transition metals part 2

what are the two types of limiting mechanisms?

dissociative and associative

what are dissociative mechanisms?

intermediate with lower coordination umber

similar to sn1

what are associative mechanisms?

intermediate has higher coordination number

similar to sn2

what is the most common substitution mechanism?

interchange

what is an interchange mechanism?

concurrent bond breaking and formation via transition state

what is I_a? what is I_d?

does bond formation or breakage dominate?

I_a = bond formation dominates over bond breakage

I_d = bond breakage dominates

which mechanism is most common for square planar?

associative

what type of mechanism is this?

interchange

bond is formed at same time as bond broken

what is the formation constant, K_f?

equilibrium constant

gives interaction strength of ligand to the metal relative to that of solvent

what does higher K_f mean in terms of stability?

higher = more thermodynamically stable complex

what is the general trend for stepwise formation constants?

decreases with each successive ligand exchange

what is the overall formation constant β_n?

product of equilibrium constants for stepwise reactions

multiply K_f’s together

what is overall equation for β_n?

what is the chelate effect?

complex with one or more 5 or 6 membered chelate rings is more stable (higher β) than similar complex with no chelate rings

considering Gibbs free energy, why is the chelating complex more stable?

• ΔH ?

• ΔS ?

• ΔH similar, as both involve breaking 6 Ni-OH₂ bonds and forming 6 Ni-N bonds

• gain in entropy favourable for cheating (more ways energy can be distributed)

entropy change of this reaction?

no change in entropy

entropy change of this reaction?

there is an increase in entropy

how does chelate effect change as number of members of ring increases?

greatest for 5 and 6 membered rings

marginal effect for 7

none for larger - entropy becomes negative as ligand loses degrees of freedom

why do smaller chelate rings not show chelate effect?

have steric strain (enthalpy unfavourable)

what does stable mean in terms of equilibrium?

K_f?

tendency to exist under equilibrium

K_f > 1

what does unstable mean in terms of equilibrium?

K_f?

tendency to not exist under equilibrium

K_f < 1

what does labile mean in terms of kinetics?

species that reacts rapidly

t<1min

what does inert mean in terms of kinetics?

species than react slowly

t>1min

what are general rules for lability?

most first row are labile

Cu(ii) and high spin Cr(ii) are extremely labile

Cr(iii), Co(iii) and low spin Fe(ii) are inert

some d⁸ and d⁶ 2nd and 3rd row metals are very inert

are stability and kinetics related?

no

what does it mean if the incoming ligand doesn't have a large effect on kinetics?

dissociative

kinetics depend on breaking M-L bond

what does it mean if incoming ligand has a large effect on kinetics?

associative mechanism

what does associative mechanism need?

what orbital is this for octahedral?

available d orbital on metal

t_2g for octahedral

what influences lability (3)?

charge density

Jahn-teller

CFSE

how does charge density affect lability?

increase in oxidation states reduce rate of exchange

how does Jahn-teller affect lability?

elongation of 1 axis for high spin Cr²⁺ and Cu²⁺ (d⁹) accounts for rapid kinetics

how does CFSE affect lability?

kinetically inert complexes have large CFSE

• electron count and Δ are important

what is crystal field activation energy?

CFSE changes as geometry changes

ligand exchange intermediate/transition state have different geometry

what are the crystal field splitting diagrams?

for octahedral, what configuration has the most negative CFSE?

what does this mean about ligand exchange?

low spin d⁶

CFSE is most affected by ligand exchange

what signifies inert complex (Δ and CFAE?)

large Δoct and positive CFAE

• less stabilisation of intermediate compared to reactant