Ruthenium spin-orbit coupling

Foyt et al. [137] interpreted the quadrupole-splitting parameters of low-spin ruthenium(II) complexes in terms of a crystal field model in the strong-field approximation with the configuration treated as an equivalent one-electron problem. They have shown that, starting from pure octahedral symmetry with zero quadrupole splitting, A q increases as the ratio of the axial distortion to the spin-orbit coupling increases. 

The emission lifetimes of the bipy and phen complexes of ruthenlum(II) at 77°K are generally in the range t = 0.5-10 ps. (Table 7). Since these values are intermediate to those generally observed for the fluorescence and phosphorescence of organic compounds, the radiative transition in the ruthenium complexes was suggested to be a heavy-atom perturbed spin-forbidden process (168,169). From a determination of the absolute quantum yields as well as lifetimes of a series of ruthenium(II) and osmium(II) complexes, the associated radiative lifetimes were calculated (170). The variations in these inherent lifetimes within the series could be rationalized with a semi-emipirical spin-orbit coupling model thus affording further evidence that the radiative transitions are formally spin forbidden in these systems. 

The emissions from the osmium(II) complexes of bipy, and so on, have in general received assignments analogous to those of ruthenium(Il), that is, to formally spin-forbidden charge transfer (ir d) transitions (79,163,170). A notable difference in the behavior of the two sets of compounds is that the lifetimes of the osmiura(II) emitters are generally shorter than those of ruthenium(II) (Table 7). This difference is due to the greater affect of spin-orbital coupling on the radiative decay rates of the excited state osmium(II) species (170). 

Referring to the short Ru-Ru distance, Gibb et al. considered the problem of the electronic structure from a molecular orbital viewpoint [58]. Such a treatment is a poor approximation in this case since it cannot explain the low temperature magnetic behavior, which mainly results from spin-orbit coupling effects. These ones prevail here as seen in other ruthenium(IV) compounds [57]. 

Figure 2. Manifold of d7r emitting levels characteristic of complexes of ruthenium(II) ana osmium-ill). Splitting is calculated in terms of a parameter related to spin-orbit coupling (ki) and exchange integrals K(flj,aJ and K(e+,aJ.

This is the typical behavior of many organic chromophores such as, for example, aromatic hydrocarbons. By contrast, the behavior of the ruthenium porphyrin is similar to that of typical inorganic chromophores, being dominated by the strong spin-orbit coupling provided by the heavy ruthenium center. Thus, ultrafast intersystem crossing prevents any measurable... 

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