Towards a low-spin configuration in extended metal atom chains. Theoretical study of trimetallic systems with 22 metal electrons.

Different electronic configurations of a series of trinuclear heterometallic chains with 22 metallic electrons, MM'M(dpa)(4)X(2) (M = Co, Rh; M' = Ni, Pd; X = Cl, NCS), have been modelled in search of new systems with novel electrical properties. For this purpose, we explore the possibility of obtaining low-spin (extensively closed-shell) states by introducing chemical changes to the reference compound CoPdCo(dpa)(4)Cl(2) (1), isoelectronic to the herein studied systems, but possessing magnetically coupled localized electrons. The discussion is based on the orbital energies obtained by the DFT methodology. Among the systems herein analysed, CoNiCo(dpa)(4)(NCS)(2) has only two unpaired electrons vs. six in the case of 1, its closed-shell configuration appearing at high energies. For Rh(2)M-based chains, changes go a step further and the RhPdRh(dpa)(4)Cl(2) and RhPdRh(dpa)(4)(NCS)(2) molecules present a closed-shell ground state in close competition with the broken symmetry solution with S = ½ on each Rh(II). One-electron reduction of the latter compounds has been computed with marked structural changes. Our calculations show that the two lowest 23-electron states are separated by 7-8 kcal mol(-1) in favour of the state with an unpaired localized electron on the δ(Pd-N)* orbital instead of the delocalized one (σ(nb))(2)(σ*)(1).