THERMODYNAMIC & KINETIC ASPECTS OF METAL COMPLEXES Inorganic Chemistry B.Sc. III Presentation By: Sitanshu Kumar

STABILITY Discussed under two main types:- Thermodynamic Stability Kinetic Stability Generally we talk about THERMODYNAMIC STABILITY, unless KINETIC or RATE OF FORMATION word is mentioned.

Refers to the faster rate of formation of the particular species. POINTS OF DIFFERENCE Thermodynamic Stability Relates to the time period of existence of a species in a particular form. Kinetic Stability Refers to the faster rate of formation of the particular species.

STABILITY CONSTANTS Higher the value of equilibrium constant for a reaction, more stable is the product formed. TYPES: Stepwise stability constant Overall stability constant

FORMATION OF MLn COMPLEX Two different ways: Stepwise formation M + L------> ML ; K1 = [ML]/[M][L] ML + L ----> ML2 ; K2 = [ML2]/[ML][L] . MLn-1 + L -----> MLn ; Kn = [MLn]/ [MLn-1][L] K1, K2, Kn are known as stepwise stability constants.

2. Overall formation: M + L------> ML ; β1 = [ML]/[M][L] M + 2L ----> ML2 ; β2 = [ML2]/[M][L]2 . M + nL -----> MLn ; βn = [MLn]/[M][L]n β1, β2, βn are known as overall stability constants.

Proceeding with rate constants, we get, β1 = K1 β2 = K1 . K2 βn = K1 . K2 . K3 ………. Kn Taking log, we get, log βn = log K1 + log K2 + log K3 +……. log Kn. Complex is stable if log β >= 8.

INERTNESS & LABILITY OF COMPLEXES Lability refers to reactivity and Inertness refers to non- reactivity of the complex. In other words, complex with ability to exchange its ligand(s) present inside co-ordination sphere is labile otherwise inert. Inert complexes have substitution reaction half life period larger than a minute & opposite for labile complexes.

FACTORS AFFECTING STABILITY OF COMPLEXES Nature of Metal Ion: +ve Charge on metal ion ↑, stability of complex↑ Size of metal ion ↓, stability of complex ↑ Charge:size ratio ↑, stability of complex ↑ more electronegative metal ions form complexes with ligand having donor atom of high electronegativity & vice versa.

2. Nature of ligand: -ve charge on ligand ↑, stability of complex ↑ Size of ligand ↓, stability of complex ↑ Basic strength of ligand ↑, stability of complex↑ Presence of chelating ligand makes the complex more stable. Ligand bulkiness↓ , stability of complex↑, because of less stearic hindrance.

SUBSTITUTION REACTIONS IN SQUARE PLANAR COMPLEXES • Associative - A (2 steps) MLnX + Y→ MLnXY→ MLnY + X • Dissociative - D (2 steps) MLnX + Y→ MLn + X + Y→ MLnY + X • Interchange (1 continuous process) MLnX + Y→ Y--MLn --X→ MLnY + X

rate of substitution of ligands opposite to it. TRANS EFFECT Definition The trans effect is best defined as the effect of a coordinated ligand upon the rate of substitution of ligands opposite to it. Or The ability of a ligand in a square planar complex to direct the replacement if the ligand trans to it. The trans effect is given as the following series: CN- > NO2- > I- = SCN- > Br- > Cl- > py > NH3 > H2O

Substitution reactions in square-planar complexes the trans effect In the above reaction, “T” is the ligand having good trans effect ; L’s are the inert ligands & “X” is the labile ligand.

MECHANISM OF Nu- SUBSTITUTION REACTION

THEORIES EXPLAINING TRANS EFFECT (∏-bonding) p-backbonding to a metal is weakened when it is trans to another good p-backbonding ligand. metal is strengthened when it is trans to a good s-donating ligand that can’t p-backbond .

Polarization theory:- This theory explains the weakening of metal-leaving group bond due to the more polarizable nature of Trans ligand, resulting in cleavage of the former bond and thus SUBSTITUTION. !!!!!!THANK YOU!!!!!! ∂+ T ∂- X ∂+ M ∂-