On the suggested correlation between the electronic absorption spectra and stereochemistry in [CuN5]2+ and [CuN6]2+ chromophores

Abstract

The diffuse reflectance spectra of a number of closely related [CuN₅]²⁺ and [CuN₆]²⁺ chromophores have been examined to test empirically recent statements by Hathaway and his co-workers that the electronic spectra could be used to assign structures to copper(II) compounds. The new compounds studies are of the type [Cu(tetramine)R]X₂, [Cu(diamine)₂R]X₂, and [Cu(tetramine)(diamine)]X₂(where tetramine = 1,6-bis-(2-pyridyl)-2, 5-diazahexane, 1,7-bis-(2-pyridyl)-2,6-diazaheptane, and 1,6-bis-(2-pyridyl)-3-methyl-2,6-diazahexane, or their 6-methyl-2-pyridyl analogues; diamine = 1,10-phenanthroline, 2,2′-bipyridyl or 2-imidazolyl-2-pyridyl; R = pyridine, γ-picoline, and imidazole; and X = ClO₄^–, BF₄^–, or PF-^–). Their diffuse reflectance spectra show that, whilst tendencies towards differences in the spectra of [CuN₅]²⁺ and [CuN₆]²⁺ chromophores can be observed, the overlap between these classes is so great that the spectra cannot be used in most cases to distingush unambiguously between five-co-ordinate and six-co-ordinate species, let alone different geometries within either class.