The structure of dichromium tetraformate

Second-order perturbation theory (CASPT2) calculations using large basis sets have been used to study the CrCr bond in dichromium tetraformate, Cr2(O2CH)4 The CrCr potential was found to have a double minimum with one short equilibrium at 1.94 Å, exhibiting considerable 3d-3d bonding, and a long equilibrium at 2.50 Å, which is essentially determined by the structure of the bridging ligands. The energy difference between the two minima is small (of the order of 0.1 eV) and the relative order depends on the level of theory used. Inclusion of relativistic effects and 3s,3p correlation (estimated from the corresponding effect in the 3Σu+ state of Cr2 locates the inner minimum 0.07 eV below the outer. The measured distance in Cr2(O2CCH3)4 is 1.966 Å. It is likely that addition of axial ligands like water will make the outer minimum more stable, thus explaining the variations in Cr{inCr} distances in dichromium complexes. The DFr approach is also applied and long, 2.296 and 2.487 \rA, CrCr bond distances are found for Cr{in2}(O{in2}CH){in4} and Cr{in2}(O{in2}CCH{in3}){in4}, respectively. Addition of axial water ligands increases the CrCr distance by about 0.19 and 0.07 \rA, respectively, for these two systems.