Geminal ligands

Usually the value of the /asym//sym ratio is used for estimating the angle between geminal ligands in the M(XY)2 " complexes, based on the approximate formula of Cotton [21] ... 

Scheme 2.22 Cu-catalysed 1,4-addition of ZnEt2 to 2-cyclohexenone with C2-S3mi-metric geminal bis(sulfoximines) ligands.

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. 

It was found that the geminal methyl substituents of chlorostannane 29 are nonequivalent in nonpolar solvents such as benzene, toluene or CCI4 at concentrations less than 0.2 M. At higher concentration peak coalescence was observed, indicating rapid interconversion of the enantiomers on the NMR time scale. The addition of DMSO, acetone or HC1 also caused coalescence, even in nonpolar media. It was concluded that inversion at tin in 29 was occurring by self-association in nonpolar solvents or through ligand addition. In each case, a transient five-coordinated Sn species is a likely intermediate. 

Fig. 2 a Symmetric model of ligand-protein complex. LI to L5 represent the ligand, while PI to P5 represent the protein. All the distances between the nearest neighbor protons in the complex are assumed to be 2.5 A. b Asymmetric model, with the L4-L5 distance set at 1.8 A (to mimic the geminal protons)... 

The STDs and, hence, the apparent epitope map can exhibit significant dependence on intraligand relaxation rates, thus necessitating caution in simple qualitative interpretations. In particular, geminal protons on a ligand can show smaller STDs despite their proximity to the protein [36]. 

---