2 NEW ANTIFERROMAGNETIC NICKEL(II) COMPLEXES BRIDGED BY AZIDO LIGANDSIN THE CIS POSITION - EFFECT OF THE COUNTERANION ON THE CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES

Two new nickel(II) end-to-end azido-bridged compounds, cis-catena-[NiL (2)(mu-N-3)](n)(ClO4)(n) . nH(2)O (1) and [Ni(2)L(4)(mu-N-3)(2)](PF6)( 2) (2), were synthesized and characterized; L is 2-(aminoethyl)pyridin e. The crystal structures of 1 and 2 were solved. Complex 1: monoclini c system, space group P2(1)/a, a = 8.637(2) Angstrom, b = 18.9995(7) A ngstrom, c = 12.3093(7) Angstrom, beta = 105.92(2)degrees, Z = 4. Comp lex 2: triclinic system, space group P (1) over bar, a = 9.139(7) Angs trom, b = 10.124(3) Angstrom, c = 12.024(2) Angstrom, alpha = 70.407(1 4)degrees, beta = 84.19(2)degrees, gamma = 67.67(4)degrees, Z = 1. In the two complexes the nickel atom is situated in a similarly distorted octahedral environment. The two complexes are different; 1 is a one-d imensional helicoidal complex with the two L ligands and the two end-t o-end azido bridges in a cis arrangement while complex 2 is a dinuclea r system with two end-to-end azido bridges, indicating the extreme imp ortance of the counteranion present (ClO4- for 1 and PF6- for 2). The magnetic properties of the two compounds were studied by susceptibilit y measurements vs temperature. The chi(M) vs T plot for 1 shows the sh ape for a weakly antiferromagnetically coupled nickel(II) one-dimensio nal complex without a maximum until 4 K. In contrast, for complex 2 th e shape of the chi(M) vs T curve shows a maximum near 40 K, indicating medium antiferromagnetic coupling. From the spin Hamiltonian -J(ij)S( i)S(j), J values for 1 and 2 were less than -1 and -29.1 cm(-1), respe ctively. The magnetic behavior for 1 and 2 may be explained in terms o f the overlap between magnetic orbitals, taking into account the torsi on of the Ni(II) atoms and azido-bridging ligands in the two structure s.