Terminology Used In Coordination Chemistry

TERMINOLOGY USED IN COORDINATION CHEMISTRY

(a) Lewis Acid

All electron acceptors are lewis acids.

(b) Lewis Base

All electron donors are lewis base.

(c) Central metal ion

In the complex ion an acceptor accepts a pair of electrons from the donor atoms. The acceptor is usually a metal / metal ion to which one (or) more of neutral molecules (or) anions are attached. The acceptor metal cation is referred to as central metal cation. Hence, central metal cation in a complex serves as a lewis acid.

(d) Oxidation state

This number denotes the charge, explaining the number of electrons it has lost to form the cation. It is oxidation number that denotes the charge, if the central metal atom would have if all the ligand in the complex were removed along with their electron pairs that were shared with the central atom. It is usually represented by Roman Numeral.

(e) Ligand (Latin word meaning to bind)

A ligand is an ion (or) a molecule capable of functioning as an electron donor. Therefore the neutral molecules or ions which are directly attached to the central metal ion are called as ligand (or) coordination groups. These coordination groups or ligands can donate a pair of electrons to the central metal ion (or) atom. Hence, in a complex compound ligands act as Lewisbases.

Types of ligands

When a ligand is bound to a metal ion through a single donor atom, as with - Cl , H2O or NH3, the ligand is said to be unidentate. Whenever a single coordinating group (or) ligand occupies two (or) more coordination position on

the same central metal ions, a complex possessing a closed ring is formed. Such ligands are called polydentate ligands. When a single ligand has two coordinating positions,itiscalledbidentateligandandwhentherearethreecoordinatingpositions available, it is called a tridentate ligand and so on. For example, ethylenediamine is a bidentate ligand because it has two amino groups each of which can donate a pair of electrons.

Name of the ligands

Positive ligands

The positive ligands are named with an ending -ium.

NH2 - NH3+ hydrazinium

This ligand, though positive can bind through the uncharged nitrogen.

Neutral ligands

The neutral ligands are named as such without any special name. But water is written as 'aqua : Ammonia is written as ammine. Note that two m's to distinguish from organic amine  CO-Carbonyl, NO-Nitrosyl, NH2 - CH2 - CH2 - NH2-ethylenediamine (en), Pyridine C5H5N.

Negative Ligands

Negative ligands end in suffix 'O'.

Example

F--Fluoro, Cl--Chloro, C2O42--Oxalato, CN--Cyano, NO2--Nitro, Br--Bromo, SO42--Sulphato, CH3COO--acetato CNS--thiocyanato, NCS--isothiocyanato, S2O32--thiosulphato.

Chelates

If a ligand is capable of forming more than one bond with the central metal atom (or) ion then the ring structures are produced which are known as metal chelates. Hence the ring forming group are described as chelating agents (or) polydentate ligands.

Coordination sphere

In a complex compound, it usually, central metal ion and the ligands are enclosed with in square bracket is called as coordination sphere. This represents a single constituent unit. The ionisable species are placed outside the square bracket.

These ions do not ionise to give the test for constituent ions.

Coordination number

The coordination number of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded. Numerically coordination number represents the total number of the chemical bonds formed between the central metal ion and the donor atoms of the ligands. For example in K4[Fe(CN)6] the coordination number of Fe(II) is 6 and in [Cu(NH3)4]SO4 the coordination number of Cu(II) is 4.

Charge on the complex ion

Charge on the complex ion is equal to the sum of the charges on the metal

ion and their ligands.

Example

1. [Cu(NH3)4]2+ can be written as [Cu2+(NH3)4]2+ since NH3 ligand is neutral. The sum of the charges on the metal ion and the ligands = +2.

This can be determined as shown below

Charge on the metal ion (Cu2+) = +2

Charge on the ligand (NH3) = 4 × 0 = 0

Net charge on the complex ion = +2 + 0 = +2

2. Similarly for [Fe(CN)6]4- (or) [Fe2+(CN)6]4-

The sum of the charge on the metal ion and the ligand = -4.

Charge on the metal ion (Fe2+) = +2

Charge on the ligand (CN-)           = 6 × (-1) = -6

Net charge on the complex = +2 - 6 = -4