RBSE Class 12 Chemistry Notes Chapter 9 Coordination Compounds

These comprehensive RBSE Class 12 Chemistry Notes Chapter 9 Coordination Compounds will give a brief overview of all the concepts.

RBSE Class 12 Chemistry Chapter 9 Notes Coordination Compounds

→ Coordination Compounds:
A compound containing central metal atom or ion bonded to a fixed number of ions or molecules. These atoms or ion are called as ligands.

→ Ligands:
An ion, atom or molecule of donating a pair of electrons to the central atom via a donor atom.

→ Unidentate Ligands:
Ligands with only one donor atom e.g., NH₃, Cl^-, F^- etc.

→ Bidentate Ligands :
Ligands with two donor atoms e.g., ethylenediamine, oxalate ion (C₂O₄^2-) etc.

→ Polydentate Ligands :
Ligands with more than two donor atoms e.g., EDTA, trien, dien etc.

→ Coordination Sphere :
The central atom and the ligands which are directly attached to it are inclosed in square brackets and are collectively termed as coordination sphere.

→ Coordination Number :
The number of coordinate bonds formed by the ligands with the metal atom i.e., num-ber of unidentate ligands or double the number of bidentate ligands etc.

→ Chelating Ligands :
Multidentate ligand simul-taneously coordination to a metal ion through more than one site is called chelating ligand.
Example: Ethylene diamine.

→ Coordination Polyhedron:
The spatial arrangement of the ligands which are directly attached to the central atom.

→ Homolaptic Complexes:
The complexes which con-tain only one type of ligands.

→ Heterolaptic Complexes:
The complexes which con-tain more than one type of ligands.

→ Stereoisomers :
The isomers which have the same position of atoms or groups of atoms but they differ in the spatial arrangement around the central metal atom.

→ The Valence Bond Theory (VBT) :
It explains the resonable success, the formation, magnetic behaviour and geometrical shapes of coordination compounds. It, however, fails to provide a quantitative interpretation of magnetic behaviour and has nothing to say about the optical proper¬ties of these compounds.

→ Crystal field Splitting:
The conversion of five degen-erate -orbitals of the metal ion into different sets of orbitals having different energies in the presence of electrical field of ligands is called crystal field splitting.

→ Constitution and Geometry
The most important factors that govern the C.N. of a complex are:

• The size of the central metal atom or ion.
• The steric interaction between ligands.
• Electronic interactions between the central atom or ion and the ligands.

Based on above mentioned factors, bulky ligands re-sult in low C.N. Higher C.N. are most common on the left of a period and lower C.N. are found on the right of the d- block.
e.g., [Mo(CN)₈]^4- has high C.N. and [PtCl₄]^2- has low C.N.

→ Complex with C.N. = 2:
These are found for Cu+ and Ag+. Examples are and [AgCl₂]^- and HgMe₂. The geometry is linear.

→ Complexes with C.N. = 3:
These are very rare among metal complexes, but are found with bulky ligands.
e.g [Pt(P(Ph)₃)₃] tricyclophenylphosphineplatinium (0). The ligands are in trigonal arrangement.

→ Complexes with C.N. = 4:
Tetrahedral complexes are favored over higher coordinated complexes if the central atom is small or the ligands are large. Square planar complexes are typically observed for metals with d⁸ configuration.
Four coordinated and p-block complexes without lone pair on the central atom, such as [BeCl₂]^2- or [BF₄]^- are always tetrahedral.

→ Complexes with C.N. = 5:
Square pyramidal five co-ordinated complexes are found in the biologically porphyrins.
Another possible geometry for C.N. 5 is trigonal bipyramidal.

→ Complexes with CN. = 6:
The majority of six coordi-nated complexes are octahedral. A coordination number of 6 is found for s, p, d and f metal coordination compounds. The deviations from octahedral symmetry are tetragonal, rhombic and trigonal distortions.

→ Higher C.N.: Large atoms particularly those of the f-block tend to form complexes with high C.N. Seven coordinated complexes are encountered in a few 3d complexes and many more 4d and 5d complexes. The geometries are pentagonal bipyramidal, capped octahedral, capped trigonal prismatic.

Relatively large ions can act as host for nine coordi-nated complexes, e.g., [Nd(H₂O)₉]³⁺.