Phosphine stabilizing sphere

Bis-ylides (Scheme 31) may function as four-electron donors and become geminally diaurated. The donor capacity of ylides is generally higher than that of most other ligands, and even powerful donors like tertiary phosphines can thus be readily replaced by non-stabilized ylides. Ylides may therefore be used as auxiliary ligands which are retained, while other components of the coordination sphere are subject to substitution. 

The electronic influence of the other ligands in the metal s coordination sphere is a major consideration when exploring the coordination chemistry of carbon dioxide. Since the metal-C02 bond is stabilized mainly by back-donative interactions, it would be expected that good donor ligands (e.g., the ubiquitous phosphine ligands) would enhance the binding ability of C02 to the metal center. This has been verified both experimentally (10) and theoretically (20), and has as well been demonstrated to be of importance in C02 insertion processes (see below). 

These are pentane-soluble, volatile tetrahedral molecules. They have an extensive chemistry involving metallacycles which will be dealt with later (Section 13.6). Although MMe4 (M = Th, U) have not been isolated, this unit can be stabilized as eight-coordinate phosphine adducts, where the role of phosphines is to saturate the coordination sphere. The coordination of a soft phosphine ligand to a hard metal ion like uranium(iv) is noteworthy. 

Bombesin was derivatized at the C terminus with bidentate chelators (Smith et al. 2003 a, b). The labeled peptide fully retained the biological activity and was stable in vitro and in vivo. Since the bidentate coordination is not optimal with respect to stability (pharmacokinetics), the coordination sphere of the metal tricarbonyl core has been saturated with a highly hydrophilic phosphine. This additional coordination is an example of the 2-Hl approach mentioned in the previous section. The mixed-ligand approach resulted in significantly higher hydrophilicity of the radioconjugates and an improved biodistribution labeled with Tc-99m and also with Re-188 (Smith et al. 2003). 

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