ON THE MECHANISM OF PHOTOCHEMICAL ALKYNE INSERTION INTO CYCLOPENTADIENYLIRONDICARBONYL DIMER - ALKYNE ADDITION TO A TIED-BACK CP-FE DIMER

The photochemical insertion of phenylacetylene with the ''tied-back'' dinuclear complex [(eta5-C5H4)2SiMe2]Fe2(CO)2(mu-CO)2 (4) yields, by X -ray structural analysis, a dimetallacyclopentenone product entirely a nalogous to that obtained from the photochemical reaction of the alkyn e with (eta5 - C5H5)2Fe2(CO)2(mu-CO)2 (1). This result is interpreted as evidence for an alternative pathway for photochemical alkyne additi on to 1. Attempts to identify the photochemical intermediates from 4 h ave been unsuccessful. A competition study between 1 and 4 indicates t hat photochemical alkyne insertion into 1 is preferred by approximate ratio of 2:1 over insertion into 4.