Electric field gradient and electronic structure of linear-bonded halide compounds

The importance of covalent metal-ligand interactions in determining hyperfine fields and energy-level structure of $M{X}_2$ linear-bonded halide compounds has been studied, using the self-consistent local-density molecular-orbital approach. We present results for Fe${\mathrm{Cl}}_2$, Fe${\mathrm{Br}}_2$, and Eu${\mathrm{Cl}}_2$ obtained using the discrete variational method with numerical basis sets. The high-spin configuration for the iron compounds, first predicted by Berkowitz et al., is verified; a successful comparison with gasphase photoelectron spectra is made. Variation of the predicted electric field gradient (EFG) with bond length $R$ is found to be rapid; the need for an extended x-ray-absorption fine structure (EX-AFS) measurement of $R$ for the matrix-isolated species and experimental determination of the sign of the EFG is seen to be crucial for more accurate determinations of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ quadrupole moment.