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 MX₂ linear-bonded halide compounds has been studied, using the self-consistent local-density molecular-orbital approach. We present results for FeCl₂, FeBr₂, and EuCl₂ 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 ⁵⁷Fe quadrupole moment.