Photoreactions of group 6 metal carbonyls with olef i ns

Abstract -A scheme is presented, based on preparative and mechanistic studies, concerning the principles which govern the course of multiple photosubstitution of group 6 metal carbonyls with olefins: (i) after initial (n -olefin)M(CO) formation, photcdetachment of CO in cis-position to the olefin is strongly favoured over frans-CO dissociation; (ii) a (n2-C=C) M subunit with frarrs-orfhogonal position of the olefins is distinctly more stable than other geometries. This is rationalized in terms of competitive demand of CO and olefin ligands for metal d electron density, whereby the single-faced n-acceptor character of the olefin plays a crucial role. -Sequential photosubstitution of M(C0)6 with olefins yields (n2-olefin)M(C0)5 and, ultimately, trans-(n -olefin) M(C0)4, as verified for all three group 6 metals. Quantum yield measurements (0.72 for W and 0.61 for Cr in the first step: ca. 0.5 for W and ca. 0.04 for Cr in the second step: at A = 302 nm) and studies revealing the role of cis-(n -olefin)2M(C0)4 were performed with E-cyclooctene, which exhibits exceptionally strong bonding to transition metals. In accord with the above principles, photosubstitution of (l14-norbornadiene)M(C0)4 complexes yields bornadiene)M(CO) . Implications of these findings with respect to photocatalytic processes are 3 briefly discussed