MAGNETIC STUDIES ON MU-AZIDO POLYNUCLEAR NICKEL(II) COMPOUNDS WITH THE 222-TET LIGAND - CRYSTAL-STRUCTURE OF 22-TET)](2)(BPH4)(2)(222-TET=TRIETHYLENETETRAMINE) AND EXAFS STRUCTURAL CHARACTERIZATION OF THE TRIANGULAR COMPOUNDS (MU-N-3)(3)[NI(222-TET)](3)(X)(3) (X=PF6(-), CLO4(-))

The nickel(II) dinuclear 1,3-azido-bridged compound (mu-N-3)(2)[Ni(222 -tet)](2)(BPh4)(2) (1) and the trinuclear compounds (mu-N-3)(3)[Ni-3(2 22-tet)(3)](PF6)(3) (2) and (mu-N-3)(3)[Ni-3(222-tet)(3)](ClO4)(3) (3) were synthesized and characterized. 222-tet is the tetraaminate ligan d triethylenetetramine. The crystal structure of 1 was solved by X-ray diffraction. 1 crystallizes in the monoclinic system: space group P2( 1)/n, a = 10.639(4) Angstrom, b = 19.770(7) Angstrom, 13.609(6) Angstr om, beta = 97.78(3)degrees, Z = 2, formula C60H76B2N14Ni2. In the abse nce of single crystals of 2 and 3, we carried out an EXAFS study of 1- 3 at the nickel K-edge, using compound 1 as a model, in order to obtai n structural information for compounds 2 and 3. The analysis of XANES and EXAFS spectra of compounds 1-3 reveals the occurrence of azido-bri dged trinuclear nickel(II) compounds for 2 and 3 with Ni-Ni separation s of 5.16 and 5.12 Angstrom, respectively. Each nickel(II) atom is pla ced in an octahedral NiN6 environment: four nitrogen atoms of the amin e and two nitrogen atoms of two azido bridges. The magnetic properties of the three compounds were studied by susceptibility measurements at variable temperatures (300-4 K). From the spin Hamiltonian H = -JS(1) S(2), the calculated J value for 1 is -83.6 cm(-1), in good agreement with the expected value. From the spin Hamiltonian H = -J(1,2)(S1S2 S1S3) -J(3)(S2S3), the obtained J values are J(1,2) = -72(3) cm(-1), J (3) = -36(3) cm(-1)for 2 and J(1,2) = -60.3(3) cm(-1), J(3) = -29.4(2) cm(-1) for 3.