Nucleophilicity and inert-labile metal complexes.pptx

It's about the effect of nucleophilicity on the compounds structure

1. NUCLEOPHILICITY Nucleophilicity is a measure how readily nucleophile is able to attack on electron deficient atom/. Nucleophilicity is a kinetic property Nucleophilic substitution Reaction [Co(NH₃)₅ Cl ]²⁺ + OH⁻ → [Co(NH₃)₅OH]²⁺ + Cl ⁻ Classified into two SN1 reactions : is a two-step nucleophilic substitution reaction where the rate-determining step depends on only one substrate. Reaction processed through carbocation intermediate The first step is the leaving of group followed by attack on carbocation by nucleophile substrate.

2. 2. SN2 reactions is a one-step, bimolecular nucleophilic substitution in organic chemistry where a nucleophile attacks a substrate from the back, simultaneously replacing a leaving group The reaction rate depends on the concentration of both the nucleophile and the substrate. MECHANISTIC PATHWAYS FOR NUCLEOPHILI SUBSTITUTION REACTIONS Associative (nucleophile adds first ) Dissociative ( Ligand leaves first Interchange ( simultaneous bond breaking and formation Associative (nucleophile adds first ) The intermediate at rate determination step has high coordination number than that in the reactant

3. 2. Dissociative ( Ligand leaves first) the intermediate is detected in the rate determination step in which the central metal has a lower coordination number than has the reactant 3. Interchange ( simultaneous bond breaking and formation) Also called concerted path Does not form intermediate Activated complex contain both substrate and the ligand STRUCTURE FACTORS AFFECTING NUCLEOPHILITY factor effect Example Charge More negative species is more nucleophilic OH - > H 2 O Steric hindrance Bulky nucleophiles react slower t-BuNH₂ < NH₃ Solvent Polar protic solvents reduce nucleophilicity H₂O < DMSO

4. Inert /labile complexes Definitions Labile complexes are metal complexes in which the rate of ligand displacement reactions is very fast and therefore show high reactivity this property is called lability 2. Inert complexes these are metal complexes in which the rate of ligand displacement reactions is very slow and therefore show slow reactivity. this property is called inertness

5. kinetic of labile and inert complexes Kinetic stability of labile and inert complexes depend on the activation energy of the reaction . Complexes are kinetically unstable if activation energy barrier is low at which reaction take place at high speed. Complex are kinetically stable if activation energy barrier is fast, at which reaction take place slowly. The diagram of reaction coordination

6. Labile and inert complexes on the basis of valence bond theory (VBT) Types of octahedral metal complexes according to VBT Outer orbital complexes have sp 3 d 2 hybridization Are labile In nature VTB proposed that orbitals in sp 3 d 2 hybridization are weaker than that of (n-1)d 2 sp 3 hybridization orbital and therefore show labile character, eg ., Mn +2 ,Fe +2 , and Cr +2 complexes shows fast ligand displacement 2. Inner orbital complexes Have d 2 sp 3 hybridization The orbitals are filled with six electrons pairs donated by ligand The d n electron of metal occupies d xy , d xz , d yz . d 2 sp 3 hybrid orbitals can form both inert or labile complexes. To show lability , one orbital of d xy , d xz , d yz must be empty so that it should accept electron pair and form seven coordinated intermediate which is a necessary step for the associative pathway of ligand displacement. to show inertness, if all d xy , d xz , d yz orbitals contain atleast one electron, and it will be unable to accept a pair of electron from the incoming ligand

7. Labile and inert complexes on the basis of Crystal Field Theory (CFT) Octahedral complexes react either by SN1 or SN2 mechanism in which the intermediates are five and seven-coordinated species, respectively. In both cases, the symmetry of the complex is lowered down which result to change of CFSE value. Labile complexes in terms of crystal field theory the complex is labile in nature If the CFSE value for the five or seven-membered intermediate complex is grater than of the reactant because activation energy barrier is zero. The reaction coordinates diagram for ligand displacement reactions in labile metal complexes

8. inert complexes in terms of crystal field theory the metal complex is inert in nature If the CFSE value for the five or seven-membered intermediate complex is than of the reactant because loss of CFSE will become the activation barrier. The diagram for reaction coordinates of ligand displacement reaction in inert metal Note: It means the gain in CFSE makes the complex labile while the loss in CFSE makes the complex inert

9. Assumptions for calculation of CFSE All the six-coordinated complexes should be treated as perfect octahedral even if the mixed ligands are present The inter-electronic repulsive forces arising from d-subshell can simply be neglected The Dq -magnitude for reacting as well as the intermediate complexes are assumed to be the same The Jahn -Teller distortion should be neglected in calculations Evidence for the lability and inertness: the ligand displacement can be dissociative or associative depending on the reaction nature. For SN1 or dissociative pathway. The 5-coordinate intermediate is square pyramidal Dissociative mechanism occurs through square-pyramidal intermediate’ The gain and loss of CFSE is calculated as: CFSE gain or loss = CFSE of square pyramidal intermediate - CFSE of Octahedral reactant (coordination No=5) (coordination No=6) Note: is the CFSE gain or loss is negative, it means that the Ae is zero because it can not be negative. Example : calculate the gain or loss CFSE of Co +2 that have -16 CFSE for octahedral reactant and -14.57 CFSE for square pyramidal intermediate?. Determine its kinetic stability

10. For SN1 or dissociative pathway. The 7-coordinate intermediate is octahedral wedge a ssociative mechanism occurs through octahedral wedge intermediate’ The gain and loss of CFSE is calculated as: CFSE gain or loss = CFSE of octahedral-wedge intermediate - CFSE of Octahedral reactant (coordination No=7) (coordination No=6) Note: is the CFSE gain or loss is negative, it means that the Ae is zero because it can not be negative. Example : calculate the gain or loss CFSE of Fe +3 that have -12 CFSE for octahedral reactant and -10.20 CFSE for octahedral wedge intermediate?. Determine its kinetic stability.