SYNTHESIS AND KINETICS OF DECOMPOSITION OF TUNGSTEN(VI) AND MOLYBDENUM(VI) 2-OXAMETALLACYCLOBUTANE COMPLEXES AND THE X-RAY STRUCTURE OF TRANS-MO[CH(TERT-BU)CH(C6F5)O](NAR)(O-TERT-BU)2

2-Oxametallacyclobutane complexes (M = Mo (2a), W (2b)) can be prepared by adding pentafluorobenzaldehyde to M(CH-t-Bu)(NAr)(O-t-Bu)2 (M = Mo (1a), W (1b)). Two isomers are observed for 2a and 2b that contain t-Bu and Ph(F) substituents trans or cis to one another (the trans isomer predominating) and that decompose to give trans- or cis-Ph(F)CH = CH(t-Bu), respectively. An X-ray structure of the major isomer of 2a (2a-trans) showed it to be a square pyramid containing an apical imido ligand and pseudo-equatorial alpha-t-Bu and beta-Ph(F) groups in a slightly puckered MoOC2 ring. It is proposed that cis isomers of 2 also contain a pseudoequatorial alpha-t-Bu group. At 25-degrees-C 2a-trans decomposes twice as rapidly as 2b-trans, and at 5.0-degrees-C 2b-cis decomposes approximately 25 times more rapidly than 2b-trans. These findings are all consistent with square-pyramidal 2-oxametallacyclobutane complexes being more stable when they contain trans alpha and beta substituents, when electron-withdrawing substituents are present on the ring, and when a bulky substituent is present in the pseudoequatorial alpha position. These results are discussed in relation to findings concerning analogous MC3 metallacyclobutane complexes.