Mixed-ligand complexes, transition metal structural studies

Of the transition metals, the nickel triad has been the most studied in terms of xanthate structures, especially nickel xanthates. Thus, binary xanthate structures are known for all elements with no less than 15 binary nickel xanthate structures known and these aggregate in the solid state in six distinct supramolecular architectures. There is also a rich diversity of adducts as well as mixed-ligand complexes, in particular for nickel. 

Infrared and UV/vis data have been used by several authors to identify the C=C, C=0, and M—O stretches in the complexes synthesized 15, 18-21, 37, 38, 41, 44, 50, 54, 56, 58, 59, 64-66, 69, 74, 78, 80, 82, 103). Except in the initial research on first-row transition metal complexes of squaric acid, where these data were used in proposing structures, IR and UV/vis analysis have been used as supporting evidence for the particular coordination mode of the ligand 19,21,22, 45, 52, 59, 65). Infrared spectroscopy has also been utilized in the study of mixed oxalate/squarate complexes 118), although not to the same extent as in complexes of the oxalate ion. For example, Scott et al. studied the IR properties of Co(III) oxalate complexes with the hgand in a variety of chelating/bridging situations 119). 

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