Presentation1 on the kinetic and inorganic chemistry group 1-1_1.pptx

Its about the chemical kinetic and inorganic reactions where it focus on the types of reactions, structure effect and formula generation

1. KINETICS AND MECHANISMS OF INORGANIC REACTIONS HARRIS JAMBO BSC/CHE/HON/34/20 WANANGWA NYIRENDA BSC/CHE/HON/18/20 JAMES CHISAMBA BSC/CHE/HON/15/20 CHIKUMBUTSO BSC/CHE/HON/10/20 WEZI KACHINGWE BSC/CHE/HON/33/20 STYWELL NGWENYA BSC/CHE/HON/19/20

2. Chemical kinetics is the study of reaction rates Reaction rate is the rate of change of concentration of reactants or products with time. For a general reaction: The reaction rate and rate of disappearance of reactants and rate of formation of products are related by Rate laws The rate law are mathematical expressions that describe the relationship between the rate of a chemical reaction and the concentrations of its reactants. Rate = k[A] m [B] n Where k is the proportionality constant called the rate constant The order of the rate of reaction is the sum of the exponents in the rate law. For example, if m = 1 and n = 2, then the rate law has an overall order of 3. However, except in the simplest cases, it is the best to describe the order with respect to individual reagents, in this example, first order in [A] and second order in [B]  

3. Types of inorganic reactions 1. Substitution reactions Substitution reactions involve the exchange of one ligand with another. The general reaction MX + Y→ MY + X. Where x and y are ligands and M is a metal. example;     [Fe(H2O)6] 2+ + SCN → [Fe(H2O)5(SCN)] 2+ + H2O They are classified as: a. Dissociation (D) b. Association (A) c. Interchange (I)

4. DISSOCIATION MECHANISM(D)/SN1 For a general reaction; M-X + Y ↔ [M + X +Y] (intermediate) ↔ MY + X. In a D mechanism , The entering ligand Y does not participate in the transition state, which is the highest energy barrier. In the transition state, the bond between the metal and the leaving ligand Is virtually broken, and the bond with the incoming ligand has not yet been established. Therefore, the reaction rate does not depend on the nature of the entering ligand (with the exception of effects that might be due to solvation). coordination number is maintained

5. . The reaction intermediate M has a reduced coordination number, and its relative stabilization (lower Gibbs free energy) is due to solvation (formation of bonds with the solvent, stabilizing the chemical species). Formation of second transition state require relatively high energy(needed for the elimination of the bound solvent). In this way the location for new coordination is freed and the weak bond M….Y is formed. The reaction is exothermic. Figure 1 in the next page shows the energy diagram for dissociation mechanism.

6. Example Aqua Substitution in Octahedral Cobalt(III) Complexes: Substitution of water by ammonia in a hexaaquacobalt Substitution of water by ammonia in a (III) complex: [Co(H2O) 6 ] 3+ + NH 3 → [Co(H 2 O) 5 (NH3)] 3+ + H 2 O Mechanism : The water ligand dissociates from the cobalt(III)  [Co(H 2 O) 6 ] 3+ → [Co(H 2 O) 5 ] 3+ + H 2 O Rate=k[Co(H 2 O) 5 ] 3+ 2.  The ammonia ligand binding to the intermediate  [Co(H2O) 5 ] 3+ + NH 3 → [Co(H2O) 5 (NH3)] 3+

7. ASSOCIATION REACTION (A)/SN2 For a general reaction; M-X + Y ↔ [Y-M-X] ↔ MY + X In associative mechanism, the intermediate will have an increased coordination number. In the transition state (TS), the bond with the entering ligand Y is largely established, while the bond towards the leaving ligand X is not essentially weakened. The reaction rate depends strongly on the nature of the entering ligand Y because Y participates in the transition state. Coordination number is maintained

8. Figure 2: shows the energy profile for this A or S N 2 mechanism.

9. Example: Substitution in Square-Planar Platinum(II) Complexes [Pt(NH3)2Cl2] + Br− → [Pt(NH3)2BrCl] + Cl− Mechanism : The incoming bromide ion coordinates to the platinum(II) center [Pt(NH3)2Cl2] + Br− → [Pt(NH3)2Cl2(Br−) - Rate=k[Pt(NH3)2Cl2][Br−] 2. The chloride ion dissociating [Pt(NH3)2Cl2(Br−)]- → [Pt(NH3)2BrCl] + Cl−

10. Interchange (I) Mechanism . Interchange mechanism, or concerted mechanisms, are the interchanges of ligands X and Y between inner and outer . coordination spheres of the metal. Occurs in one step that involves a transitional state being formed – the Transitional State cannot be traced or seen. . Attachment of the incoming ligand and detachment of the leaving ligand occurs simultaneously. example M- X + Y → X ----M---- Y → M- Y + X . The interchange can be either of the I d  type (interchange, dissociative), or of the l a  type (interchange, associative). . The I d  mechanism is identical to S N 1 mechanism, and l a  to S N 2 mechanism .

