Evidence for increased exchange interactions with 5d compared to 4d metal ions.: Experimental and theoretical insights into the ferromagnetic interactions of a series of trinuclear [{M(CN)8}3-/NiII] compounds (M = MoV or WV)

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 {Mo-V(CN)(8)-Ni-II} and {W-V(CN)(8)Ni-II} 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 (J(MoNi)) 26.9 cm(-1) and J(WNi) = 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.