low-lying singlet Cp2Fe2(CS)(CO)2 and Cp2Fe2(CS)2(CO) structures with four-electron donor bridging η2-μ-CS groups and formal Fe–Fe single bonds are also found. The lowest lying Cp2Fe2(CS)(CO) and Cp2Fe2(CS)2 structures have two bridging groups and very short FeFe distances of ∼2.14 Å, suggesting formal triple bonds. Several higher energy four-electron donor η2-μ-CS bridged structures are also found for Cp2Fe2(CS)(CO) and Cp2Fe2(CS)2. In addition, singlet and triplet structures are found for Cp2Fe2(CS)2 in which the two CS ligands have coupled to form a bridging SCCS group with a carbon–carbon bond. Only a η2-μ-CS bridged singlet structure is predicted for Cp2Fe2(CS), rather than the normal bridged structure with a FeFe quadruple bond such as that predicted for the carbonyl analog Cp2Fe2(CO).