Intermediate field complexes

If talking in terms of fractions of electrons seems strange, remember that these fractions refer to the probability distribution of the electron density. Physically, because electron repulsion is larger than the crystal field in weak field complexes, this repulsion forces some electron density into the orbitals. 

In the vast majority of transition metal complexes the energies associated with electron repulsion and the crystal field are of the same order of magnitude. This means that neither the weak field limit (electron repulsion crystal field) nor the strong field limit (crystal field electron repulsion) are met with in practice. There is no separate theory for the intermediate, real life, region. It is approached from either the strong or the weak field end. 

The ground and low-lying weak field terms of the d, d, d and d configurations are given in Fig. 7.24. Again, this figure summarizes the relevant discussion in the previous sections. In particular, the interaction between the and terms has been included and is responsible 

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The data also bear on the validity of the two models that have been proposed to describe the mechanism of ionic chain propagation in the gas phase. In review, Lampe, Franklin, and Field (23) have proposed that the polymerization proceeds through the reactions of long-lived, undissociated, intermediate reaction complexes,... 

In Chapter 6 we presented an expression for the transition probability (or intensity, amplitude) of field-swept spectra from randomly oriented simple 5=1/2 systems (Equation 6.4), and we could perhaps tacitly assume (as is generally done in the bioEPR literature) that the expression also holds for effective S = 1/2 systems, such as for the high-spin subspectra defined by the rhombograms discussed in Chapter 5. But what about parallel-mode spectra And how do we compute intensities in complex situations like for systems in the B S B B intermediate-field regime Clearly, we need a more generic approach towards intensity calculations. 

Fig. 57 Calculated energies (level-6) of the lowest multiplets relative to the ground state for high-spin/weak- and intermediate-field Mn(III) complexes F4(xy) and F4(z) ranging between 4000-14000 cur1...

The bimolecular racemization of ethylenedinitrilotetraacetatocobalt-ate(III) 29j 36) should be noted. This racemization suggests that bimolec-ularity should not be excluded in mechanistic considerations of octahedral complexes. Base hydrolysis studies of other complexes without ionizable protons would be of considerable value, provided they are of intermediate field strength. 

HS complexes are associated with the condition D0 < P and low spin complexes with D0 > P. For complexes in which the energy difference between A0 and P is relatively small, an intermediate field... 

This has restricted many of our studies of Pt(II) complexes to intermediate fields (e.g. 4 7 T). Pt(IV) complexes are less anisotropic and the effect is less marked It is probable that CSA relaxation is partly responsible for our failure to observe Pt signals directly from Pt bound to macromolecules. CSA relaxation of Pt can also lead to the disappearance of Pt satellites from C or spectra. These are normally used as indications of binding sites. Pt couplings appear in the spectra of coupled ligand nuclei as 1 4 1 multiplets only if relaxation times are the same in Pt(I 0) and Pt species. Indications that differences could exist were noted in NMR studies by Erickson et al ( ) on 1,2-diaminoethane complexes, and Lallemand et al ( ) for nucleoside complexes. We have recently shown ) for trans-Pt(ethene)(2-carboxy-pyridine)Cl2 that the broadening of satellites at high field arises from Pt relaxation via the CSA mechanism. The effect on the satellite linewidths is proportional... 

It is now recognized that yet another situation pertains to many compounds of the actinides. This may be termed the intermediate-field case, for which spin-orbit c crystal field electronic repulsion. Obviously this situation is more complex theoretically, in the sense that none of the usual perturbation-type calculations are applicable here. Since the advent of highspeed digital computers, however, this need not be a serious limitation. 

The stereochemical parameters of these three low-spin species are presented in Table II. It can be seen, as in the five-coordinate intermediate-spin complexes, that there is significant variation in the parameters. The general decreases in Fe-Np and displacement follow the binding and ligand field strength of the axial ligand. A particularly noteworthy feature is the variation in the out-of-plane displacement of the low-spin iron(III) atom. 

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