Reduction Pathway of End-On Coordinated Dinitrogen. 3. Electronic Structure and Spectroscopic Properties of Molybdenum/Tungsten Hydrazidium Complexes

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Abstract
The spectroscopic properties and electronic structure of the hydrazidium complexes [MF(NNH3)(depe)2](BF4)2, M = Mo and W, are investigated (depe = 1,2-bis(diethylphosphino)ethane). Vibrational spectroscopic data for both compounds are evaluated with a quantum-chemistry-assisted normal coordinate analysis, giving an N−N force constant of 6.03 mdyn/Å and metal−N force constants of 8.01 (Mo−N) and 7.31 mdyn/Å (W−N), respectively. On the basis of these results and DFT calculations on a [MoF(NNH3)(PH3)4]2+ model system, the N−N bond order in these systems is 1 (single σ bond) and metal−N bonding corresponds to a triple bond. The metal centers are assigned a +IV oxidation state (d2 configuration) and the NNH3 ligand is assigned a −1 formal charge which by σ- and π-donation to the metal is reduced to +0.48. The two metal-d electrons are located in the nonbonding (n) dxy orbital. This bonding description is supported by the results of optical absorption spectroscopy showing the n → (metal−ligand)π* transition at 536 nm (not observed in the tungsten compound) and the (metal−ligand)π → (metal−ligand)π* transition at 251 nm for the MoNNH3 and at 237 nm for the WNNH3 complex. The activation enthalpy for splitting of the N−N bond in these systems to generate NH4+ is estimated to be larger than 40 kcal/mol. Hydrazidium complexes with diphosphine coligands are therefore inert with respect to N−N cleavage and thus represent the ultimate stage of N2 reduction at six-coordinate d6 metal centers in the absence of external reductants.

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