Chemical origin of C–O blue shift and nature of Cu+…C bonding in non-classical copper carbonyl Cu+(CO)n (n = 1–8) complexes

Natural bond orbital (NBO) method and atoms-in-molecules (AIM) theory are used to study the chemical origin in the direction and ordering of C–O frequency shift, as well as Cu+…C bond nature in copper carbonyl cations Cu+(CO)n (n = 1–8). This study emphasises the role of π-back donation in explaining the ordering of C–O blue shift. NBO analyses show that the interplay of two competing factors, including π-back donation and rehybridization, is responsible for both the direction and the ordering of C–O stretching frequency shift in Cu+(CO)n (n = 1–8) complexes. In addition, the Cu+…C interaction is interesting because Cu+(CO)n (n = 1–4) structure has one-sphere CO ligands but Cu+(CO)n (n = 5–8) has two-sphere CO ligands. Topological analyses of electron density are applied to characterise the Cu+…C interactions of first- and second-sphere CO with Cu+ and to explore the Cu+…C interactions in the nature.