On copper–copper bond in hydrated cupric acetate

Broken symmetry (BS) calculations within the framework of density functional theory are performed on hydrated copper(II)acetate and its anhydrous variety using LACVP∗(Cu)/6-31G∗(rest) and LACVP*(Cu)/6-311G++**(rest) basis sets. The experimentally known Cu Cu distance of 2.617 A in hydrated copper(II)acetate is reproduced well as 2.619 A. Using the second basis set, the singlet–triplet separations in both the compounds come out as 497 cm−1. The Cu Cu distance in anhydrous copper(II)acetate is calculated as 2.480 A. Bader’s AIM calculations show presence of a bond critical point (BCP) of the type (3, −1) between the two copper atoms in both the compounds indicating the existence of a chemical bond between the metal atoms. The sign of the Laplacian associated with the electron density at the BCP is found to be negative indicating a covalent bond between the two copper centers. For the Cu Cu bond in both the compounds, Wiberg’s bond index comes out as ∼0.3 suggesting a reasonably strong metal–metal bond in cupric acetate. Calculations of the energies of the BS and the triplet states around the Cu Cu distance of 2.6 A (to be specific, in the range of 2.4–2.9 A) reveal that the observed magnetic behavior of hydrated cupric acetate is the resultant of two opposing pathways for the interaction between the two unpaired metal electrons – (1) through the ligand framework and (2) through a direct Cu Cu bond. The Cu Cu bond results in an antiferromagnetic exchange while the acetato framework provides a means of ferromagnetic interactions.