Iron Bispentazole Fe(η5-N5)2, a Theoretically Predicted High-Energy Compound: Structure, Bonding Analysis, Metal-Ligand Bond Strength and a Comparison with the Isoelectronic Ferrocene

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Abstract
Quantum‐chemical calculations with gradient‐corrected (B3LYP) density functional theory have been carried out for iron bispentazole and ferrocene. The calculations predict that Fe(η5‐ N5)2 is a strongly bonded complex which has D5d symmetry. The theoretically predicted total bond energy that yields Fe in the 5D ground state and two pentazole ligands is Do=109.0 kcal mol−1, which is only 29 kcal mol−1 less than the calculated bond energy of ferrocene (Do=138.0 kcal mol−1; experimental: 158±2 kcal mol−1). The compound Fe(η5‐N5)2 is 260.5 kcal mol−1 higher in energy than the experimentally known isomer Fe(N2)5, but the bond energy of the latter (Do=33.7 kcal mol−1) is much less. The energy decomposition analyses of Fe(η5‐N5)2 and ferrocene show that the two compounds have similar bonding situations. The metal–ligand bonds are roughly half ionic and half covalent. The covalent bonding comes mainly from (e1g) η5‐N5−→Fe2+ π‐donation. The previously suggested MO correlation diagram for ferrocene is nicely recovered by the Kohn–Sham orbitals. The calculated vibrational frequencies and IR intensities are reported.

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