SO2 – yet another two-faced ligand

Abstract

Experimentally known adducts of SO₂ with transition metal complexes have distinct geometries. In the present paper, we demonstrate by a bonding analysis that this is a direct consequence of sulfur dioxide acting as an acceptor in one set, square-planar complexes of d⁸ and linear two-coordinated complexes of d¹⁰ transition metals, and as a donor with other compounds, well-known paddle-wheel [Rh₂(O₂CCF₃)₄] and square-pyramidal [M(CO)₅] (M = Cr, W) complexes. Bonding energy computations were augmented by the natural bond orbital (NBO) analysis and energy decomposition analysis (EDA). When the SO₂ molecule acts as an acceptor, bonding in the bent coordination mode to the axial position of the d⁸ or the d¹⁰ metal center, the dominant contributor to the bonding is LAO(S) (Lewis Acidic Orbital, mainly composed of the p_x-orbital of the S atom) as an acceptor, while a d_z² orbital centered on the metal is the corresponding donor. In contrast, the distinct collinear (or linear) coordination of the SO₂ bound at the axial position of [Rh₂(O₂CCF₃)₄] and/or [M(CO)₅] is associated with a dominant donation from a lone pair localized on the sulfur atom, σ*(Rh–Rh) and/or empty LAO(M) (mainly composed of the d_z² orbital of the metal), respectively, acting as an acceptor orbital. The donor/acceptor capabilities of the SO₂ molecule were also checked in adducts with organic Lewis acids (BH₃, B(CF₃)₃) and Lewis bases (NH₃, N(CH₃)₃, N-heterocyclic carbene).