Stability Of Complexes

Question 1

When is deltaG negative during ligand exchange

Answer 1

1. If a ligand displacing another ligand has a high K, deltaG = negative = spontaneous
2. Also if the ligand-acceptor bond is much stronger

Question 2

Explain the concept of stepwise stability constants (Kn)

Answer 2

1. Each time the same ligand substitutes a different ligand from a complex, up to six equilibrium constants exist.

E.g. mono substituted, disubstituted, tri substituted —>

Question 3

How to calculate the overall stability constant and what it means

Answer 3

1. Equals the product of stepwise stability constants e.g. B2 = K1 x K2
2. Can be generalised to [MLn] / [M][L]^n when the general equation is M + nL <—> MLn
3. Large B indicates concentration of complex&raquo_space; concentration of constituents, so delta G = 0 for the reaction i.e. ligand substitution favoured
4. B is usually big so logBn used often
5. If logB > 0 (B>1), we know deltaG is negative and vice versa

Question 4

What factors can influence the likelihood of spontaneous reactions

Answer 4

1. Statistical factor - successive substitutions limit the number of places the ligand can further substitute
2. Steroids - bulky ligands and hinder approach of an attacking ligand
3. Electrostatics - adding charged ligands can reduce or increase the charge on the whole complex, which will influence attraction or repulsion e.g. adding anionic ligands to a -ve charged complex = less favourable

Question 5

How is chelation linked to ligand substitution

Answer 5

1. Chelating ligands aka polydentate ligands that form rings around acceptor e.g. ethylenediamine, EDTA form more stable complexes compared to monodentate ligands
2. Higher stability constants, K and B values
3. The increase in entropy from reactants to products for chelating agents result in the lower deltaG and higher stability i.e. only 1 chelating ligand needed to add to complex compared to 2 monodentate ligands +complex. And three molecules produced in both reactions so there’s an increase in 1 molecule for chelation reaction

Question 6

Macrocyclic effect explained

Answer 6

1. When there are 3 or more donor atoms in a ring of at least 9 atoms
2. Macrocyclic ligand containing complexes more stable than acyclic ligand containing complex
3. Macrocyclic effects increase stability by both entropy or enthalpy, not usually both

Question 7

Why is the macrocyclic effect favourable

Answer 7

Due to steric favourability, meaning bonding optimisation and therefore a large negative enthalpy change