Consideration of spin states in determining the structure and decomposition of the transition metal pentazoles FeClN5, Fe(N5)2, Fe(H2O)4ClN5, and Fe(NH3)4ClN5Electronic supplementary information (ESI) available: energies for all structures with the various basis sets. See http://www.rsc.org/suppdata/cc/b3/b315812h/

Optimised geometries and energies have been calculated for the unidentate, bidentate, and ferrocene-like structures of Fe(II) pentazoles of the form MN(5) and M'(N(5))(2), (M = FeCl, M'= Fe) using the UB3LYP DFT method with basis sets up to 6-311+G(3df) in order to study the effect of an unfilled d shell on the stability of the singlet, triplet, and quintet states; the ground state is a quintet and the quintet has a lower barrier to decomposition than the singlet.