The results of valence bond calculations for the ground-state of H2 with (a) 30 nuclear-centred 1s, 2s, 3s, 4s, 2p, 3p and 3d atomic orbitals and (b) 32 s and p atomic orbitals (16 nuclear-centred and 16 midbond) give energies of (a) −1.17245 a.u. and (b)

The biomimetic dioximatoiron complexes [Fe(Hdmed)]+ and [Fe(H2dmdt)]2+ act as catecholase models but the latter also catalyzes the intradiol cleavage of 3,5-ditert-butylcatechol. This remarkable selectivity was ascribed to the stable, hydrogen-bonded, macrocyclic coordination sphere of [Fe(Hdmed)]+, precluding bidentate catechol binding, as opposed to the less stable, mobile structure of [Fe(H2dmdt)]2+. This hypothesis is supported by the structures of [Fe(Hdmed)]+ and [Fe(H2dmdt)]2+ determined by X-ray diffraction in 1 M methanol solution. This study is a demonstrative example for the capabilities of the X-ray diffraction technique applied for complexes in the solution phase. The characteristic bond lengths of the complex and the number of solvent molecules in the solvation shell have been determined. To examine the structure of the complexes, gas phase density functional geometry optimizations were performed by using the adf-2004.01 and gaussian-98 packages. The PCM methodology was applied to estimate the effect of solvent on the relative energetics of the low-spin and high-spin electronic states of the complexes. The calculations confirmed that the electronic ground state corresponds to the triplet state for both complexes.