Spin-forbidden bands

The second example in Fig. 4-4 shows how a (spin-allowed or spin-forbidden) band lying close to a charge transfer band may acquire unusually high intensity. We shall discuss charge-transfer bands more in Chapter 6. For the moment, we note that they involve transitions between metal d orbitals and ligands, are often fully allowed and hence intense. On occasion, the symmetry of a charge transfer state... 

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

Cerny, and Maxova (44, 45) have adopted larger values for C/B, Of these latter the ferrocene assignment (32) is dubious since subsequent reexamination has failed to establish the existence of two of the three spin-forbidden bands there claimed, whilst valid alternative assignments of the spin-forbidden transitions of Ni(Cp)2 and V(Cp)2, on which the higher C/B values were based (44, 45) are readily made (vide infra). 

The assignment of the shoulder at 25 kK. is though rather problematical no other spin-allowed d-d band is expected in this region, and since the crude extinction coefficient no doubt exaggerates its true intensity, it may tentatively be assigned as a spin-forbidden band, the most likely candidate being the 4E (a 52) -> 2A(a2 5) transition. This in turn leads to a B value of about 650 cm 1, with 0 not unreasonably at about 0.70. 

For AgFl the intensities of the spin-forbidden bands, relative to those of the allowed transitions, are actually smaller than for the analogous 3d complex, CuFi This however is because the larger Dq of the Ad series, and the smaller B value, results in a greater separation between... 

For ruthenium and the following elements of the 4d series the free-ion f values become in excess of 1000 cm-1, and apart from the exceptions mentioned previously, the relative intensities of the spin-forbidden bands are found to be significantly larger than for the 3d elements. For the intra-subshell (t g) transitions 3T g XT%g and 3T g 1Eg some 2%... 

Once again two spectroscopic studies have been made — due to Brown et al. (32) and to Allen et al. (12) respectively — and the same comments apply as for the two investigations of the RuFspin-allowed transitions at 19—21 kK. and at 26.0 kK., but no other absorptions were found below 40 kK. On the other hand the spectrum of Allen et al., although broadly mirroring these findings, revealed extra absorptions at 32.8,39.6, and 44.6 kK., together with a strong indication of a band below 4 kK. (Fig. 2). 

In some papers, only the spin-allowed bands have been used in the analysis for d3 and d8 systems, this obviates the need to consider the Racah parameter C. Where the spin-forbidden bands have been included, C has sometimes been allowed to find a value which best fits the experimental data, along with the other parameters others have assumed a fixed value of the ratio B/C, such as 0.2S. The treatment of interelectron repulsion introduces some uncertainty into the orbital splitting parameters. Although it is well-known that the d-d spectra of Oh chromophores cannot be perfectly fitted within a model which allows only one value of B (1-3,... 

Spin selection rule The spin selection rule, AS = 0, specifies that there should be no change in the spin multiplicity. Weak spin-forbidden bands may occur when spin-orbit coupling is possible. Spin-forbidden transitions are more intense in complexes of heavy atoms as these lead to a larger spin-orbit coupling. 

Assignments of spin-forbidden bands in the spectra of [Cr(S2CNR2)3] and [Cr(S2COR)3] (R = Me or Et) have been made and the emission spectra studied.1018... 

The coupling mechanism given above for mixing excited u terms into g electronic terms through vibrational modulation of the ligand field is likely to be less efficient if terms of different spin are involved. In accordance with this, it is observed that spin-forbidden bands are a good deal narrower than are the spin-allowed ones dealt with above, corresponding to reduced overlap of available vibrational structure. Half-widths of a few hundred to one thousand cm-1 seem to be involved. 

The interpretation of the spectrum of PtCIi8- is complicated by the presence erf one or more spin-forbidden bands. However, PtCU4-" seems to exhibit the three spin-allowed d-d bands, and one charge transfer band. The spectra of the AuCh" and AnBr complexes clearly show the two charge transfer bands.10... 

---