Possible ligand field effects in metal-oxygen vibrations of some first-row transition metal alkoxides

Abstract

Although metal alkoxides, particularly alkyl titanates and their hydrolysis products, have been the subject of several infrared studies,l-5 little is known of the vibrational spectra of transition metal alkoxides in the far infrared region.Kriegsmann and Lichtl have assigned Ti-0 vibrational modes in titanium(1v) alkoxides in the range 400-650 cm-1 and similar assignments have been made by Barraclough et a1.4 for Ti(OEt)4.We report here far infrared spectra of the methoxides of chromium(~~), manganese(~~), iron(11), cobalt(11), nicke1(11), copper(11), and zinc(11) within the region 600-200 cm-1 (Table 1).Two bands, separated by 40-140 cm-1, are found for all the compounds except C~(O;l\le)~, where a third band is observed.For Pb(OMe)z6 and some alkyl tin methoxides, RzSn(OMe)2,7 the two metal-oxygen vibrations have been assigned to asymmetric and symmetric stretching modes, v,,(M-0) and vS(M-0).The pairs of bands listed in Table 1 can likewise be assigned to asymmetric and symmetric stretching modes although they may equally well arise from distortions of the MOs octahedra which produce more than one M-0 bond distance.The two (Cu-X) bands in copper(11) halide pyridinates are believed to be due to distortions of this kind.8Whichever interpretation is correct, it is likely that within the present Methoxide Cr(OM.e)zMn(0Me)z Fe(0Me)z Co(0Me)z