Photoreactions of Group 6 metal carbonyls with ethene: syntheses of trans-(.eta.2-ethene)2M(CO)4 (M = chromium, molybdenum, tungsten

Photolysis of Cr(CO)6 in ethene-doped liquid xenon at -78 °C has been reported1 to yield (7j2-ethene)Cr(CO)5 (la), trans- (772-ethene)2Cr(CO)4 (2a), and m-(?j2-ethene)2Cr(CO)4 (3a). All three compounds were said to be thermally unstable under these experimental conditions, their stability decreasing in the order la > 2a > 3a. More recently2 la was photochemically generated from Cr(CO)6 and ethene in the gas phase. Subsequent pulsed-laser photolysis produced a transient species, detected by means of time-resolved IR spectroscopy, which decayed on the ms time scale and was assigned as m-(t)2-ethene)2Cr(CO)4 (3a). The lability and cis structure of 3a was set in contrast with the stability and trans configuration of the known (?z2-olefin)2M(CO)4 complexes of molybdenum and tungsten.