Tetranuclear iron carbonyl complexes with a central tin atom: relationship to iron carbonyl carbides

Abstract

The two tetranuclear iron carbonyl systems EFe₄(CO)_n (E = Sn, C) containing central group 14 interstitial atoms differ in that spiropentane-like SnFe₄(CO)₁₆ has been synthesized in the tin system whereas the butterfly CFe₄(CO)₁₃, with three fewer carbonyl groups is the carbonyl-richest tetranuclear iron carbonyl carbide that has been synthesized. In order to clarify this point, the complete SnFe₄(CO)_n (n = 16, 15, 14, 13, 12) series has been studied by density functional theory for comparison with earlier similar studies on their CFe₄(CO)_n analogues. The experimentally observed spiropentane-like Sn[Fe₂(CO)₈]₂ structure is found to be the lowest energy structure for the SnFe₄(CO)₁₆ system as it is for the experimentally unknown CFe₄(CO)₁₆ system. Loss of a CO group from Sn[Fe₂(CO)₈]₂ joins the two Fe₂(CO)₈ units by a third Fe–Fe bond to give an SnFe₄(CO)₁₅ structure with a bonded four-atom Fe–Fe–Fe–Fe chain. Further CO loss from SnFe₄(CO)₁₅ adds a fourth Fe–Fe bond in the lowest energy SnFe₄(CO)₁₄ structure. The lowest energy SnFe₄(CO)₁₃ structure is analogous to that of the experimentally known iron carbonyl carbide CFe₄(CO)₁₃ with a central Fe₄ butterfly having five Fe–Fe bonds. The energetics of CO dissociation from the EFe₄(CO)_n (E = C, Sn; n = 16, 15, 14, 13) species account for the experimentally observed differences between the systems with central tin and central carbon atoms. Thus for the tin systems the CO dissociation energy from SnFe₄(CO)₁₆ is relatively high at ∼50 kcal mol⁻¹ consistent with its experimental observation as a stable species. However, for the tetranuclear iron carbonyl carbides CFe₄(CO)_n, the CO dissociation energies of the species with more than 13 CO groups are all very small or even negative suggesting CFe₄(CO)₁₃ to be the carbonyl-richest viable iron tetracarbonyl carbide consistent with experiment.