Ligands with central atoms

There are many ligands in addition to water, for example Cl , NH3, CN , N02, and transition metal ions, in particular, form a large number of complex ions with different ligands. The number of ligands surrounding the central atom, or ion, is called the coordination number. The numerical value of the co-ordination number depends on a number of factors, but one important factor is the sizes of both the ligands and central atom, or ion. A number of complex ions are given below in Table 2 9. The shape of complex... 

There are rare cases of molecules with central atoms surrounded by more than seven ligand atoms or groups. The rules of VSEPR theory apply to such cases and examples are to be found in more advanced texts. 

However, this definition cannot be applied to the many organometallic compounds in which two or more adjacent atoms of a ligand interact with the central metal atom through what is often a combination of cr, n and 8 bonding (the labels cr, ji or 8 referring to the symmetry of the orbital interactions between ligand and central atom). 

Fig. 2. Overlap of ligand a orbital with central atom p orbital as a function of angle.

Tetrahedral Tetrahedral structures are also associated with CN of 4 and are more common than square planar structure. In tetrahedral or tetragonal structures the four ligands and central atom are not in the same plane. The tetragonal shape can form for all the nontransition metals, and some transition metals. Examples include CH, CCl, and SO . (The T-metals on the right side of the d-block have no electron in the valence shell and can only form sigma bonds to the ligands. For more about a (sigma) and k (pi) bonds, see Chapters 6 and 7.)... 

In compounds with mixed ligands, the central atom can adopt a valence state which is not stable otherwise. Stabilization of such states is caused by introduction into the structure of a component which reduces the concentration of structural tension. For example, C11I2 and Auls do not exist, whereas CuICl, CuIBr, and AUI2CI, Aul2Br and AuIBrCl are quite stable [303, 304]. The stabilization in these cases is caused by a shift of the electron density from metals to F, Cl or Br ligands, preventing the reduction of the metal cation by the electrons of the M-I bond. 

---