Isolated CH stretching frequencies, methyl group geometry, and methyl CH bond lengths and strengths in tricarbonyl(η5-cyclopentadienyl)methyl-chromium(II), -molybdenum(II), and -tungsten(II)

Abstract

‘Isolated’ methyl CH stretching frequencies, ν(CH^is), are reported for [M(CHD₂)(η⁵-C₅H₅)(CO)₃](M = Cr, Mo, or W) and are used to predict CH bond lengths, bond-dissociation energies, and mean HCH angles. The methyl groups are subject to appreciable internal rotation energy barriers and each contain two different types of CH bond, with non-equivalent orientations in the molecule and with significantly different bond lengths and strengths. The CH bond strengths decrease in the order Cr > Mo > W, reflecting a corresponding increase in metal–carbon bond strength estimated to be of the order of 55–70% from Cr to W. The methyl groups in [Cr(CHD₂)(η⁵-C₅H₅)(CO)₃] and [Cr(CD₃)(η⁵-C₅H₅)(CO)₃] are affected by extensive H/D exchange, probably with the 1,2-dimethoxyethane solvent used in the preparative reaction.