Spin-1/2 metal

In this section, the acquisition of NMR spectra for spin-1/2 nuclei in diamagnetic compounds is discussed see Section 5 for a discussion of NMR spectroscopy on paramagnetic complexes. Obtaining NMR spectra for spin-1/2 isotopes in solid samples became practical with the development of two widely used techniques MAS and CP under the Hartmann-Hahn matching condition. These techniques have been discussed extensively. Below, the acquisition of NMR spectra for abundant-spin ligand nuclei ( H and F), as well as the direct observation of the NMR signals from spin-1/2 metal nuclei, are discussed. 

CADMIUM-113 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY IN BIOINORGANIC CHEMISTRY. A REPRESENTATIVE SPIN 1/2 METAL NUCLIDE... 

Identify high-spin metal complexes of carotenoids Establish the effect of distant high-spin metals on Tt-radicals properties... 

When an organic radical is located near a high-spin metal ion, the g-tensor of the radical depends on the exchange interaction between the radical and the metal ion. 

C- hyperfine satellites are detectable in natural abundance (Figure 3) and their intensities indicate a formulation Cr(C0)4 for the carrier of unpaired spin. Slight anisotropy in the 13C hyperfine structure of the 95 13C- enriched species could only be correctly reproduced in simulations under the assumption of tetrahedral geometry. The centre is thought to be Cr(C0)4+ with a 6At ground state in symmetry, a rare example of a high-spin metal carbonyl. 

Interestingly, this compound was known for some years [38, 39] before MCE research came back into vogue. Here, the maximum — ASM for a decoupled system is 42 J kg-1 K-1 and is almost met for AH = 0 - 7 T and 1.8 K. So, we have a reasonably high metal content, with a small, though ferromagnetic, interaction, with the appropriate high spin metals. Heat capacity data allow the adiabatic temperature change to be calculated here, this was found to be 12.7 K below 2 K, one of the best by this measure until recently. 

Fig. 8. Potential energy surfaces for ligand addition. Case (c) weakly repulsive potential energy surface for approach of the incoming ligand to the high-spin metal fragment.

In the case of octahedral geometry, the steric effect of the hydrogens at the 6-positions in bipy and the 2,9-positions in phen are less severe (see Figure 20), because the bulky groups are directed to the outside of the molecule. In fact the octahedral species such as [M(bipy)3] + are very stable and often kinetically inert (especially with d and d6 low-spin metal ions). With Fe2+, low-spin species are observed,127 and with the m-octahedral species [Fe(bipy)2(NCS)2], high-spin to low-spin transitions may be observed upon cooling.72... 

FIGURE 10.12 Typical potential energy surface topologies for spin crossover (a) and spin-forbidden addition of a ligand to a high-spin metal center (b). In many cases, AE is quite small.33... 

Both FeF and Ki[CoF,] contain six-coordinate high spin metal ions. The electronic spectrum of the former shows absorptions at 6990 and 10.660 cm , while the latter has absorptions at 10.200 and 14.500 cm. For which complex is A largest Why How many multiplicity allowed electronic transitions would you expect for these complexes How can you account for the pre.sence of two bands in each spectrum ... 

There is a problem in that the ground state of the metal atoms are all high-spin, except for Pd, which is (4d). The metal carbonyls are all low-spin with all electrons paired up in d orbitals, as far as possible. Fortunately, in most cases it is possible to correct I and A for this change in configuration, using spectroscopic data." Although DFT is not really valid for these excited states, we can assume that a low-spin metal atom, M, is a good model for a low-spin M(CO) i, which then reacts with CO to form M(CO) . 

Examples of other polynuclear clusters that have more metals, dissimilar metals and/or higher spin metals will be of greater complexity than the simple example above, as they will have a larger number of energy levels to consider and it may be more laborious to specify the correct Van Vleck coefficients for each. The resulting expressions may then involve multiple g values, many exponential terms in the denominator and numerator and appear superficially complex however, the principles for the derivation of such equations are the same as those presented here. Those interested in the methods required to treat the general case are referred to the literature. ... 

Certain generalizations may be made about the dependence of the magnitudes of A values on the valence and atomic number of the metal ion, the symmetry of the coordination shell and the nature of the ligands. For octahedral complexes containing high-spin metal ions, it may be inferred from the accumulated data for a large number of systems that ... 

For a half-filled system, the low-spin metallic band filling (125) is appropriate when U < W. The energies for the low-lying excited states of the metallic state (125) are well... 

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