Solution and solid state electronic spectra of technetium (vi) tetrahalonitrido complexes

Full text: A striking feature of the electronic spectra of the isoelectronic dl species [TcvINCl4]- and [MovOCl4]- in HCl solution and of [TcNBr4]- and [MoOBr4]- in HBr is the similarity in peak shape, width and intensity (and hence oscillator strength); the [TcNX4]- spectra are essentially red-shifted versions of the [MoOX4]- spectra. However, whilst the [MoOX4]- spectra in HX are similar to those in organic solvents, the spectrum of Nbu4[TcNCl4] in MeCN contains additional structure (bands 2 and 3) and differences in position and intensity (band 1) to the [TcNCl4]- in HCl spectrum (Type II). The main band (band 4) is little affected. Similar differences are observed in the [TcNBr4]- spectra The spectrum of Nbu4[TcNCl4] in dioxane was similar to the spectrum in 7.5 M HCl, but the spectra in anisole and bis(2-methoxyethyl)ether were the same as those in MeCN. In order to explain these differences it is noted that the [TcNCl4]- species in aqueous HCl solution may exist as the readily interconvertible five-coordinate [TcNCl4]-, and the six-coordinate [TcN(OH2)Cl4]- and [TcNCl5]2-. The solid state (Nujol mull) spectrum of Nbu4[TcNCl4] was found to be essentially the same as the spectrum in MeCN (Type I), and the spectrum of Net4[TcN(OH2)Cl4] to the Type II spectrum. These results indicate that the solution species is [TcNCl4]- in most organic solvents, [TcN(OH2)Cl4]- in aqueous HCl and [TcN(dioxane)Cl4] in dioxane. Progressive addition of 2-butanone to a dioxane solution of Nbu4[TcNCl4] in dioxane resulted in the conversion of the Type II spectrum to Type 1, and the reverse transformation occurred on the addition of dioxane to a 2-butanone solution