Structural studies metal atom shielding

In the bifunctional radiopharmaceuticals, Tc and Re complexes that are suitable for coupling to antibodies are almost square-pyramidal oxo complexes of tetradentate ligands, as mentioned earlier. Spies et al. [65] recently studied the syntheses and structures of rhenium complexes with a tetradentate NS3 tripod ligand 2, 2,, 2"-nitrilotris(ethanethiol) (l in Fig. 9) as an interesting alternative to the oxo complex. Such ligands could lead to weakly polar, trigonal-bipyramidal complexes in which the metal atom is more strongly shielded than in the square-pyramidal oxo complexes. Schemes for the syntheses... 

Thus, Brook and coworkers prepared a series of tetrakis(trimethylsilyl)ethynyl derivatives of Si, Ge, Sn and Pb and studied their structure by means of NMR, Mossbauer and X-ray methods. The combined results clearly indicated that there exists an electronic interaction between the central metal atom and distal MesSi groups, leading to a strong shielding of the central atom which was clearly demonstrated by chemical shifts of respective nuclei (Figure 4). The presence of the four distal MesSi groups serves to increase substantially the upheld shifts of the central atom. The chemical shifts reported here are -101.6, -188.5, -384.5 [-356.3 for (HC=C)4Sn], and -760.7 ppm for si, oe, Sn and Pb respectively. 

Additional information on electronic structure comes from a NMR study of a closely related series of compounds of the type (Cp )2Hf(X)(H), where X is H, Me, OH, NH2, NHMe, or NMe2. Bercaw and co-workers observed that in the H-NMR spectrum the metal-bonded H atom becomes more shielded as the tt donor ability of X increases (51). The hydride chemical shifts corresponding to X are 8 15.6, 13.1, 10.2, 9.3, 9.1, and 11.5, respectively. Clearly, the tt interaction of the main group ligand with the metal is important in compounds of this type. 

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