A ligand is an ion or molecule, which donates a pair of electrons to the central metal atom or ion to form a coordination complex. The word ligand is from Latin, which means “tie or bind”. Ligands can be anions, cations, and neutral molecules. Ligands act as Lewis bases (donate electron pairs) and central metal atoms viewed as Lewis acid (electron pair acceptor). The nature of bonding between metal to ligand varies from covalent bond to ionic bond.

Explain Werner's theory of coordination compounds with suitable examples.

Werner's theory: Postulates: (1) Every complex compound has a central metal atom (or) ion. (2) The central metal shows two types of valencies namely primary valency and secondary valency. (A) Primary valency: The primary valency is numerically equal to the oxidation state of the metal. Species or groups bound by primary valencies undergo complete ionization. These valencies are identical with ionic bonds and are non-directional. These valencies are represented by discontinuous lines (.....). Eg.: CoCl 3 contains Co 3 + and 3 Cl - ions. There are three Primary Valencies or three ionic bonds. (B) Secondary Valency: Each metal has a characteristic number of Secondary Valencies. They are directed in space around the central metal. The number of Secondary Valencies is called Coordination number ( C . N . ) of the metal. These valencies are directional in Nature. For example in CoCl 3 . 6 NH 3 . Three Cl - ions are held by primary Valencies and 6 NH 3 molecules are held by Secondary Valencies. In CuSO 4 . 4 NH 3 complex SO 4 2 - ion is held by two Primary Valencies and 4 NH 3 molecules are held by Secondary Valencies. (3) Some negative ligands, depending upon the complex, may satisfy both primary and secondary valencies. Such ligands, in a complex, which satisfy both primary and secondary valencies do not ionise. (4) The primary valency of a metal is known as its outer sphere of attraction or ionizable valency while the Secondary valencies are known as the inner sphere of attraction or coordination sphere. Groups bound by secondary valencies do not undergo ionization in the complex.

Explain geometrical isomerism in coordination compounds giving suitable examples.

Geometrical isomerism in coordination compounds: This type of isomerism arises in heteroleptic complexes due to different possible geometric arrangements of the ligands. Important examples of this behaviour are found with coordination numbers 4 and 6 . In a square planar complex of formula [ MX 2 L 2 ] ( X and L are unidentate), the two ligands X may be arranged adjacent to each other in a cis isomer, or opposite to each other in a trans isomer, for example geometrical isomers of Pt [ ( NH 3 ) 2 Cl 2 ] . Each geometrical isomer has a central platinum surrounded by the same four ligands ( 2 chloro and 2 amine in each case) which lie at the corners of a square. But they differ in the special positions of the ligands. In cis-isomer two similar ligands occupy positions at 90 ° to one another. In trans-isomer two similar ligands occupy positions opposite to one another at 180 ° apart.

Using IUPAC norms write the formulas for the following: Tetrahydroxozincate II .

The formula of complex compounds are written as per of the following rules: (i) Central atoms is written first. (ii) It is followed by the symbols of the ligands which are written in the order-negative ligands, neutral ligands followed by positive ligands. (iii) If more than one type of ligands are there then their formulae are written according to the alphabetical order of the first symbol of their formulae. (iv) The number of ligands linked to metal atom are indicated by putting its number as subscript written after simple brackets in which the ligands are placed. (v) The coordination sphere is enclosed in square brackets. (vi) No space is kept between representations of ions in the formula, whether the coordination sphere is a cation or anion. (vii) If the formula of coordination sphere is written without the counter ion then the charge on the coordination sphere is written as a superscript on the right-hand side, outside the square brackets. Based on the above rules, the formula of the given coordination compound is [ Zn ( OH ) 4 ] 2 - .

Explain the terms. Central metal atom/ion.

In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion. For example, the central atom/ion in the coordination entities: [ NiCl 2 ( H 2 O ) 4 ] , [ CoCl ( NH 3 ) 5 ] 2 + and [ Fe ( CN ) 6 ] 3 - are Ni 2 + , Co 3 + and Fe 3 + , respectively. These central atoms/ions are also referred to as Lewis acids.

Explain the term coordination entity.

A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, [ CoCl 3 ( NH 3 ) 3 ] is a coordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions. Other examples are [ Ni ( CO ) 4 ] , [ PtCl 2 ( NH 3 ) 2 ] , [ Fe ( CN ) 6 ] 4 - , [ Co ( NH 3 ) 6 ] 3 + .

Explain the term coordination number.

The coordination number ( CN ) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded. For example, in the complex ions, [ PtCl 6 ] 2 - and [ Ni ( NH 3 ) 4 ] 2 + , the coordination number of Pt and Ni are 6 and 4 respectively. Similarly, in the complex ions, [ Fe ( C 2 O 4 ) 3 ] 3 - and [ Co ( en ) 3 ] 3 + , the coordination number of both, Fe and Co , is 6 because C 2 O 4 2 - and en (ethane - 1 , 2 - diamine) are didentate ligands.

Explain the term Ligand.

The ions or molecules bound to the central atom/ion in the coordination entity are called ligands. These may be simple ions such as Cl - , small molecules such as H 2 O or NH 3 , larger molecules such as H 2 NCH 2 CH 2 NH 2 or N ( CH 2 CH 2 NH 2 ) 3 or even macromolecules, such as proteins.

How is the variability in oxidation states of transition metals different from that of the non-transition metals? Illustrate with examples.

In case of transition elements, the variability in the oxidation state is due to participation of ( n - 1 ) d orbitals and ns orbitals. The oxidation states differ by unity. Thus, V shows + 2 , + 3 , + 4 and + 5 oxidation states and Mn shows + 2 , + 3 , + 4 , + 5 , + 6 and + 7 oxidation states. On the other hand, the variable oxidation states shown by some p block elements differ by two units. Thus, tin shows + 2 and + 4 oxidation state and indium shows + 1 and + 3 oxidation states.