Syntheses and structures of six and seven-coordinate diene complexes of molybdenum and tungsten(II). The crystal and molecular structures of [WBr2(CO)(2)(PMePh2)(norbornadiene)] and [WBr2(CO)(N CCH=CH2)(norbornadiene)]

Reactions of [WI2(CO)(3)(NCMe)(2)] with norbornadiene (NBD) and 1,5-cycloct adiene (COD) give six-coordinate dicarbonyls [WI2(CO)(2)(NBD)] and [WI2(CO) (2)(COD)], respectively. The related dibromo complex [WBr2(CO)(2)(NBD)] rea cts with tertiary phosphines L to give unstable seven-coordinate adducts [W Br2(CO)(2)(L)(NBD)] (L = PPh3, PMePh2, PEt3), which in the case of L = PMeP h2 was shown to undergo CO loss to give the six-coordinate derivative [WBr2 (CO)(PMePh2)(NBD)]. X-ray diffraction studies of [WBr2(CO)(2)(PMePh2)(NBD)] reveal a seven-coordinate structure (distorted pentagonal bipyramid) with the diene C=C bonds on the pentagonal plane lying approximately parallel to the W-CO axes. Reactions of [WBr2(CO)(2)(NBD)] with organonitriles NCR at room temperature afford CO substitution products (R = Me, Pr-i, Ph, C6H4Me- 4, CH=CH2) directly with no evidence for seven-coordinate intermediates. [W Br2(CO)(2)(COD)] and acetonitrile similarly give [WBr2(CO)(NCMe)(COD)]. X-r ay diffraction studies of [WBr2(CO)(NCCH=CH2)(NBD)] confirmed a distorted o ctahedral structure where the carbonyl lies trans to the acrylonitrile liga nd. (C) 1999 Elsevier Science S.A. All rights reserved.