11. EXAMPLE: Substitution in Octahedral Cobalt(III) Amine Complexes [Co(NH 3 ) 5 Cl] 2+ +OH − →[Co(NH 3 ) 5 OH] 2+ +Cl − Rate=k[Co(NH 3 ) 5 Cl][OH − ] This substitution follows an interchange dissociative mechanism, where the leaving group starts to dissociate as the incoming ligand (OH - ) starts to coordinate: [Co(NH 3 ) 5 Cl] 2+ → [Co(NH 3 ) 5 (OH − ⋅Cl − )] 2+ → [Co(NH 3 ) 5 OH] 2+ +Cl −

12. Interchange associative Ia mechanism The bond formation between the metal and the incoming ligand is more important than the bond breaking with the leaving ligand (X) in determining the activation energy of the transition state. The incoming ligand is therefore significant to the reaction rate. Unlike the A and D mechanism, the Ia mechanism does not form a distinct, long lived intermediate. The substitution happens in a single step via a transition state. Example is the solvent exchange reaction of the hexaaquovanadium (II) ion [V(H 2 ​O) 6 ​] 2+ +H 2 O→[V(H 2 ​O) 5 ​(H 2O )] 2+ +H 2 ​O

13. 2. Hydrolysis Reactions Hydrolysis is a specific type of substitution where water is the incoming ligand. The kinetics provide a classic way to distinguish between mechanisms. Acid Hydrolysis ( Aquation ) :   [Co(NH₃)₅ Cl ]²⁺ + H₂O → [Co(NH₃)₅(H₂O)]³⁺ + Cl ⁻ Rate Law for Co(III):  Rate = k [Complex] [H⁺] Sₙ1CB (Conjugate Base) Mechanism:  This rate law is evidence for a dissociative pathway that is acid-catalyzed. Pre-equilibrium:  A proton is removed from an ammonia ligand to form a conjugate base (CB). [Co(NH₃)₅ Cl ]²⁺ + OH⁻ ⇌ [Co(NH₃)₄(NH₂) Cl ]⁺ + H₂O (fast) Rate-Determining Step:  The amido ligand (NH₂⁻) is a strong π-donor, which labializes the  trans  chloride ligand, facilitating its dissociation. [Co(NH₃)₄(NH₂) Cl ]⁺ → [Co(NH₃)₄(NH₂)]²⁺ + Cl ⁻ (slow)

14. ii. Fast Protonation:  The highly reactive intermediate is rapidly protonated by water. [Co(NH₃)₄(NH₂)]²⁺ + H₂O → [Co(NH₃)₅(H₂O)]³⁺ Base Hydrolysis: [Co(NH₃)₅ Cl ]²⁺ + OH⁻ → [Co(NH₃)₅(OH)]²⁺ + Cl ⁻ Observed Rate Law:  Rate = k [Complex] [OH⁻] This was historically thought to be an associative attack by OH⁻, but it is now widely accepted to also proceed via the  Sₙ1CB mechanism  described above. The observed second-order kinetics arise because the deprotonation step (which is fast) is directly linked to the concentration of OH⁻.

15. 3. Redox Reactions (Electron Transfer) These reactions involve the transfer of at least one electron from the reducing agent to the oxidizing agent. Classificlassific ation Oxidation-reduction 1. Stoichiometric Classification i. Complementary Reactions The change in oxidation state of the reducing agent is the same as the change in oxidation state of the oxidizing agent. Cr 2+ + Ag + → Ag + Cr 3+ ii. Noncomplementary Reactions The oxidizing agent and the reducing agent undergo different net changes in oxidation state. 2Cr 2+ + Ti3+ → 2Cr 3+ + Ti +

