Labile & inert and substitution reactions in octahedral complexes

The first part includes a definition of labile and inert. lability and inertness on the basis of VB theory and CFT and also factors affecting inertness and lability of the complexes.
And also the second part includes Substitution Reactions in Octahedral Complexes like mechanisms and their evidence.

1. labile & inert and SUBSTITUTION REACTIONS IN OCTAHEDRAL COMPLEXES Presented by: K. Muthu Kannan

2. Contents: 1 . Labile and inert 1.1 Definition 1.2 Lability and inertness on the basis of VB theory 1.3 Lability and inertness on the basis of CFT 1.4 Factors affecting lability inertness of complexes ( i ) Size of the central metal ion (ii) Charge on the central metal ion (iii) d-electron configuration 2 . Substitution reactions in octahedral complexes ( i ) Dissociative Mechanism (ii) Associative Mechanism (iii) Interchange Mechanism 2.1 Evidence for dissociative Mechanism (Water exchange)

3. Labile and inert: Definition: The ability of a complex to engage in reaction that results in replacing one or more ligands it’s co-ordination sphere is called lability and the complex in which the ligands are rapidly replaced by others are called labile complexes . The inability of a complex to engage in such reaction is termed as inertness and the complexes which exhibit such property are called inert complexes .

4. The complexes as labile if they have half life ( t 1/2 ) of reaction under one minute while the reactions having half life greater than one minute are termed as inert. t 1/2 < 1 minute [ labile complexes ] t 1/2 > 1 minute [ inert complexes ] Ex: [ Thermodynamically stable and kinetically unstable ] [Cr(CN) 6 ] -3 + 6 C*N - [Cr(C*N) 6 ] -3 + 6 CN - [ t 1/2 = 24 days ] [ Thermodynamically stable and kinetically stable ] [Hg(CN) 4 ] -2 + 4 C*N - [Hg(C*N) 4 ] -2 + 4 CN - [ t 1/2 = very small ]

5. Lability and inertness on the basis of VB theory: According to valance bond theory, if the transition metal complexes undergoing substitution reactions through dissociation mechanism, then all outer orbital complexes are labile and inner orbital complexes are inert. Outer orbital complexes ( Sp 3 d 2 ) - Outer d-orbitals - Labile complexes Inner orbital complexes ( d 2 sp 3 ) - Inner d-orbitals - Inert complexes (M-L bond weak) (M-L bond strong)

6. Ex:1 [Cr(CN) 6 ] 4- - Cr 2+ - d 4 d 4 3d 4s 4p d 2 sp 3 hybridisation complex 3d 4s 4p There is no empty d-orbital. It is inert complex. ↑ ↑ ↑ ↑ ↑ ↓ ↑ ↑ ×× ×× ×× ×× ×× ××

7. Ex: 2 [V(NH 3 ) 6 ] 3+ - V 3 + - d 2 d 2 3d 4s 4p d 2 sp 3 hybridisation complex 3d 4s 4p There is one empty d-orbital. It is also labile complex. It does not explain by VB theory. ↑ ↑ ↑ ↑ ×× ×× ×× ×× ×× ××

8. Lability and inertness on the basis of CFT: Acco rding to crystal field theory, octahedral complexes react either by S N 1 or S N 2 mechanism in which the intermediates are five and seven coordinated species, respectively. In both cases, the symmetry of the complex is lowered down and due to this change in crystal field symmetry, the CFSE value also changes. The cases for lability and inertness are; If the CFSE value for the five and seven membered intermediate complex is greater than that of the reactant, the complex will be of labile nature.

9. If the CFSE value for the five or seven membered intermediate complex is less than that of the reactant, the complex will be of inert nature. For isoelectronic metal cations, the inertness increases with increase of charge on the metal. Because high charge strengthen the M-L bond. Ex: Cr 3+ (d 3 ) more inert than V 2+ (d 3 ) . Crystal field activation energy (CFAE) = CFSE of intermediate – CFSE of reactant

10. Factors affecting lability and inertness of complexes: Size of the central metal ion: Smaller the size of the metal ion, greater will be the inertness because the ligands are held tightly by the metal ion. Ex: [Cs(H 2 O) 6 ] + < [Rb(H 2 O) 6 ] + < [K(H 2 O) 6 ] + < [Na(H 2 O) 6 ] + (ii) Charge on the central metal ion: Greater the charge on the metal ion, greater will be the inertness of the complex. Since the M-L bonds are stronger. Ex: [AlF 6 ] 3- < [SiF­ 6 ] 2- < [PF 6 ] - < [SF 6 ]

11. (iii) d-electron configuration: If elec trons are present in the anti-bonding e g orbitals, the complex will be labile. The ligands will be weakly bonded to the metal and hence can be substituted easily. Complexes with empty t 2g orbitals, will be labile because ligands can approach easily without much repulsion. In short, if the complex contains less than three d-electrons, it will be labile. If one or more e g electrons are present, it will be labile and half filled or more than half filled t 2g orbitals are inert.

