Metal–Nitrogen Infrared Stretching Frequencies in Dialkylamido-transition Metal Compounds

Abstract

THE infrared and Raman spectral analysis of Ti(NMe₂)₄ reported by Bürger et al.¹ prompts us to disclose some of our infrared data on a number of dialkylamido derivatives of transition metals². Of particular interest are the stretching frequencies of the metal–nitrogen bonds. All the compounds examined had a characteristic strong band in the 590–700 cm⁻¹ region (Table 1) which was absent in the parent amine and which we believe is predominantly metal–nitrogen stretching in character. For the tetrakisdialkylamido–metal (IV) compounds, assuming local T_d symmetry of the MN₄ system, an infrared active band (ν₃) should occur involving the asymmetric metal–nitrogen stretching vibration and it is suggested that this corresponds to the “characteristic band” shown in Table 1 although it is recognized that coupling with other modes of vibration may be involved. The work of Bürger et al.¹ confirms our assignment in the case of Ti(NMe₂)₄. Considering the titanium, zirconium, hafnium compounds M(NR₂)₄, it is interesting to note the definite increase in frequency with increase in chain length of the alkyl group R. This is interpreted as indicative of a significant degree of ligand→metal π-electron donation involving the lone-pair of electrons on nitrogen and the vacant d-orbitals on the metal. Other factors such as the mass-effect of the ligand or its steric effect would lead to a decrease in frequency with increase in size of the alkyl groups contrary to the observed results.