Structure, Energetics, and Bonding of First Row Transition Metal Pentazolato Complexes:  A DFT Study

Quantum chemical calculations with gradient-corrected (B3LYP) density functional theory for the mono- and bispentazolato complexes of the first row transition metals (V, Cr, Mn, Fe, Co, and Ni), the all-nitrogen counterparts of metallocenes, were performed, and their stability was investigated. All possible bonding modes (e.g. η¹, η², η³, and η⁵) of the pentazolato ligand to the transition metals have been examined. The transition metal pentazolato complexes are predicted to be strongly bound molecules. The computed total bond dissociation enthalpies that yield free transition metal atoms in their ground states and the free pentazolato ligands were found in the range of 122.0−201.9 (3.7−102.3) kcal mol^-1 for the bispentazolato (monopentazolato) complexes, while those yielding M²⁺ and anionic pentazolato ligands were found in the range of 473.2−516.7 (273.6−353.5) kcal mol^-1. The electronic ground states of azametallocenes along with their spectroscopic properties (IR, NMR, and UV−vis) obtained in a consistent manner across the first transition metal series provide means for discussion of their electronic and bonding properties, the identification of the respective azametallocenes, and future laboratory studies. Finally, exploring synthetic routes to azametallocenes it was found that a [2 + 3] cycloaddition of dinitrogen to a coordinated azide ligand with nickel(II) does not seem to provide a promising synthetic route for transition metal pentazolato complexes while the oxidative addition of phenylpentazole and fluoropentazole to Ni(0) bisphosphane complexes merits attention for the experimentalists.