Linear trimetallic complexes with 24 d electrons: Magnetic interactions in [Ni3]6+ and [Ni-Pd-Ni]6+ dipyridylamide chain complexes from density functional theory

Linear chains of cobalt and nickel atoms stabilized by polypyridylamide equatorial ligands and completed at both ends with σ-donor ligands in axial position define a class of molecules in which the atomic configuration of a metal atom may depend on its position in the chain. Metal atoms occupying a central or inner position are in a square-planar environment and tend to be low spin, whereas the atoms in contact with the axial ligands at both ends of the chain experience a square-pyramidal environment and tend to be high spin. The sequence of metal orbitals arising from a trinuclear chain M3(dpa)4Cl2 is reviewed and the population of these orbitals according to the nature of the transition metal M is sketched for M = Cr, Co, Ni and Cu. For M = Ni, the high spin state (S = 1) of the terminal atoms gives rise to an antiferromagnetic interaction involving four electrons, and couples these atoms via the central nickel (direct exchange) and via the dipyridylamide ligands (superexchange). The strength of this antiferromagnetic coupling, defined as the −2J factor of the Heisenberg Hamiltonian, can be calculated within Noodleman's broken-symmetry density functional theory formalism using the B3LYP functional. Replacing the central Ni(II) atom by an isoelectronic Pd(II) does not modify the antiferromagnetic nature of the ground state, but dramatically increases the −2J factor, suggesting that the properties of metal chains involving atoms of the second transition row could differ from their first row homologues. It is, however, not possible yet to decide which coupling, direct exchange, or superexchange, or both, is responsible for the surge of −2J.