Ligands

INTRODUCTION

Complexes are also called coordination compounds. They result from a donor-acceptor mechanism or Lewis acid-base reaction between two or more different constituents. These products of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) bond to a central metal ion (or ion) by coordinate covalent bonds. There are certain important points which are related to the coordination complexes. They are-

Ligands are Lewis bases- they contain at least one pair of electrons to donate to a metal atom/ion. Ligands are also called complexing agents.

Metal atoms/ions are Lewis acids- they can accept pair of electrons form Lewis bases. Within the ligand, the atom that is directly bonded to the metal atom/ion is called the donor atom.

A coordinate covalent bond is a covalent bond in which one tom (i.e. the donor atom) supplies both electrons. This type of bonding is different form a normal covalent bond in which each atom supplies one electron.

If the coordination complex caries a net charge, the complex is called a complex ion.

Compounds that contain coordination complexes are called coordination compounds.

Coordination compounds and complexes are distinct chemical species. Their properties and behavior is different from the metal atom/ion and ligands from which they are composed.

LIGANDS

A ligand is an ion or molecule that binds to a metal tom to form a complex. Ligands have electron pair available. They may be neutral or negatively charged. They are electron donor attracted to the metal (electron acceptor) at the center of the complex.

Water is a common ligand. An electron pair from the ligand, such as water, provides both of the electrons for the bond that forms between itself and the central metal atom or ion. Here a single ligand L, which could be water, donates a pair of electrons to form a bond with a metal atom M. (L-›M).

 MONODENTATE LIGANDS

Monodentate ligands have only one atom capable of binding to a central metal atom or ion.

H2O and NHs are examples of neutral monodentate ligands.

When H2O is a ligand, oxygen is the donor atom binding to the metal. When NH3 is a ligand, nitrogen is the donor atom binding to the metal.

Examples of electrically charged monodentate ligands are halide ions, such as: F, CI, Br, I, and cyano, CN.

The overall charge on a complex is the arithmetic sum of the oxidation state of the metal in the center plus the charge(s) brought to the complex by each ligand.

For example, if a complex forms between Fe and six CN ligands, the complex will have a -4 charge, and the formula is written [Fe(CN)]6*4.

POLYDENTATE LIGANDS

A ligand molecule with more than one donor atom is a called a polydentate ligand. These are given specific names, depending on how many donor atoms they contain.

(a) Bidentate Ligands: Bidentate ligands have two atoms capable of binding to a central metal atom or ion. Ethane-1,2-diamine (shown in the image) is an example of a bidentate ligand. both of the nitrogen atoms in this molecule can act as electron donors, binding with a central metal atom or Ion.

(b) Tridentate Ligands and Higher Polydentate Ligands: Tridentate ligands have three atoms capable of binding to a central metal atom or ion. 

 Molecules with four donor atoms are called tetradentate ligands; five donor atoms, pentadentate; and six donor atoms hexadentate.

(c) Ambidentate Ligands: This is a monodentate ligand that can bind in two possible places. For example, the nitrate ion NOz can bind to the central metal atom/ ion at either the nitrogen atom or one of the oxygen atoms. The thiocyanate ion, SCN can bind to the central metal at either the sulfur or the nitrogen.