12. d - t 2g e g - Labile d 1 - t 2g 1 e g - Labile d 2 - t 2g 2 e g - Labile d 3 - t 2g 3 e g - Inert d 4 (high spin) - t 2g 3 e g 1 - Labile d 4 (low spin) - t 2g 4 e g - Inert d 5 (high spin) - t 2g 3 e g 2 - Labile d 5 (low spin) - t 2g 5 e g - Inert d 6 (high spin) - t 2g 4 e g 2 - Labile d 6 (low spin) - t 2g 6 e g - Inert d 7 - t 2g 6 e g 1 - Labile d 8 - t 2g 6 e g 2 - Labile d 9 - t 2g 6 e g 3 - Labile d 10 - t 2g 6 e g 4 - Labile

13. Note: All the complexes of co-ordination number four are labile. They easily participate in substitution reaction which involves form intermediate which involves associative mechanism. We add more ligands to increase co-ordination number. Complexes of 4d and 5d group metals are maximum inert.

14. Substitution reactions in octahedral complexes: Replacement of one ligand in co-ordination sphere by another without changing co-ordination number and oxidation state of metal cation. [ML 5 X] + Y [ML 5 Y] + X Mechanism: Dissociative mechanism (D) Associati ve mechanism (A) Interchange mechanism (I)

15. Dissociative Mechanism: One of the ligands dissociates from the reactant, to form a reaction intermediate with lower co-ordination number than reactants. [ML 5 X] C.N=6 [L 5 M] C.N=5 [L 5 MY] C.N=6 The dissociative mechanism predicts that rate of overall substitution reaction depends on only the concentration of the original complex [ML 5 X], and is independent of the concentration of the incoming ligand [Y].

16. Rate = k 1 [ML 5 X]. It follows S N 1 mechanism. The intermediate can be either square pyramidal (most probable) or trigonal bipyramidal.

17. (ii) Associative Mechanism (A): Associative of an extra ligand with the complex to give an intermediate of higher co-ordination number: one of the original ligands is then lost to restore the initial co-ordination number. [ML 5 X] + Y L 5 C.N=7 C.N=6 [ML 5 Y] C.N=6 Rate determining step is the collision between the original complex ML 5 X and the incoming ligand Y to produce a seven co-ordinated intermediate.

18. The second faster step is dissociation of the X ligand to produce the desired product. The associative mechanism predicts that the rate of reaction depends on the concentration of ML 5 X and Y. It follows S N 2 mechanism. The formed intermediate can be either mono-capped octahedron or pentagonal bipyramidal. Rate = k 1 [ML 5 X][Y]

19. (iii) Interchange Mechanism (I): It is a continuous single step process. It takes place in one step without forming stable intermediate. Two steps exist; ( i ) Interchange associative (I A ) (Bond making is more important) (ii) Interchange dissociative (I D ) (Bond breaking is more important) ML 5 X + Y [Y-------ML 5 ------X] [ML 5 Y] + X (Activated complex)

20. It follows formation of activated complex or transition state. As Y begins to bond X begins to leave. i.e. the bond making to Y and bond breaking to X occur simultaneously. The terms associative and dissociative are reversed for situations where 7 and 5 co-ordinate intermediates have actually been isolated and positively identified. Evidences for dissociative (S N 1 ) mechanism: Water Exchange: Water molecule in the co-ordination sphere are exchanged with isotopically labelled bulk water(H 2 O 18 ).

21. [M(H 2 O) 6 ] n+ + H 2 O 18 [M(H 2 O) 5 (H 2 O 18 )] n+ + H 2 O EX: [Cr(H 2 O) 6 ] 2+ + 6 H 2 O 18 [Cr(H 2 O 18 ) 6 ] 2+ + 6 H 2 O It depends on charge density. The water exchange mechanism is directly proportional to charge density. Charge density increases the metal and ligand bond strength also increases therefore it difficult to break. In group the size of the atom increases the charge density will be decrease. So, the metal and ligand bond strength decreases. Now substitution reactions happens in it.

22. Order of water exchange: Cu 2+ > Cr 2+ > Zn 2+ > Mn 2+ > Fe 2+ > Ni 2+ > V 2+ Ti 3+ > Fe 3+ > V 3+ > Cr 3+ Di-positive metal complexes are more water exchange order compare to tri-positive metal complexes. Alkali and alkaline earth metals are also very high water exchange and Be 2+ , Mg 2+ are exceptional cases.

23. References: 1) https://www.slideshare.netchemsantreactions-of-complexes . 2) https://www.slideshare.netsaikumardarsiniinert-and-labile-complexes-and-substitution-reactions . 3) https://www.dalainstitute.com/books/a-textbook-of-inorganic-chemistry-volume-1/inert-and-labile-complexes . 4) https://youtu.be/k4ilpQmGlHo .

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