title: Nature of magnetic interactions in 3D {[M(II)(pyrazole)4]2[Nb(IV)(CN)8].4H2O}n (M = Mn, Fe, Co, Ni) molecular magnets.
abstract: The self-assembly of [Nb(IV)(CN)(8)](4-) with different 3d metal centers in an aqueous solution and an excess of pyrazole resulted in the formation of four 3D isostructural compounds {[M(II)(pyrazole)(4)](2)[Nb(IV)(CN)(8)].4H(2)O}(n), where M(II) = Mn, Fe, Co, and Ni for 1-4, respectively. All four assemblies crystallize in the same I4(1)/a space group and show identical cyanido-bridged structures decorated with pyrazole molecules coordinated to M(II) centers. All four compounds show also long-range magnetic ordering below 24, 8, 6, and 13 K, respectively. A thorough analysis of the structural and magnetic data utilizing the molecular field model has allowed for an estimation of the values of coupling constants J(M-Nb) attributed to the one type of M(II)-NC-Nb(IV) linkage existing in 1-4. The J(M-Nb) values increase monotonically from -6.8 for 1 through -3.1 for 2 and +3.5 for 3, to +8.1 cm(-1) for 4 and are strongly correlated with the number of unpaired electrons on the M(II) metal center. Average orbital contributions to the total exchange coupling constants J(M-Nb) have also been identified and calculated: antiferromagnetic J(AF) = -21.6 cm(-1) originating from the d(xy), d(xz), and d(yz) orbitals of M(II) and ferromagnetic J(F) = +15.4 cm(-1) originating from d(z(2)) and d(x(2)-y(2)) orbitals of M(II). Antiferromagnetic interaction is successively weakened in the 1-4 row with each additional electron on the t(2g) level, which results in a change of the sign of J(M-Nb) and the nature of long-range magnetic ordering from ferrimagnetic in 1 and 2 to ferromagnetic in 3 and 4.
authors.affiliation: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland. pinkowic@chemia.uj.edu.pl
authors.forename: Dawid
authors.initials: D
authors.lastname: Pinkowicz
authors.forename: Robert
authors.initials: R
authors.lastname: Pełka
authors.forename: Olga
authors.initials: O
authors.lastname: Drath
authors.forename: Wojciech
authors.initials: W
authors.lastname: Nitek
authors.forename: Maria
authors.initials: M
authors.lastname: Bałanda
authors.forename: Anna Małgorzata
authors.initials: AM
authors.lastname: Majcher
authors.forename: Giordano
authors.initials: G
authors.lastname: Poneti
authors.forename: Barbara
authors.initials: B
authors.lastname: Sieklucka