16. 2. Mechanistic Classification These reactions involve a change in the oxidation state of the metal. The two primary mechanisms are  Inner-Sphere  and  Outer-Sphere . A. Inner -Sphere Electron Transfer Mechanism (Taube's Experiment): The classic experiment: [Co(NH₃)₅ Cl ]²⁺ + [Cr(H₂O)₆]²⁺ + 5 H₃O⁺ → [Co(H₂O)₆]²⁺ + [Cr(H₂O)₅ Cl ]²⁺ + 5 NH₄⁺ Bridge Formation:  The substitution ally labile Cr(II) complex loses a water molecule and coordinates to the chloride ligand of the inert Co(III) complex, forming a chloride-bridged dimer. [ (NH₃)₅Co- Cl ]²⁺ + [ Cr(H₂O)₆ ]²⁺ → [ (NH₃)₅Co- Cl -Cr(H₂O)₅ ]⁴⁺ + H₂O Electron Transfer:  The electron moves from Cr(II) to Co(III) through the bridging chloride ligand. Bridge Decomposition:  The products dissociate. The key evidence is that the radioactive chloride ( Cl ⁻) originally bound to Co is found in the coordination sphere of the Cr product, [Cr(H₂O)₅* Cl ]²⁺. Kinetics:  The rate law often reflects the initial substitution step. For the reaction above, it would be: Rate = k [Co(NH₃)₅Cl²⁺] [Cr(H₂O)₆²⁺]

17. Example

18. B. Outer-Sphere Electron Transfer The coordination spheres of the oxidant and reductant remain intact during the electron transfer, as shown in the following reaction: Prerequisites:  At least one complex must be substitutionally inert (e.g., Co(III), Cr(III)) to prevent an inner-sphere pathway. The other can be labile (e.g., Co(II), Cr(II)). The reactants are considered as hard charged spheres, and an electrostatic approach can be used to anticipate the rates of these reactions. Kinetics:  The rate law can be complex but generally depends on the concentrations of both reactants.

19. 2 . THE REACTION ORDER Is the sum of the powers of the concentration terms in the rate laws. It provide information about which species are involved in the rate determining step(RDS)

20. 3. Activation energy The minimum energy that the colliding molecules must posses to react. It is derived from the Arrhenius equation:

21. The Transition State Theory (TST) Basic idea: A chemical reaction proceeds through formation of a transition state (activated complex) , denoted as [AB] ǂ

22. The rate of reaction depends on how many activated complexes are formed and how quickly they proceed to products. The equilibrium between reactants and the transition state gives rise to the Eyring equation .   DERIVATION OF EYRING RELATIONSHIP According to TST, the rate constant (k) is given by:

23. LINEAR (EYRING) PLOT Taking natural logs:

24.   INTERPRETATION OF ACTIVATION PARAMETERS (a) Activation Enthalpy (ΔH‡) Indicates energy barrier height. High ΔH‡ → slow reaction (requires more energy to reach transition state). Low ΔH‡ → fast reaction. Example: For [Co(NH₃)₆]³⁺ substitution, ΔH‡ ≈ 100 kJ/ mol → very inert. For [Ni(H₂O)₆]²⁺ substitution, ΔH‡ ≈ 30 kJ/ mol → more labile ΔS‡ Interpretation Mechanistic Implication Negative (–) Transition state more ordered Associative (A) mechanism (ligands crowd around metal) Positive (+) Transition state less ordered Dissociative (D) mechanism (ligand leaves first) ~0 Minimal change Interchange (I) mechanism (b) Activation Entropy (ΔS‡) Indicates degree of order change when forming transition state.

25. APPLICATION IN BIOINIORGANIC CHEMISTRY

26. STRUCTURE -REACTION CORRELATIONS THE TRANS EFFECT Refers to the tendency of an already attached group to direct the incoming ligand to its trans- position in ligand displacement reactions in square-planar complexes. Different ligands have different trans-directing effects and they are arranged in increasing order of their trans effect. CN - > C 2 H 4 >CO> NO> SCN - > I - > Br - > Cl - > Py >NH3> OH - > H 2 O

