Transition-metal-substituted diphosphenes. 29. Cycloaddition reactions of the diphosphenyl complex (.eta.5-C5Me5)(CO)2Fe-P:P-Mes* (Mes* = 2,4,6-tert-Bu3C6H2) with hexafluoroacetone. X-ray structure analyses of (.eta.5-C5Me5)(CO)Fe-P(:PMes*)OC(CF3)2C(O) and (.eta.5-C5Me5)(CO)2Fe-P-P(Mes*)OC(CF3)2