Structures, metal-ligand bond strength, and bonding analysis of ferrocene derivatives with group-15 heteroligands Fe(η⁵-E₅)₂ and FeCp(η⁵-E₅) (E = N, P, As, Sb). A theoretical study

Quantum chemical DFT calculations using B3LYP and BP86 functional have been carried out for the title compounds. The equilibrium geometries and bond dissociation energies are reported. The metal-ligand bonding was analyzed with an energy partitioning method. The strongest bonded homoleptic complex with a heterocyclic ligand is Fe(η⁵-P₅)₂. The bond dissociation energy yielding the Fe atom and two cyclo-P₅ ligands (D₀ = 128.3 kcal/mol) is nearly the same as for ferrocene (D₀ = 131.3 kcal/mol). The nitrogen, arsenic, and antimony analogues of Fe(η⁵-E₅)₂ have significantly weaker metal-ligand bonds, which, however, should still be strong enough to make them isolable under appropriate conditions. The calculated heats of formation show also that the phosphorus complex is the most stable species of the heterocyclic Fe(η⁵-E₅)₂ series. The Fe-(η⁵-E₅) bonding in the mixed sandwich complexes FeCp(η⁵-E₅) is much stronger compared to the homoleptic molecules. The heterocyclic ligands cyclo-E₅ in the mixed complexes FeCp(η⁵-E₅) bind as strongly or in case of phosphorus even stronger than one Cp ligand does in FeCp₂ except for E = Sb. The metal fragments Fe(η⁵-E₅)⁺ have a pyramidal geometry except for E = Sb, which is predicted to be a planar ion with D_5h symmetry. The energy partitioning shows that the binding interactions between the closed shell cyclo-E₅^- ligand and the Fe(η⁵-E₅)⁺ fragment do not change very much for the different ligand atoms E in the homoleptic and heteroleptic complexes. The bonding comes from 53%-58% electrostatic attraction, while 42%-47% come from covalent interactions. The latter contribution comes mainly from the donation of the occupied e₁ (π) orbital of the ligand into the empty orbital of the metal fragment.