Orientation of a ligand

In the present article, it will be assumed that a molecule within a skeletal class is completely determined for our purposes by specifying the nature of the ligand on each site, though a ligand is permitted to have two enantiomeric (mirror image) forms. If more than one orientation of a ligand on a site is possible, this means that one of the following must hold either... 

If more than one orientation of a ligand is possible this means that, in relation to the time scale of the experiment, one of the following must hold either... 

The deviation from a true tetrahedral geometry that we find for the molecules Be(OX)42, B(OX)4, and C(OX)4 is common to all A(OX)4, A(NX2)4, and A(CX2Y)4 molecules, all of which have two bond angles smaller than, and four greater than, 109.5° or two angles larger than 109.5° and four smaller than 109.5°. In each case the overall symmetry of the molecule, which depends on the relative orientation of the ligands, is D2d or S4. Some examples... 

Figure 2.6 Types of orientations between the electronic pair of a ligand and a lanthanide cation, (a) The lone pair is directly oriented towards the lanthanide cation.

Re and Si Labels used in stereochemical descriptions of heterotopic faces. If the CIP priority of the three ligands a, b, and c is assigned as a > b > c, the face that is oriented clockwise toward the viewer is called Re, while the face with a counterclockwise orientation of a — b — c is called Si, as shown in Figure 1-34 ... 

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