Ligands electronegative

Silanes with electronegative ligands did not react, except in the presence of the tertiary base EtjN [e.g., Eq. (19)] (62, 63). This then provides an attractive route to Si—Pt(II) compounds since the starting materials are readily available and the reaction gives pure products in high (60-90%) yields. Only aromatic phosphine Pt complexes reacted readily, and the use of an excess of silane failed to bring about replacement of the second halogen atom. 

For the most electronegative ligand, fluorine, we expect a relativistic destabilization in the Au—F bond, which was indeed determined to be —0.36eV at the coupled cluster level [182,183], Nevertheless, AuF has a sufficiently high dissociation energy of about 3.17 eV and has been identified recently in the gas phase [184]. In solution or in the solid state it would disproportionate to metallic Au and compounds of Au (AuF3 for the solid). However, a carbene-stabilized Au(I) fluoride was synthesized only very recently (see discussion in the next section) [185]. 

For gold compounds in the high oxidation state (e.g., AuCU , AuFe , AuFs etc.,) we And rather small bond contractions as the electronegative ligands reduce the 6s... 

Figure 4.22 The less electronegative ligands always preferentially occupy the less crowded equatorial sites of a trigonal bipyramid, leaving the more electronegative ligands in the more crowded axial sites.

Figure 9.13 The geometry of some phosphorus chlorofluorides and some phosphorus methyl fluorides in which the larger and less electronegative ligands occupy the equatorial sites.

Apical bonds are stabilized by X whose electronegativity (x(X)) is larger than x(E) the more electronegative ligands show a trend to occupy apical... 

The principle can also be stated in corollary form expressing the enriched p character of hybrids toward electronegative ligands ... 

In the above picture, the primary factor that dictates allocation of hybrid p character toward electronegative ligands is the promotion energy Asp, the valence-shell s-p energy separation... 

---