27. TYPES OF TRANS EFFECT REACTIONS Kinetic Trans Effect (KTE ) This effect describes how a ligand can accelerate the rate of substitution of another ligand located opposite to it in a  square planar  complexes The ligand with a strong kinetic trans effect stabilizes the transition state of the reaction by accepting electron density from the incoming nucleophile through  backbonding .   Ligands like carbon monoxide (CO),  Cyanide (CN-) and  Ethylene (C2H4) are strong kinetic trans effect ligands. Thermodynamic Trans Effect (Trans Influence) This is a thermodynamic factor that impacts the ground state of a metal complex.    Ligands with a strong trans-influence weaken the M-L bond trans to them in the ground state. This weakening makes the trans-positioned ligand more labile (prone to substitution).  Strong σ-donors like hydride (H⁻), iodide (I⁻), and alkyl/aryl groups (like CH₃⁻ or Ph ⁻) have a significant trans-influence.  APPLICATION OF THE TRANS EFFECT Synthesis of cis and trans platin Synthesis of the isomers of [ Pt ( Cl )(NH3)(Br)( Py )] Differenciating between cis and trans isomers of [PtCl2(NH3)2]  

28. Nucleophilicity Nucleophilicity refers to the capacity of a Lewis base to participate in a nucleophilic substitution reaction  Lewis base is a chemical species, like a molecule or ion, that can donate a pair of electrons to form a coordinate covalent bond with a Lewis acid . Factors affecting Nucleophilicity Charge: Negatively charged species are usually more  nucleophilic   than their neutral counterparts , ie , OH⁻ (hydroxide) is more nucleophilic  than H₂O (water). Basicity:  Stronger bases tend to be better nucleophiles. For example, NH₃ (ammonia) is a better nucleophile than H₂O. Solvent : In polar  protic  solvents, nucleophiles can be hindered because  these solvents stabilize nucleophiles through hydrogen bonding. In polar aprotic solvents, nucleophiles are less hindered and can react  more readily . Steric hindrance:  Bulkier nucleophiles can be less reactive due to  steric hindrance. For example, tertbutoxide  ( tBuO ⁻) is less  nucleophilic   than ethoxide  ( EtO ⁻)

29. LABILITY AND INERTNESS Definitions: Labile complexes are metal complexes in which the rate of ligand displacement reactions is very fast and hence 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 hence show slow reactivity. this property is called inertness

30. kinetic and thermodynamic stability Metal complexes Recall: in CHE413 stability of the metal complexes depends on 1. nature of the central metal atom 2. nature of the ligands 3. reaction conditions The stability of metal complexes is discussed under two main types Thermodynamic stability Kinetic stability Thermodynamic stability Is the measure of the extent in which the complex will form or will be transformed into another complex when the system is at equilibrium Thermodynamic stability depends on the energy difference of the reactant and product . if the complex product is lower in energy than the reactant, then it is thermodynamically stable and if the complex product is higher in energy than the reactant, then it is thermodynamically unstable Thermodynamically stable product may be labile or inert.

31. Kinetic stability Refers to the speed at which the transformation of reactant complex to product complex take place. Kinetic stability of metal complexes depend on the activation energy of the reaction . activation energy is the minimum energy reactant molecule must overcome to form product If the activation energy barrier is high, substances will react slowly and will be called kinetically stabilized or inert I f activation energy barrier is low, substances will react fast and it will be call kinetically unstable or labile . Note : The re is no correlation between thermodynamic stability and kinetic stability of metal complexes

32. 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 The M─L bond are weaker and long VTB proposed that bonds 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 ., [Fe(H 2 O ) 6 ] +2 and [MnCl 6 ] -3 2. Inner orbital complexes Have d 2 sp 3 hybridization The M─L bonds are stronger and short Generally d 2 sp 3 hybridization form inert complexes The d 2 sp 3 hybrid orbitals are filled with six electrons pairs donated by ligand and the d n electron of metal will occupy d xy , d xz , d yz . e g ., [Cr(CN) 6 ] -3 In some case the d 2 sp 3 form 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 .

33. Labile and inert metal complexes on the basis of Crystal Field Theory (CFT ) According to crystal field theory , 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 and due to this change in crystal field symmetry the CFSE value also changes. T he cases for lability and inertness If the CFSE value for the five or seven-membered intermediate complex is grater than of the reactant, the complex will be of labile nature . 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 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 formula for calculating CFSE Crystal field stabilization energy = CFSE of intermediate – CFSE of reactant

34. Examples 1. C alculate the CFSE of d 2 configuration that have octahedral reactant CFSE of -8 and octahedral intermediate CFSE of -9.14 andDetermine its kinetic stability? Factors that affect 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 help tightly by the metal ion. Ex: [Cs(H 2 O) 6 ] + < [ Rb (H 2 O) 6 ] + Charge on the 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 ] - 3. Geometry tetrahedral and square planar – labile octahedral complexes – either labile or inert Thank you