Macrocyclic selenoether complexes of nickel(II).: Synthesis and properties of [NiX2([16]aneSe4)] ([16]aneSe4 = 1,5,9,13-tetra-selenacyclohexadecane, X = Cl, Br or I) and [NiX2(MeSeCH2CH2-SeMe)2]

Reaction of NiX2 (X = Cl, Br or I) with [16]aneSe(4) (1,5,9,13-tetraselenacyclohexadecane) under anhydrous conditions in n-butanol yielded trans-[NiX2([16]aneSe(4))]. Similar reactions using MeSeCH2CH2SeMe produced trans-[NiX2(MeSeCH2CH2SeMe)(2)]. The complexes have been characterised by IR and UV/VIS spectroscopy and magnetic measurements. The crystal structure of [NiCl2(MeSeCH2CH2SeMe)(2)] showed an octahedral nickel centre co-ordinated to trans chlorines [2.370(2)-2.376(2) Angstrom] with the diselenoethers present as chelating ligands in the DL conformation [2.522(1)-2.5623(8) Angstrom]. Structural data on the very poorly soluble [16]aneSe(4), complexes were obtained via nickel K-edge EXAFS (extended X-ray absorption fine structure), and the results compared with related complexes. A ligand field analysis of the UV/VIS spectra of [NiX2([16]aneSe(4))] and of the tetrathioether analogues [NiX2([16]aneS(4))] ([16]aneS(4) = 1,5,9,13-tetrathiacyclohexadecane) showed that the ligand fields produced by 16-membered ring macrocycles [Dq(xy)] lie in the order N-4 > S-4 > Se-4, with MeSeCH2CH2SeMe affording a stronger field that [16]aneSe(4) due to the smaller chelate ring size.