complexation

Complexation and Protein Binding
Complexes or coordination compounds, according to the classic definition, result from a donor–acceptor mechanism or Lewis acid–base reaction between two or more different chemical constituents.
Any nonmetallic atom or ion, whether free or contained in a neutral molecule or in an ionic compound, that can donate an electron pair can serve as the donor. The acceptor, or constituent that accepts a share in the pair of electrons, is frequently a metallic ion, although it can be a neutral atom.
Complexes can be divided broadly into two classes depending on whether the acceptor component is a metal ion or an organic molecule; these are classified according to one possible arrangement in Table 10-1. A third class, the inclusion/occlusion compounds, involving the entrapment of one compound in the molecular framework of another, is also included in the table.
Intermolecular forces involved in the formation of complexes are the van der Waals forces of dispersion, dipolar, and induced dipolar types. Hydrogen bonding provides a significant force in some molecular complexes, and coordinate covalence is important in metal complexes.
Table 10-1 Classification of Complexes
I. Metal ion complexes
A. Inorganic type
B. Chelates
C. Olefin type
D. Aromatic type
1. Pi (?) complexes
2. Sigma (?) complexes
3. “Sandwich” compounds
II. Organic molecular complexes
A. Quinhydrone type
B. Picric acid type
C. Caffeine and other drug complexes
D. Polymer type
III. Inclusion/occlusion compounds
A. Channel lattice type
B. Layer type
C. Clathrates
D. Monomolecular type
E. Macromolecular type

Inorganic Complexes
The ammonia molecules in hexamminecobalt (III) chloride, as the compound [Co(NH3)6]3+ Cl3- is called, are known as the ligands and are said to be coordinated to the cobalt ion. The coordination number of the cobalt ion, or number of ammonia groups coordinated to the metal ions, is six. Other complex ions belonging to the inorganic group include [Ag(NH3)2]+, [Fe(CN)6]4-, and [Cr(H2O)6]3+.
Each ligand donates a pair of electrons to form a coordinate covalent link between itself and the central ion having an incomplete electron shell. For example,

Hybridization plays an important part in coordination compounds in which sufficient bonding orbitals are not ordinarily available in the metal ion.
Table 10-2shows some compounds in which the central atom or metal ion is hybridized differently and the geometry that results. Ligands such as H2? H3 , C -, or l- donate a pair of electrons in forming a complex with a metal ion, and the electron pair enters one of the unfilled orbitals on the metal ion.
Table 10-2 Bond Types of Representative Compounds

For example, the ground-state electronic configuration of Ni2+ can be given as

In combining with 4C - ligands to form [Ni(CN)4]2-, the electronic configuration of the nickel ion may become either


in which the electrons donated by the ligand are shown as dots. The dsp2 or square planar structure is predicted to be the complex formed because it uses the lower-energy 3d orbital.
In the case of divalent copper, Cu(II), which has the electronic configuration


the formation of the complex [Cu(NH3)4]2+ requires the promotion of one d electron of Cu2+ to a 4p level to obtain a filled 3d configuration in the complexed metal ion, and a dsp2 or planar structure is obtained: