Fig. 6: Electronic basis for the valence trapping in ildc, compared with the [Fe₄S₄]²⁺ cubane.

a, Qualitative molecular orbital diagram for the electronic structure of ildc calculated with DFT. The α spins are drawn in green and β spins in purple. Local minority spins pertinent to SDD are drawn in grey and the corresponding molecular orbital is circled in grey. The spin and valence topologies of the complex are schematically drawn in separate (symmetric) halves of the molecule. For completion, the former is inverted, while the latter is mirrored over the symmetry plane indicated as a grey line. The experimental ⁵⁷Fe Mössbauer isomer shift values associated with each Fe site are shown alongside, with the respective theoretical DFT-derived isomer shift value given in brackets. b, Calculated spin-density isosurface plot (isodensity cut-off 0.015) for ildc (left). c, Isosurface plot (isodensity cut-off 0.04) of one of the four equivalent orbitals relevant to the evaluation of SDD, which are highlighted in a grey circle in a. The corresponding atomic populations (φ) are φ(Fe1) = 16% and φ([Fe3,Fe4]) = 77%. d, Comparative qualitative molecular orbital diagrams for [Fe₄S₄(DmpS)₄]²⁻ and its associated measured (and calculated) ⁵⁷Fe Mössbauer isomer shifts determined at equivalent temperature (refer to Supplementary Fig. 103 for the spectra and simulations), and at the equivalent level of theory. The Fe₂S₂ subclusters across which SDD occurs are highlighted in grey. e,f, Comparative spin-density isosurface plot (e) and one of the two equivalent orbitals relevant to SDD for [Fe₄S₄(DmpS)₄]²⁻ (f). The corresponding atomic populations (φ) are φ(Fe1) = 47% and φ(Fe2) = 48%.