Chemical Bonds: Coordination Compounds, Ligands and Chelates

A molecule is a stable union of two or more atoms. The formation of the bonds that exist between the atoms comprising a molecule requires less energy than that needed to keep the atoms apart.

The breakage of the bonds that join atoms consumes energy. If a large amount of energy is required to break the bond, the bond is called a strong bond. If small amounts of energy are required, the bond is a weak bond.

Four major types of chemical bonds may be identified covalent bonds, ionic bonds, hydrogen bonds, and hydrophobic bonds. The energies of these bonds are given in Table 3-9.

Covalent bonds are the bonds formed by the sharing of one or more electron pairs by the atoms comprising the molecule. These are generally the strongest bonds formed between atoms. Whereas ionic bonds formed between oppositely charged atoms have a high energy level in crystals (such as in crystal­line NaCl, KCl, etc.; see Table 3-9), the ionic bonds that are formed in aqueous solution (the usual case for cells) have much lower energy levels and are consider­ably weaker than covalent bonds.

Very weak bonds called hydrogen bonds occur when hydrogen is shared by two electronegative at­oms. Hydrogen bonds most commonly occur between the oxygen atoms of water molecules, but they can also be formed between two nitrogen atoms and be­tween nitrogen and oxygen.

Hydrophobic bonds are associations of molecules or parts of molecules that have non-polar groups. They are not chemical bonds in the usual sense. The attrac­tion of water molecules for one another is far greater than their attraction to hydrophobic groups and this causes the hydrophobic groups to aggregate and have minimum contact with the surrounding water. The as­sociation of the hydrophobic groups is stabilized by van der Waals interactions, which are weak attrac­tions between all uncharged atoms or molecules that are in close proximity.

Coordination Compounds, Ligands, and Chelates:

In addition to the types of bonds discussed above, oxy­gen and nitrogen can form a special kind of covalent association, called a coordination bond, with certain divalent and trivalent metal ions such as Ca²⁺, Co²⁺, Cu²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, and Zn²⁺. In this type of bond, all of the shared electrons that form the bond are donated by the nitrogen or oxygen atom. No elec­trons are donated by the metal. The energy of such bonds is about 251 kJ or 60 kcal per mole.

Molecules that contain nitrogen or oxygen atoms capable of do­nating electron pairs to metals or metal ions are called ligands. If two or more electron pair donating atoms are present in the same molecule, the molecule is called a chelate or chelating agent.

The metal bound by the chelating agent is said to be sequestered. Water may also coordinate metal ions, as in the hydrated forms of Mg, Ca, and Al salts. Figure 3-7 shows the chemical structure of cyanocobalamin (vitamin B₁₂) in which the cobalt ion forms coordination bonds (de­picted as arrows) with several atoms. The oxygen- transporting protein hemoglobin is a prime example of an iron chelate. Some enzymes are activated by chelation of a metal.