Abstract
Several new series of chromium(II) compounds have been prepared in anaerobic conditions, and investigated by various physical techniques. The double sulphates, M[2]SO[4].CrSO[4].xH[2]O (M = NH[4],Na,K,Rb,Cs; x varies from 2 to 6), except the caesium dihydrate, are high-spin. Since it has a low magnetic moment, the caesium dihydrate presumably has a sulphate-bridged structure analogous to that of chromium(II) acetate. The high-spin compounds have reflectance spectra indicative of tetragonal distortion. The reflectance spectra of the high-spin complexes of ethylenediamine, Cr(en)xX2(x = 2, X = Br, I,SO4; x = 3, X = Cl, Br, SO4) provide the first spectral evidence for tetragonal distortion in chromium(II) chelates. Infrared spectra have been used to distinguish between coordinated and ionic sulphate, and between cis and trans structures. The bis(diethylenetriamine) complexes, Cr(dien)2X2 (X = Cl, Br, I), closely resemble the tris(en) complexes. The low, temperature-dependent, moments of mono(dien) complexes Cr(dien)X2 (X = Cl, Br, I), have been ascribed to interaction between the chromium atoms in halide-bridged binuclear structures. The single, broad reflectance bands of the mono complexes near 690mu apparently arise from superposition of the three transitions expected for tetragonally-distorted structures. The complexes of 2-picolylamine: Cr(pic) xX2, (x = 1, X = Cl, Br; x = 2, X = Cl, Br, I), and of 8-aminoquinoline: Cr(8-AQ) xX2, (x = 1, X = Cl; x = 2, X = Cl, Br, I), have been prepared. The reflectance spectra of the high-spin, bis (pic) complexes resemble those of bis(en) complexes except that the main band has moved to shorter wavelength. The spectra of the bis(8-AQ) complexes are obscured by charge transfer bands. The mono complexes behave similarly to the mono(dien) complexes. Various complexes of 2,2'-bipyridyl and 1,10-phenanthroline have been studied. The bis compound, Cr(bipy)2I2, and the tris(phen) complexes are low-spin. Their magnetic moments are less than predicted by Kotani's theory, probably due to electron delocalisation and to departures from octahedral symmetry. Mono complexes are high-spin. The amines above contain -N-C-C-N- groups which form five-membered rings with the metal. Apparently conjugation in the chelate rings, and at least two such rings, are necessary to cause spin-pairing.