Effect of Cation on the Mössbauer Spectra of Tetrahaloferrate Anions

Abstract

The tetrahaloferrates are high-spin, tetrahedral Fe (III) complexes and as such, they should have spherical electric field symmetry. Thus expects to observe only a small (or zero) quadrupole splitting derived from lattice effects for these anions. However, if chemical effects, such as hydrogen bonding, are present to destroy or distort the tetrahedral symmetry one can expect to see larger quadrupole splittings due to the distortion of the electric field symmetry. This paper describes the Mössbaure spectra at liquid-nitrogen temperature for a series compounds of the form (R)_4−nNH_nFeC1₄, where R is a straight-chain alkane. In the methyl series where R is -CH₃, a single narrow line was observed for (CH₃)₄NFeCl₄ and (CH₃)NH₃FeCl₄. A quadrupole splitting of approximately 0.33 mm/sec was observed for (CH₃)₂NH₂-FeCl₄, and a single, broadened peak (Г = 0.54 mm/sec) was found for (CH₃)₃NHFeC1₄. For the corresponding butyl series, where R is -(CH₂)₃CH₃, a splitting of approximately 0.33 mm/sec was observed in all cases except tetrabutyl, [CH₃(CH₂)₃]₄NFeCl₄, where a narrow singlet was again obtained. To rule out the Possibility that the splittings were arising from lattice effects, the butyl compounds were dissolved in benzene and the spectra repeated at liquid-nitro temperatures. The Mössbauer spectrum of each compound in solution was identical with that obtained for the crystalline solid. Thus it appears that the quadrupole interactions are not of lattice origin but are caused by cation diatortion of the spherical symmetry. The most reasonable explanation appears to be hydrogen bonding, which exists both in the solid state and in solution. Correlations of the observed Mössbaure spectra with far-infrared data (where metal-halide stretching and bending vibration can be seen) are presented. Also other cations and their effects are discussed,as well as effects observed for other tetrahaloferrates such as FeBr₄ ^-.