Reduction Pathway of End-On Terminally Coordinated Dinitrogen. IV. Geometric, Electronic, and Vibrational Structure of a W(IV) Dialkylhydrazido Complex and Its Two-Electron-Reduced Derivative Undergoing N−N Cleavage upon Protonation

The molybdenum and tungsten dialkylhydrazido complexes [M(dppe)2(NNC5H10)]2+ (M = Mo, W; compounds AMo
and AW) and their two-electron-reduced counterparts [M(dppe)2(NNC5H10)] (compounds BMo and BW) are characterized
structurally and spectroscopically. The crystal structure of BW indicates a geometry between square pyramidal and
trigonal bipyramidal with the NNC5H10 group in the apical position and in the trigonal plane of the complex, respectively.
Temperature-dependent 31P NMR spectra of BMo show that this geometry is present in solution as well. At room
temperature, rapid Berry pseudorotation between the “axial” and “equatorial” ligand positions gives rise to a singlet
in the 31P NMR spectrum. This exchange process is slowed at low temperature, leading to a doublet. The N−N
distance of BW is 1.388 Å, and the W−N distance is 1.781 Å. Infrared and Raman spectroscopy applied to AW, BW,
and their 15N isotopomers reveals extensive mixing between the N−N and W−N vibrations of the metal−N−N core
with the modes of the piperidine ring. The N−N force constant of AW is determined to be 6.95 mdyn/A, which is
close to the values of the Mo and W NNH2 complexes. In BW, the N−N force constant decreases to 6.4 mdyn/Å,
which is between the values found for the Mo/W NNH3 and NNH2 complexes. This allows us to attribute N−N
double bond character to AW and intermediate character between the double and single bonds for the N−N bond
of BW. These findings are supported by DFT calculations. More importantly, the HOMO of BW corresponds to a
linear combination of the metal dσ orbital with a ligand orbital having N−N σ* character, inducing a weakening of
the N−N bond. This contributes to the cleavage of the N−N bond taking place upon protonation of BW at the Nβ
atom of the NNC5H10 group.