81 A stereoisomeric rrans-alkyi-cis-hydroxysuinodimethane can hardly be considered to compete with the trans-/runs-auinodimethane i l l ) as a cycloaddition partner because of its short lifetime and for steric reasons; cf. Ref. (71. I91 According to F. Nerdel and W. Brodowski 171 the photoaddition of o-methylbenzophenone to maleic and fumaric acids proceeds stereospecifically with preservation of the olefinic configuration.The great importance of monoolefin n-complexes formed as intermediates in homogeneous catalysis (e. g . during oxidation of ethylene to acetaldehyde, hydroformylation, dimerization of olefins, Ziegler-Natta POIYmerization, and hydrogenation of olefins) calls uarticularly for model study of stable olefin x-complexes. Both the factors that influence the formation of x-complexes by an olefin, and the extent of interaction between the olefin and its neighboring ligands have been investigated for the case of complex formation by olefins with photochemically produced carbonylmetal fragments (ICr(CO)51, IC H~M n~C 0~~1 ,~C~H~C r~C O~~N O~~