No abstract

Density functional calculations for full structures of dimers of Cu(II) complexes linked via O-H...O hydrogen bonds provide exchange-coupling constants that are in excellent agreement with experimentally reported values. Magneto-structural correlations between the exchange-coupling constant and the O...O distance or the separation between the coordination planes of the two monomers are analyzed. The calculations support the orbital models usually employed in qualitative interpretations of magneto-structural correlations, showing excellent correlations between the calculated coupling constants and the square of the orbital gap or of the overlap between the two magnetic orbitals. The orbital gap responsible for the antiferromagnetic coupling is seen to result from direct through-space overlap between the oxygen atoms of the two monomers, whereas the hydrogen bonds play an essentially structural role by holding these oxygen atoms in close proximity.

Two main questions are addressed in this study: (i) What increase of exchange interaction can be expected when replacing a paramagnetic metal ion with a heavier congener located farther down the periodic table (i.e., 3d-4d-5d), and (ii) for a molecular unit with higher coordination numbers, eight in the present case, how is the magnetic information transferred from the metal ion to its ligand set? Qualitative and quantitative investigations on a series of trimetallic cyano-bridged {MoV(CN)8-NiII} and {WV(CN)8-NiII} compounds revealed ferromagnetic interactions but with a strength modulated by the spin organization and their nature. DFT calculations have been used to examine the mechanism and strengths of the exchange coupling, as well as the influence of the local symmetry of the cyanometalate unit on the spin density distribution. Both the experimental and the calculated behaviors underline a noticeable difference between the Mo and the W derivatives (JMoNi = 26.9 cm(-1) and JWNi = 37.3 cm(-1)) that is correlated to the spin density transferred from the metal center to its ligand set. It is also shown that the shape of the {M(CN)8} polyhedron may lead to nonequivalent CN sites and, consequently, to different strengths of the exchange interaction as a result of the position of the bridging ligands.