topic 10 - ligand exchange and stability in d-block complexes

reactions with acid

• most d-block metals react with acids to liberate H₂ and form a metal salt

• this involves the oxidation of the metal

• the process is favourable with a negative delta G

• can also be seen as the reduction of the metal ion is unfavourable - putting a negative standard electrode potential in the equation (delta G = -nFE^o) leads to a positive delta G

delta G

= -nFE^o

• n = the number of electrons transferred per mole of reaction

• F = Faraday’s constant: 96,485 C mol^-1

a thermodynamically favourable process requires E^o > 0 and delta G < 0

standard electrode potentials

• under standard conditions - standard states, 1atm, solutes at unit activity

• always reported as reduction ie. forming the solid metal

• values are relative to the reduction of hydrogen ions 2H⁺ + 2e^- (eqm) H₂

• standard electrode potentials aren’t per mole so can’t be used in the same way as delta g when adding of subtracting equations

• _{(when working with redox reactions its safer to work in delta g to calculate whether a reaction is favourable)}

calculating K → delta G = ?

delta G = -(RT)lnK

• R = 8.314 J K-1 mol-1

• T = temperature in kelvin 298K

• lnK = natural log (base e) of k

a thermodynamically favourable process requires K > 1

reaction with O₂, S [S₈], X₂

• d-block metals are reactive (usually at elevated temps) and readily produce binary compounds with O₂, S [S₈], X₂

• stereochemistry depends on the available oxidation states

• disproportion reactions are also possible

E^o(Cu⁺/Cu) = 0.52V

E^o(Cu²⁺/Cu) = 0.34V

calculate E for: Cu²⁺ + e^- → Cu⁺

delta G = -0.68F - (-0.52F) = -0.16F

• E^o = -deltaG/nF = +0.16V

hydrolysis of ions in solution

• dissolution of metal-salts e.g. MX_n , in water produces complex ions ligated by H₂O molecules

• strongly polarising metal-aqua cations (eg Cr³⁺ (high charge density)water complexes) readily lose a proton to form an acidic solution and a metal-hydroxide species

• as metal cation charge increases (above 4 ish) it becomes even more polarizing and two protons are lost to form an oxo (O^2-) ligand

• hydroxide and oxo ligands can lead to hydroxy or oxo bridged species with higher nucleophilicity (shown as (mu-OH)₂)

stability of M²⁺ ions to oxidation

• determining whether an ion will be oxidised or not cannot be done just by looking at electrode potentials - need to calculate delta G

why can Cu aqua complexes not displace all water ligands with ammonia

• only 4 of the water ligands can be displaced as the gibbs free energy value for the 5th ammonia ligand is > 0 due to Jahn Teller effects

equilibrium stability constants

K₁ represents the relative stability of the new complex

K₂ = ? (Step wise stability constant)

K₆ = ? (Stepwise stability constant)

Overall stability/formation constant = ?

beta₆ = ?

• overall stability constant

• K₁ x K₂ x K₃ x K₄ x K₅ x K₆

beta₄ = ?

• K₁ x K₂ x K₃ x K₄

log beta₃ = ?

logK₁ + logK₂ + logK₃

trend in stepwise stability constants

• decrease as the constant number increases -based on probability

• 1st binding constant is higher bc whichever ligand is displaced the desired product will be formed but with the 2nd, 1/6 times you will end up back at the starting reactant rather than the desired product with two ligands substituted

• K₁ > K₂ > K3 etc…

what causes deviation to the typical trend for stepwise stability constants?

1. changes in geometry (notably oct to tet)

2. for Cu^II (d⁹ Jahn Teller effects) 5th and 6th won’t go on

deltaG₁⁰ = ?

= -(RT)lnK₁

_{the K in the original equation is equal to beta6}

deltaG_beta2 ⁰ = ?

= -(RT)lnK(beta₂)

effects of CFSE on the kinetics of ligand exchange

• to exchange a ligand an oct metal complex must go via a 5 or 7 coordinate intermediate

• the crystal field splitting in 5 and 7 coordinate is very small so the CFSE in the intermediate is very small

• 1st row TM’s tend to go via 5 coordinate (dissociative mechanism) as they’re very small - hard to fit 7 ligands around the metal

• 2nd and 3rd row TM’s can do either (associative or dissociative) as they’re larger

crystal field activation energy

• if the CFSE is very high in the starting complex then there is a big loss of CFSE in the intermediate, and this is an energy barrier to ligand exchange

• CFAE is very large for d⁶ low spin complexes (Fe²⁺, Co³⁺) which means they are particularly kinetically inert towards ligand exchange

• Zn²⁺ (d¹⁰) has no CFAE and exchanges ligands very rapidly - kinetically liable - ideal for catalysis

• d⁶ low spin has the highest CFSE

what is the chelate effect

• when k1 for a bidentate ligand > beta2 for a similar monodentate ligand eg. en vs NH3

• two bonds are formed in each case but the formation of a chelate complex is more thermodynamically favourable

explaining the chelate effect

general

• when the first bond is broken on a chelating ligand the ligand is still held near the metal and so will re-bond

• the bidentate ligand is held more tightly and so more difficult to remove

entropy

• delta G = delta H - T.deltaS

• the stability of chelates relates to entropy ie. disorder in the system

• the number of free molecules in the system is increased by forming the chelating system - 7 molecules rather than 4

• entropy always favours the chelate complex

• enthalpy is related to bond strength and can favour or disfavour the chelate

lone pairs

• enthalpy may favour the chelate complex if both bonds are Ni-N amine as bonding the NH3 ligands created new lone pair repulsions where as with the chelate ligand the repulsions are already built in (in part)

effect of chelate ring size

• 5 membered > 6 membered > anything else

• 5 membered rings are the ideal and most stable

• bigger than 6 means the binding atoms are too far away- compete with bulk solvent

• exception to rule: acac - pseudo-aromatic 6 membered ring - very stable

the macrocyclic effect (not examined)

• you get a further enhancement in binding if you join al the donors up in a ring as it means the ligands are still anchored in even if any bond breaks