PARTIAL PARAMAGNETISM OF THE CR-CR QUADRUPLE BOND

Variable-temperature NMR measurements show that Cr2(O2CR)4L2 compounds (L = MeOH, H2O, py, MeCN) and closely related ones possess inherent, temperature-dependent partial paramagnetism. This may be attributed to a Boltzmann distribution between a ground state with S = 0 and a low-lying (approximately 400-1000 cm-1) state with S = 1. When R is kept constant (as CH3) and the Cr-Cr distance is changed by changing L, the singlet-triplet separation varies inversely with the Cr-Cr distance, suggesting that the low-lying triplet state may be the 3A2u state arising from a sigma2pi4deltadelta* configuration. Other explanations may also be considered, and cannot be falsified conclusively. The carbamato compound Cr2(O2CNEt2)4(NEt2H)2 has also been studied. Despite the fact that the Cr-Cr distance is similar to those in the acetates, the singlet-triplet gap is much smaller, ca. 600 cm-1. This may be attributed to a different interaction of the carbamato ligand with the delta orbital, as is shown by SCF-Xalpha calculations.