Excited-state properties of [(OC)₅W(L)W(CO)₅] [L = 4,4′-bipyridyl (4,4′-bipy) or pyrazine] and [(OC)₅W(4,4′-bipy)]

Fast time-resolved infrared (TRIR) spectroscopy has been employed to probe the electron distribution in the lowest metal-to-ligand charge-transfer excited states of [(OC)₅W(4,4′-bipy)W(CO)₅] 1, [(OC)₅W(4,4′-bipy)] 2 (4,4′-bipy = 4,4′-bipyridyl) and [(OC)₅W(pyz)W(CO)₅] 3 (pyz = pyrazine). The excited state of 1 (i.e. 1*) shows a ν(CO) band pattern, compared with the ground state, in which some ν(CO) bands increase and some decrease in frequency; those which increase match very closely those observed for the excited state of 2, where the increase in frequency is readily assigned to the increase in the effective oxidation state of W on electron transfer to the 4,4′-bipy ligand (schematically W⁺L^-). The ν(CO) IR bands of the electrochemically reduced 1, i.e. 1^-, are lower in frequency than those of 1, and nearly match the low-frequency bands of 1*. The interpretation is that the excited state of 1 is localised (schematically ≈W⁺L^-W) on the IR time-scale, and that the degree of coupling between the two halves of the excited molecule is very small. Similar conclusions are obtained for the excited state of 3, based on TRIR and spectroelectrochemistry of this complex.