Structure and magnetism of [M3]6/7+ metal chain complexes from density functional theory:: Analysis for copper and predictions for silver

The ground-state electronic structure of the trinuclear complex Cu-3(dpa)(4)Cl-2 (1), where dpa is the anion of di(2-pyridyl) amine, has been investigated within the framework of density functional theory (DFT) and compared with that obtained for other known M-3(dpa)(4)Cl-2 complexes (M = Cr, Co, Ni) and for the still hypothetical Ag-3( dpa) 4Cl2 compound. Both coinage metal compounds display three singly occupied x(2)-y(2)-like (delta) orbitals oriented toward the nitrogen environment of each metal atom, generating antibonding M-(N-4) interactions. All other metal orbital combinations are doubly occupied, resulting in no delocalized metal-metal bonding. This is at variance with the other known symmetric M3( dpa) 4Cl2 complexes of the first transition series, which all display some delocalized bonding through the metal backbone, with formal bond multiplicity decreasing in the order Cr > Co > Ni. An antiferromagnetic coupling develops between the singly occupied MOs via a superexchange mechanism involving the bridging dpa ligands. This magnetic interaction can be considered as an extension to the three aligned Cu-II atoms of the well-documented exchange coupling observed in carboxylato-bridged dinuclear copper compounds. Broken-symmetry calculations with approximate spin projection adequately reproduce the coupling constant observed for 1. Oxidation of 1 removes an electron from the magnetic orbital located on the central Cu atom and its ligand environment; 1(+) displays a much weaker antiferromagnetic interaction coupling the terminal Cu-N-4 moieties via four ligand pathways converging through the x(2)-y(2) orbital of the central metal. The silver homologues of 1 and 1(+) display similar electronic ground states, but the calculated magnetic couplings are stronger by factors of about 3 and 4, respectively, resulting from a better overlap between the metal centers and their equatorial ligand environment within the magnetic orbitals.