Vibrational spectroscopy and analysis of pseudo-tetrahedral complexes with metal imido bonds

A number of assignments have been previously posited for the metal-nitrogen stretch (ν(M-NR)), the N-R stretch (ν(MN-R)), and possible ligand deformation modes associated with terminally bound imides. Here we examine mononuclear iron(III) and cobalt(III) imido complexes of the monoanionic tridentate ligand [PhBP₃] ([PhBP₃] = [PhB(CH ₂PPh₂)₃]^-) to clarify the vibrational features for these trivalent metal imides. We report the structures of [PhBP₃]Fe≡N^tBu and [PhBP₃] Co≡N^tBu. Pseudo-tetrahedral metal imides of these types exhibit short bond lengths (ca. 1.65 Å) and nearly linear angles about the M-N-C linkages, indicative of multiple bond character. Furthermore, these compounds give rise to intense, low-energy visible absorptions. Both the position and the intensity of the optical bands in the [PhBP₃]M≡NR complexes depend on whether the substituent is an alkyl or aryl group. Excitation into the low-energy bands of [PhBP₃]Fe≡N^tBu gives rise to two Raman features at 1104 and 1233 cm^-1, both of which are sensitive to ¹⁵N and ²H labeling. The isotope labeling suggests the 1104 cm^-1 mode has the greatest Fe-N stretching character, while the 1233 cm^-1 mode is affected to a lesser extent by ¹⁵N substitution. The spectra of the deuterium-labeled imides further support this assertion. The data demonstrate that the observed peaks are not simple diatomic stretching modes but are extensively coupled to the vibrations of the ancillary organic group. Therefore, describing these complexes as simple diatomic or even triatomic oscillators is an oversimplification. Analogous studies of the corresponding cobalt(III) complex lead to a similar set of isotopically sensitive resonances at 1103 and 1238 cm^-1, corroborating the assignments made in the iron imides. Very minimal changes in the vibrational frequencies are observed upon replacement of cobalt(III) for iron(III), suggesting similar force constants for the two compounds. This is consistent with the previously proposed electronic structure model in which the added electron resides in a relatively nonbonding orbital. Replacement of the ^tBu group with a phenyl ring leads to a significantly more complicated resonance Raman spectrum, presumably due to coupling with the vibrations of the phenyl ring. Polarization studies demonstrate that the observed modes have A₁ symmetry. In this case, a clearer resonance enhancement of the signals is observed, supporting a charge transfer designation for the electronic transitions. A series of isotope-labeling experiments has been carried out, and the modes with the greatest metal-nitrogen stretching character have been assigned to peaks at ∼960 and ∼1300 cm^-1 in both the iron and cobalt [PhBP₃]M≡NPh complexes. These results are consistent with a multiple M-N bond for these metal imides.