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(CH2PPh2)3]-) to clarify the vibrational features for these trivalent metal imides. We report the structures of
[PhBP3]Fe⋮NtBu and [PhBP3]Co⋮NtBu. 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⋮NtBu 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.