DFT modelling of the [M–Pd–M]6+ metal atom chains (M = Ni, Pd): Structural, electronic and magnetic issues

Following the recent findings on heterometallic string complexes, we extend the recently published work on NiPdNi(dpa)4Cl2 to discuss the Ni–Pd–Ni and Pd3 chains with equatorial ligands (L) being dipyridylamide (dpa), 2,6-bis(phenylamido)pyridine (BPAP) and N,N′-bis-(p-toluenesulfonyl)-pyridyldiamide (Lpts), using the DFT formalism. The analysis of such a hypothetical series of linear trimetallics anticipates that, for NiPdNi(dpa)4Cl2, the extended valence shell of palladium strengthens the antiferromagnetic coupling between high-spin terminal nickel atoms. For L = BPAP the system, as expected, becomes diamagnetic, and antiferromagnetism reappears for L = Lpts. The theoretical modelling of the coupling following the Heisenberg Hamiltonian applied to two magnetic centres (H^=-2JABS^A·S^B) gives −2J = 320 and 497 cm−1 for L = dpa and Lpts, respectively. Pd3 chains display an enhanced tendency to be diamagnetic with various ligands. More specifically with dpa, Pd3(dpa)4Cl2—should it be synthesized—could be magnetically inactive since the strongly antiferromagnetic state generated by the coupling of two terminal, high-spin Pd atoms (−2J = 1393 cm−1) is computed to be in close competition with the diamagnetic, closed-shell state. For L = Lpts as for BPAP, the hypothetic [Pd3]6+ chain is predicted to be diamagnetic, resulting from the high energy of the antibonding d(Pd)–p(N) orbital. The shift toward diamagnetism induced by the replacement of Ni by Pd in terminal position is therefore assigned to a stronger N → M donation interaction with M = Pd.