On Cu-Cu bonding in paddle-wheel copper(II) acetates

Starting from the X-ray structure of [Cu₂(µ₂-O₂CCH₃)₄]⁰•2H₂O, the structures of [Cu₂(CH₃COO)_n]^q, n = 1 → 4, with charges q = 4-n and 2-n for the oxidation states Cu(II) and Cu(I), respectively, were optimized in the singlet and triplet ground spin states. Their electronic structures were evaluated in terms of Quantum Theory of Atoms-in-Molecule (QTAIM). Molecular graphs of Cu(II) complexes with two or fewer acetate bridges exhibit no Cu-Cu bonding. Their Cu-Cu distances decrease with the number of acetate ligands. The O-O distances in the same carboxyl group are always shorter than the Cu-Cu ones, therefore, it can be concluded that two or fewer carboxylate bridges are not sufficient to press both Cu atoms together to form a bond critical point (BCP) between them. Hypothetical dicopper(I) acetates exhibit the BCP between both Cu atoms even in ³[Cu₂(µ₂-O₂CCH₃)]⁺ unlike its unbridged singlet counterpart. The instability of the hypothetical [Cu₂(µ₂-O₂CCH₃)₄]²⁻ complex can be ascribed to the limited coordination number of Cu(I) central atoms (with subsequent Cu-O bond splitting). The QTAIM characteristics of the BCPs between both Cu atoms in all complexes under study are not sufficient to distinguish true Cu-Cu bonding from an artifact of pressing both copper atoms together by carboxylate bridges.