CBSE Class 12 Chemistry Coordination Compounds Notes Set 07

Official Class 12 Chemistry Revision Material: CBSE Class 12 Chemistry Coordination Compounds Notes Set 07

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POINTS TO REMEMBER:

1. Coordination compounds

Coordination compounds are compounds in which a central metal atom or ion is linked to a number of ions or neutral molecules by coordinate bonds or which contain complex ions.

Examples- K4[Fe(CN)6]; [ Cu(NH3)4]SO4; Ni(CO)4

 

2.The main postulates of Werner’s theory of coordination compounds

i) In coordination compounds metals show two types of linkages or valencies- Primary and Secondary.

ii) The primary valencies are ionisable and are satisfied by negative ions.

iii) The secondary valencies are non- ionisable and are satisfied by neutral molecules or negative ions. The secondary valence is equal to the C.N and is fixed for a metal.

iv) The ions or groups bound by secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination nos.

 

3.Difference between a double salt and a complex

Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, double salts such as carnallite, KCl.MgCl2.6H2O, Mohr‘s salt, FeSO4.(NH4)2SO4.6H2O, potash alum, KAl(SO4)2.12H2O, etc. dissociate into simple ions completely when dissolved in water. However, complex ions such as [Fe(CN)6]4– of K4[Fe(CN)6], do not dissociate into Fe2+ and CN– ions

 

IMPOTANT TERMINOLOGY

(i) Coordination entity: It constitutes the central metal ion or atom bonded to a fixed number of ions or molecules represented within a square bracket.

(ii) Central 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.

iii) Ligands: The neutral or negative ions bound to the central metal or ion in the coordination entity. These donate a pair/s of electrons to the central metal atom /ion. Ligands may be classified asa)

 

Monodentate/Unidentate: Ligands bound to the central metal atom/ion through a single donor atom. Ex- Cl- ; H2O ; NH3 ; NO2-.

b) Didentate: Ligates through two donor atoms. Ex- C2O4 2- (ox); H2NCH2CH2NH2(en)

c) Polydentate: which ligates through two or more donor atoms present in a single ligand. Ex-(EDTA)4-

d) Chelating ligands: Di- or polydentate ligands that uses two or more donor atoms to bind to a single metal ion to form ring- like complexes. (Ox); (edta)

e) Ambidentate ligand: A ligand that can ligate through two different atoms, one at a time.                          Ex-NO2- ; SCNv)

 

Coordination number: The no. of ligand donor atoms to which the metal is directly bonded through sigma bonds only. It is commonly 4 or 6.

vi) Counter ions: The ionisable groups written outside the square bracket.

Ex- K+ in K4[Fe(CN)6] OR 3Cl- in [Co(NH3)6]Cl3

vii) Coordination Polyhedron: The spatial arrangement of the ligand atoms which are directly attached to the central metal atom/ion. They are commonly Octahedral, Square-planar or Tetrahedral

 

Oxidation number: The charge that the central atom would carry if all the ligands are removed along with their pairs of electrons shared with the central atom. It is represented in parenthesis.

viii) Homoleptic complexes: Complexes in which a metal is bonded to only one kind of donor groups.       Ex- [Co(NH3)6] 3+

ix) Heteroleptic complexes: Complexes in which a metal is bonded to more than one kind of donor groups. Ex- [Co(NH3)4 Cl2]+ 

 

5. NAMING OF MONONUCLEAR COORDINATION COMPOUNDS 
The principle of additive nomenclature is followed while naming the coordination compounds. The following rules are used
 
i The cation is named first in both positively and negatively charged coordination entities. 
ii The ligands are named in an alphabetical order before the name of the central atom/ion 
iii The name of the anionic ligands end in –o, those of neutral and cationic ligands are the same except aqua for H2O, ammine for NH3, carbonyl for CO and nitrosyl for NO. these are placed within enclosing marks . 
iv When the prefixes mono, di, tri, etc., are used to indicate the number of the individual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris , tetrakis are used, the ligand to which they refer being placed in parenthesis. 
v Oxidation state of the metal in cation, anion, or neutral coordination entity is indicated by roman numeral in parenthesis. 
vi If the complex ion is a cation , the metal is same as the element. 
vii The neutral complex molecule is named similar to that of the complex cation. 
 
6.NAMES OF SOME COMMON LIGANDS

CBSE Class 12 Chemistry notes and questions for Co-ordination Compounds Part D 1

CBSE Class 12 Chemistry notes and questions for Co-ordination Compounds Part D 2

 

7. ISOMERISM IN COORDINATION COMPOUNDS 
Two or more substances having the same molecular formula but different spatial arrangements are called isomers and the phenomenon is called isomerism. Coordination compounds show two main types of isomerism- 
A) Structural Isomerism  B) Stereoisomerism 
 
STRUCTURAL ISOMERISM:- It arises due to the difference in structures of coordination compounds. It is further subdivided into the following types- 
 
1) Ionisation isomerism : This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionization isomers [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4. 
 
2) Hydrate or solvate isomerism: This form of isomerism is known as ‗hydrate isomerism‘ in case where water is involved as a solvent. This is similar to ionisation isomerism. Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice. An example is provided by the aqua complex [Cr(H2O)6]Cl3 (violet) and its solvate isomer [Cr(H2O)5Cl]Cl2.H2O (grey-green). 
 
3) Linkage Isomerism: Linkage isomerism arises in a coordination compound containing ambidentate ligand. A simple example is provided by complexes containing the thiocyanate ligand, NCS–, which may bind through the nitrogen to give M–NCS or through sulphur to give M–SCN. 
 
4) Coordination isomerism: It arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex . 
Example [Co(NH3)6][Cr(CN)6] & [Cr(NH3)6][Co(CN)6] 
 
STEREOISOMERISM: Stereo isomers have the same chemical formula and chemical bonds but they have different spatial arrangement. They are of two kinds 
A. Geometrical isomerism 
B. Optical isomerism 
 
GEOMETRICAL ISOMERISM- 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 [MX2L2] (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 [MABXL]-Where A,B,X,L are unidentates Two cis- and one trans- isomers are possible

 

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