Puzzling absence of hyperfine interaction in the D4h RhCl4-6 centre:: role of the 4d-5s hybridization in Rh2+ centres

The amazing absence of hyperfine interaction in EPR spectra of the tetragonally elongated RhCl64- centre in NaCl is explored through the present work. In 6 addition to the core polarization contribution it is shown that there is another isotropic contribution from the small 5s admixture in the a(1g)* (approximate to3z(2) - r(2)) level. These two contributions together with the anisotropic ones are shown to lead to a null hyperfine tensor for a 5s admixture of only similar to1.5%. Density functional calculations reveal that such an admixture is strongly sensitive to the tetragonality of the RhCl64- unit. Writing the axial (R,,) and equatorial (R-eq) Rh2+-Cl- distances as R-ax = R-0 + 2a and R-eq = R-0 - a it is shown that (i) the 5s admixture is proportional to a(2) (for a < 25 put), a fact which can be accounted for through a simple model; (ii) relative variations of R-eq and R-ax of only -0.4 and 0.8% lead to a hyperfine constant of 7 MHz. The present results are shown to shed some light on EPR data of the orthorhombic RhCl64- complex 6 with two cis-equatorial Na+ vacancies in the fourth shell. It is concluded that the hyperfine interaction measured for this centre can be explained in terms of the small strain induced by equatorial Na+ vacancies upon the RhCl64- unit. This idea can also explain the similar to2% decrement of the isotropic axial superhyperfine constant when compared with that for the tetragonal RhCl64- centre.