High- and low-spin states

In Section 20.5, we stated that paramagnetism is a characteristic of some r/-block metal compounds. In Section 21.9 we consider magnetic properties in detail, but for now, let us simply state that magnetic data allow us to determine the number of unpaired electrons. In an isolated first row rf-block metal ion, the 3d orbitals are degenerate and the electrons occupy them according to Hund s rules e.g. diagram 21.1 shows the arrangement of six electrons. 

Although VB theory (see Sections 2.1, 2.2 and 5.2) in the form developed by Pauling in the 1930s is not much used now in discussing /-block metal complexes, the terminology 

However, magnetic data for a range of octahedral d complexes show that they fall into two categories paramagnetic 

2 Bonding in d-block metai compiexes valence bond theory 

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Photoionization can also access excited electronic states of the ion that are difficult to study by optical methods. The photoionization yield of FeO increases dramatically 0.36 eV above the ionzation energy. This result corresponds to the threshold for producing low spin quartet states of FeO. These states had not been previously observed, as transitions to them are spin forbidden and occur at inconveniently low energy. Because the FeO + CH4 reaction occurs via low spin intermediates, accurately predicting the energies of high and low spin states is critical. 

Removal of calcium from HRP C has a significant effect not only on enzyme activity and thermal stability, but also on the environment of the heme group. The calcium-depleted enzyme has optical, EPR, and H NMR spectra that are different from those of the native enzyme (211). Temperature dependence studies indicate that the heme iron exists as a thermal admixture of high- and low-spin states. Kinetic measurements at pH 7 show that ki, the rate constant for compound I formation, is only reduced marginally from 1.6 0.1 x 10 to 1.4 x lO M s , whereas k, the rate constant for compound II reduction, is reduced from 8.1 1.6 x 10 to 3.6 x lO M s (reducing substrate p-aminobenzoic acid), 44% of its initial value (211). There can be little doubt that this is the main reason for the loss of enzyme activity on calcium removal. 

B-and g-bands, 36 214-215 CT bands, 36 214-216 deprotonation of histidine groups bound to low-spin heme, 36 230-233 ground and excited states, 36 215-216 high-and low-spin states, 36 214-215... 

From these qualitative pictures, the spin states of the first transition series are predicted as in Figure 5. Mn(Por) and Co(Por) show an unusually strong tendency to take high and low spin states respectively, and do not follow the general trends. Exceptions also arise for strong n donors (oxo... 

The resonance positions in solutions of Mbm(N3) were found to be independent of the excess of azide ion added. This would appear to indicate that the equilibrium between high and low spin states is not directly related to the extent of the complex formation with azide ion. It appears rather that there might be a rapid equilibrium between Mbm(N3) (S = 1/2) and Mbm(N3) (S = 5/2). 

Selected from R. D. Shannon. Acta Crysiallogr. A32, 751—67 (1976). Notations HS and LS refer to high- and low-spin state, respectively. 

In this, my third contribution, I apply freeon dynamics to problems of interest in chemistry and physics and compare with the results obtained by the spin paradigm. In particular I will apply freeon dynamics to the following "spin phenomena" i) spin exchange, ii) spin superexchange, iii) spin polarization, iii) spin density, iv) high-and low-spin states of the transition-metal ions, v) the periodic table, vi) ferromagnetism, vii) spin waves and viii) high-Tc superconductivity. 

The lettering for these cytochromes, cc, arises as the haem in them is bound into the protein in the same way as it is in cytochrome c. However the absorption spectra are much more closely related to that of myoglobin, Table 6. In fact it was suggested that they were a mixture of fairly equal amounts of the high- and low-spin state when their spectra were first discussed (126). Subsequently Ehrenberg and Kamen (127) confirmed this description by measurements of the ESR spectra. It is... 

For transition metals that can have both high- and low-spin states, for example, Fe2+, the radius is larger for the high-spin ion. 

A clear case of cyanide linkage isomerism was established for the pentanuclear TBP complex [Fe tmphen)2]3[Cr (CN)g]2 (174a). Unlike the similar complexes [Fe°(tmphen)2l3 [M ° CN)g]2 (M = Co or Fe), which exhibit temperature induced transitions between the high- and low-spin state at the equatorial Fe(II)... 

A book including a survey of NMR studies of porphyrins and metalloporphyrins has appeared. (637) Iron porphyrins may involve Fe(ii) and Fe(iii) with both high and low spin states of iron. All are paramagnetic except those of low-spin Fe(ii). 

A simple ionic bonding model accounts for many properties of transition metal complexes, including variations in the hydration and lattice enthalpies and the ionic radii of the metal ions. The observation of high- and low-spin states for complexes of some metal ions can also be explained. 

The hexacoordinate complexes with adamantane, norcamphor and camphane exhibit different populations of high- and low-spin state (100%, 46%, 46%) [132]. Structure refinement gives some evidence for a variation of the Fe - O distance the largest value is found for the pure high-spin case (adamantane) [132]. 

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