The reactions of alkyl chromium carbene complexes with alkynes have been found to give cyclopentenones. Mechanisms are proposed to account for the formation of these products that involve metal hydride intermediates. As has been previously reported for tungsten, molybdenum alkyl complexes have been found to give 1,3-dienes rather than cyclopentenones. The difference between chromium and molybdenum and tungsten may be that a metal hydride intermediate can re-add to an olefin in the case of chromium rather than undergo reductive elimination