Coordination Compounds

Question

1. What is the name of the phenomenon that makes the stability of a complex increase when two or more donor atoms are connected by a backbone that allows them to coordinate simultaneously to a metal ion?

2. Arrange the following complexes in order of increasing stability:

A [Cu(en)3]^2

B [CuF6]^2

C [Cu(en)(H2O)4]^2

D [Cu(NH3)6]^2

Answer 1

1. Stability of a complex mainly depends upon stability constant, thermodynamic stability and kinetic stability.  Stability constant (Ks) is the equilibrium constant i.e. an equilibrium exists between the ligand and central metal ion.  Grater the value of Ks, more is the stability of a complex.  In case of thermodynamic stability, when the entropy of the system is more positive, the complex will be more stable.  Kinetic stability refers to the speed with which the transformations occur in order to attain the equilibrium.  So, when two or more donor atoms are involve, depending on the nature of the ligand the stability constant, thermodynamic and kinetic factors of the system must be described to explain the stability of the complex.  Probably, the strong ligand (i.e. readily donating unshared pair of electron) displaces the weaker one.  It can be brought either by substitution reactions.

Ans. 2) I don't think the copper could able to form octahedral complexes because it has the d^9 system and copper itself in +2 oxidation state.  But the stability of a complex mainly depends upon basic character of the ligand (more base, more easy to donate the electrons), charge on ligands (higher the charge, higher the stability), and chelate effect ( ring like structures)

Based on the above facts, the stability among A, B, C and D is A>C>B>D