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Introduction to Inorganic Reaction Mechanisms

Scope of the course

  • Limited to solution reactions

  • "classical inorganic" reactions which occur at metal centres in coordination complexes

    (mechanisms of organometallic reactions given in 3rd year Lecture course)

    Elementary Reaction Kinetics and Mechanism

    Transition State Theory

    Note: only applies when transition state in equilibrium with the reactants

    does not apply in reactions which are at diffusion controlled limit

    Therefore: fast reactions are favoured by

    Note: since the rate is exponentially dependent on both S and Eact, small rate changes are "not significant" for the interpretation of mechanism with these very crude theories.

    Effects of Pressure on the Rate of Reactions

    Measurements of Rates of Reactions

  • "inert" species i.e. t1/2 > 1 min

    We can use classical static techniques, e.g. light absorption, pH measurements

  • "labile" species: i.e. t1/2 ca. 1 min-1ms

    Use stop flow measurements, rapid mixing, fast spectroscopy

  • "rapid"reactions: -relaxation techniques + fast spectrophotometry

    General Comments:

    We cannot conclude a mechanism from a rate law !

    e.g.

    (i) involvement of solvent (pseudo-first order behaviour)

    (ii) complex reactions with only one rate limiting step.

    Finally remember you can only disprove a mechanism you can never prove a mechanism.

    A rate law can at best only be consistent with a mechanistic scheme, it can never prove it.

    Definitions:

    (1) "Lability" and "Inert" are kinetic terms !!

    (2) "Stable" and "Unstable" are thermodynamic statements

    (3) "Intimate" Mechanism refers to the details of the mechanism on the molecular scale.