Iron bispentazole Fe(eta(5)-N-5)(2), a theoretically predicted high-energycompound: Structure, bonding analysis, metal-ligand bond strength and a comparison with the isoelectronic ferrocene

Quantum-chemical calculations with gradient-corrected (B3LYP) density funct ional theory have been carried out for iron bispentazole and ferrocene. The calculations predict that Fe(eta (5)- N-5)(2) is a strongly bonded complex which has DM symmetry. The theoretically predicted total bond energy that yields Fe in the D-5 ground state and two pentazole ligands is D-o = 109.0 kcal mol(-1), which is only 29 kcal mol(-1) less than the calculated bond e nergy of ferrocene (D-o = 138.0 kcal mol(-1) experimental: 158 +/- 2 kcal m ol(-1)). The compound Fe(eta (5)-N-5)(2) is 260.5 kcal mol(-1) higher in en ergy than the experimentally known isomer Fe(N-2)(5), but the bond energy o f the latter (D-o = 33.7 kcal mol(-1)) is much less. The energy decompositi on analyses of Fe(eta (5)-N-5)(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 (e(1g)) et a (5)-N-5(-) --> Fe2+ alpha -donation. The previously suggested MO correlat ion diagram for ferrocene is nicely recovered by the Kohn-Sham orbitals. Th e calculated vibrational frequencies and IR intensities are reported.