Structures of homoleptic triply bonded M-2(OR)(6) compounds where the alkoxide is tertiary: the effect of steric bulk and alkoxide conformation on structural parameters

The structures of three homoleptic metal-metal triply-bonded M-2(OR)(6) com pounds {Mo-2(OCMe2CMe2O)(3), 1; Mo-2(OCMe2Ph)(6), 2; and W-2[OCMe(CF3)(2)]( 6), 3} containing tertiary alkoxides are reported. The data demonstrate tha t, overall, the M2O6 cores are isomorphous regardless of the substitution p attern of the alkoxide. Dimer 3 is the first example of a W-2(OR)(6) compou nd containing terminal tertiary alkoxides to be structurally characterized; it is of interest as a close structural analogue of the chemically remarka ble W-2(OCMe3)(6). The hexa(hexafluoro-t-butoxide) dimer, though slightly d isordered, displays distances and angles similar to other W-2(OR)(6) specie s despite the poor electron-donor capacity of the fluoroalkoxide group. The data from these structures, combined with literature data, show that proxi mal alkoxide ligands generally exhibit shorter M-O bond distances and large r M-M-O and M-O-C angles than do distal alkoxides regardless of metal type, alkoxide steric bulk, and alkoxide pi donor ability in a range of compound s. This suggests electrostatic and conformational considerations are of gre ater importance to the bonding in these dimers than are M-O rr interactions . (C) 1999 Elsevier Science Ltd. All rights reserved.