Metal chemical shift

The computational prediction of not the ligand but the metal, particularly transition-metal chemical shifts poses an even more severe challenge to any method. Electron corre-... 

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

Transition metal NMR spectroscopy is not only a valuable source of structural information, but metal chemical shifts may also permit predictions of the reactivity and possibly even catalytic activity of a complex.51 The key to such analyses is frequently the observation of correlations of metal chemical shifts... 

Analysis of trends in transition metal chemical shifts was in most cases attempted by exploring statistical correlations with other observable spectroscopic quantities such as ligand atom chemical shifts or metal-ligand coupling constants... 

Even though the outlined approach allowed the successful rationalisation of many experimentally observed shift/structure and shift/reactivity correlations, Leitner et al. have pointed out that such relations cannot be expected to be universally valid and require that structural variations are modest and avoid large simultaneous changes in parameters that may have opposite effects on metal chemical shifts.61 To overcome these drawbacks and establish a more rational interpretation of chemical shift trends, they used a combination of experimental and computational efforts to assess the importance of different electronic and structural factors on the metal chemical shifts of a series of rhodium complexes with bidentate chelating bisphosphine ligands. The basis of their approach is first the validation of experimentally observed metal shifts by... 

With the advent of appropriate DFT-based methods, NMR properties of transition-metal compounds have now become amenable to theoretical computations (8). Suitable density functionals have been identified (9) which permit calculations of transition-metal chemical shifts with reasonable accuracy, typically within a few percent of the respective shift ranges. Thus, it is now possible to investigate possible NMR/reactivity correlations for transition-metal complexes from first principles several such studies have already been undertaken (10,11,12). 

Nakatsuji, H., K. Kanda, K. Endo, and T. Yonezawa (1984). Theoretical study of the metal chemical shift in nuclear magnetic resonance Ag, Cd, Cu and Zn complexes. J. Amer. Chem. Soc. 106, 4653-60. 

The U-shaped dependence of the metal chemical shifts on the number x of electronegative ligands is well known for Si and Sn chemical shifts in series like SiMe4 t,CU and Sn. le, CU- Therefore, a similar U-shaped relationship for the GeMe4 t,CU series is expected. In Fig. 10, the theoretical results for the Ge. le, CI, and GeMe4 t H c (x = 0-4) series are plotted against x. 

Indirect observation of transition metal chemical shifts via C was reported for the determination of 5( Fe) in a variety of ferrocenes and ferrocenyl carbenium ions, as well as in a natural myoglobin carbonyl complex and two synthetic model compounds. All measurements were made by selective double-resonance methods using either [ Fe]-labelled or... 

PZC was found to be linearly correlated with DO + DM, where DO and DM are oxygen and metal chemical shifts derived from XPS specira of metal oxides [825,3097]. The following correlation was found in [3071] ... 

Although this theory predicts the temperature dependence of the metal chemical shifts, it also predicts, for example, that an isotope shift should be independent of the remoteness of substitution, since only the vibrational frequencies of the whole molecule are considered. In practice a large dependence of the isotope on the position of substitution is observed experimentally. A theory which successfully explains both the intrinsic temperature dependence of the chemical shift and the observed isotope shifts is based on the expansion of the nuclear shielding as a function of powers of displacement coordinates. The intrinsic temperature-dependent nuclear shielding can be expressed as ... 

Linear organomercurials provide a convenient basis for the investigation of the effect of susbstitution in the organic moiety on the metal chemical shift. Increase in the shielding of 150 ppm are observed for each substitution of Me for a H in Hg-CH3 derivatives, while smaller decreases in shielding ( 30 ppm) occur on substitution of )S-Hs. Cadmium alkyls show a similar behavior, but for silyl mercury compormds, replacements at both a and position results in deshielding. 

M. Buehl, DFT Computations of Transition-Metal Chemical Shifts , Annu. Rep. NMR Spectrosc., 2008, 64, 77. 

Sn, but the former is of too low abundance to be of significance. Relative sensitivities for the other two are 4.52 X 10" and 5.18 X 10 respectively. Compounds are invariably diamagnetic. Metal chemical shifts are usually observed for Sn only (data reported relative to Me4Sn = 0). 

This discussion shows that methods for the quantitative computational evaluation of transition metal chemical shifts on a wider scale are just now beginning to emerge. We will doubtlessly see many more applications in the future. As CHF results for complexes of early transition metals in high oxidation states appear to be reasonably accurate (cf. above), it is worthwhile to briefly discuss the insight provided by two recent applications in this field. 

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