Carbonyl analogues? Analysis of Fe–E (E = B, Al, Ga) bonding in cationic terminal diyl complexes by density functional theory

Abstract

A series of DFT calculations has been carried out with the aim of characterizing the metal - group 13 element interaction in the novel cationic borylene complex [(η⁵-C₅Me₅)Fe(CO)₂(BMes)]⁺ (1) and related species of the type [(η⁵-C₅R₅)M(L)₂(EX)]ⁿ⁺. In addition, comparisons have been made with charge neutral borylene complexes and with related group 14 based ligand systems (e.g. cationic metal carbonyls, carbenes and vinylidenes) for which models of bonding have previously been established. In this regard particular attention has been focused on the interpretation of (i) molecular orbital composition; (ii) bond dissociation energies (BDEs) and the ratio of ionic to covalent contributions (ΔE_elstat/ΔE_orb); and (iii) σ and π symmetry covalent contributions. The molecular orbital compositions for the prototype borylene complex 1 and for related cationic and neutral systems {e.g. [(η⁵-C₅H₅)Fe(PMe₃)₂(BMes)]⁺ and [(η⁵-C₅H₅)Mn(CO)₂(BMes)]} are consistent with the presence of bonding interactions between metal and borylene fragments of both σ and π symmetry. Furthermore, on the basis of BDEs, ΔE_orb values and σ/π covalent ratios, the bonding in cationic terminal borylene complexes such as 1 appears to have as much right to be termed a ME double bond as does that in archetypal Fischer carbene and related complexes such as [(η⁵-C₅R₅)Fe(CO)₂(CCMe₂)]⁺ and [(η⁵-C₅R₅)Fe(CO)₂(CH₂)]⁺.