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/Angstrom and metal-N force constants of 8.01 (Mo-N) and 7.31 mdyn/Angstrom (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 a bond) and metal-N bonding corresponds to a triple bond. The metal centers are assigned a +IV oxidation state (d(2) configuration) and the NNH3 ligand is assigned a-1 formal charge which by sigma-and pi-donation to the metal is reduced to +0.48. The two metal-d electrons are located in the nonbonding (n) d(xy) orbital. This bonding description is supported by the results of optical absorption spectroscopy showing the n-->(metal-ligand)pi* transition at 536 nm (not observed in the tungsten compound) and the (metal-ligand)pi--> (metal-ligand) pi* 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 N-2 reduction at six-coordinate d(6) metal centers in the absence of external reductants.