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) 4 Cl 2 compound. Both coinage metal compounds display three singly occupied x 2 -y 2 -like (δ) 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 M 3 (dpa) 4 Cl 2 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" 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.