Isotropic Non-Heisenberg Behavior in M3(dpa)4Cl2 Extended Metal Atom Chains

Isotropic deviations to the standard Heisenberg Hamiltonian have been extracted for a series of trinuclear extended metal atom chain complexes, namely, [Ni-3(dpa)(4)Cl-2], and the hypothetical [NiPdNi(dpa)(4)Cl-2] and [Pd-3(dpa)(4)Cl-2], following a scheme recently proposed by Labeguerie and co-workers (J. Chem. Phys 2008, 129, 154110) within the density functional theory framework. Energy calculations of broken symmetry monodeterminantal solutions of intermediate M-s,M-tot values can provide an estimate of the magnitude of the biquadratic exchange interaction (lambda) that accounts for these deviations in systems with S = 1 magnetic sites. With the B3LYP functional, we obtain lambda = 1.37, 13.8, and 498 cm(-1) for the three molecules, respectively, meaning that a simple Heisenberg Hamiltonian is enough for describing the magnetic behavior of the Ni3 complex but definitely not for Pd-3. In the latter case, the origin of such extreme deviation arises from (i) an energetically affordable local non-Hund state (small intrasite exchange integral, K similar to 1960 cm(-1)) and (ii) a very effective overlap between Pd-4d orbitals and a large J. Furthermore, this procedure enables us to determine the relative weights of the two types of magnetic interactions, sigma- and delta-like, that contribute to the total magnetic exchange (J = J(sigma) + J(delta)). In all of the systems, J is governed by the a interaction by 95-98%.