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

Abstract

Photolysis of Cr(CO)/sub 6/ in ethene-doped liquid xenon at -78 /sup 0/C has been reported to yield (eta/sup 2/-ethene)Cr(CO)/sub 5/ (1a), trans-(eta/sup 2/-ethene)/sub 2/Cr(CO)/sub 4/ (2a), and cis-(eta/sup 2/-ethene)/sub 2/Cr(CO)/sub 4/ (3a). All three compounds were said to be thermally unstable under these experimental conditions, their stability decreasing in the order 1a > 2a > 3a. More recently 1a was photochemically generated from Cr(CO)/sub 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 cis-(eta/sup 2/-ethene)/sub 2/Cr(CO)/sub 4/ (3a). The lability and cis structure of 3a was set in contrast with the stability and trans configuration of the known (eta/sup 2/-olefin)/sub 2/M(CO)/sub 4/ complexes of molybdenum and tungsten. This conflicts with our experience with olefin-substituted group 6 metal carbonyls and led us to suspect that the apparent disconformity should not be associated with the particular metal but rather originates from differences in the reaction conditions, i.e., gas-phase photochemistry in the chromium case vs photoreactions of the molybdenum and tungsten compounds in solution. Recent experiments allow us to expand on this point and prompt us to report on themore » photoreaction of Cr(CO)/sub 6/ with ethene in alkane solution, which provides convenient preparative access to the stable trans-(eta/sup 2/-ethene)/sub 2/Cr(CO)/sub 4/ (2a).« less