The reaction first proceeds through the nucleophilic attack of the methanol molecule to the C u1 atom (structure II). At a C•••O distance of 1.74 Å, the inter-molecular interaction is evidenced by a deformation of the ELF around the oxygen atom, away from the typical lone pairs shape (see Supplementary Information, Fig. , panel a). The approach of the oxygen doublet induces a transfer of electronic density from the C u1 atom to the C u2 and C u3 ones. This can be appreciated from a decrease of population of attr1 from 0.982 to 0.646 and a simultaneous increasing of attr2 from 0.72 to 0.94.