EXCITED-STATE PROPERTIES OF [(OC)(5)W(L)W(CO)(5)] [L=4,4'-BIPYRIDYL (4,4'-BIPY) OR PYRAZINE] AND [(OC)(5)W(4,4'-BIPY)]

Fast time-resolved infrared (TRIR) spectroscopy has been employed to p robe the electron distribution in the lowest metal-to-ligand charge-tr ansfer excited states of [(OC)(5)W(4,4'-bipy)W(CO)(5)] 1, [(OC)(5)W(4, 4'-bipy)] 2 (4,4'-bipy = 4,4'-bipyridyl) and [(OC)(5)W(pyz)W(CO)(5)] 3 (pyz = pyrazine). The excited State of 1 (i.e. 1) shows a nu(CO) ban d pattern, compared with the ground state, in which some nu(CO) bands increase and some decrease in frequency; those which increase match ve ry closely those observed for the excited state of 2, where the increa se in frequency is readily assigned to the increase in the effective o xidation state of W on electron transfer to the 4,4'-bipy ligand (sche matically W(+)L(-)). The nu(CO) IR bands of the electrochemically redu ced 1, i.e. 1(-), are lower in frequency than those of 1, and nearly m atch the low-frequency bands of 1. The interpretation is that the exc ited state of 1 is localised (schematically approximate to W+C-W) on t he IR time-scale, and that the degree of-coupling between the: two hal ves of the excited molecule is very small. Similar conclusions are obt ained for the excited state of 3, based on TRIR and spectroelectrochem istry of this complex.