Analysis of the magnetic coupling in M3(dpa)4Cl2 systems (M = Ni, Pd, Cu, Ag) by ab initio calculations.

The magnetic exchange parameter (J) of a series of neutral and oxidized trinuclear extended metal atom chain complexes [M(3)(dpa)(4)Cl(2)](0/1+), where dpa is the anion of di(2-pyridyl)amine, has been extracted employing the complete active space second-order perturbation theory (CASPT2). The computed magnetic coupling constant for Ni(3)(dpa)(4)Cl(2) (-98 cm(-1)) and the mono-oxidized Cu(3) complex (-35 cm(-1)) are in excellent agreement with the experimental estimates. This consistence is though not achieved for Cu(3)(dpa)(4)Cl(2), with three S = 1/2 magnetic centers, for which we surprisingly obtain 1/3 of the experimental value only. This worrying result and the possible sources for such large discrepancy are analyzed. The magnetic coupling has been also analyzed for the hypothetical complexes with Pd(3) and Ag(3) metal chains. Much larger couplings are obtained, which is ascribed to the larger spatial extent of the magnetic orbitals. Furthermore, we analyze the relative importance of the magnetic coupling paths through the sigma-system (through metal coupling) and the delta-system (through ligand coupling).