The Principle of Microscopic Reversibility can be stated in several different ways:
If a certain series of steps constitutes the mechanism of a forward reaction, the mechanism of the reverse reaction (under the same conditions) is given by the same steps traversed backwards.
(Note: The phrase "under the same conditions" means that this applies only to thermal reactions, not photochemical ones.)
The sequence of transition states and reactive intermediates in the mechanism of a reversible reaction must be the same, but in reverse order, for the backward reaction as for the forward reaction
If the mechanism in one direction is known, then the mechanism in the opposite direction is known.
The lowest-energy pathway in the forward direction will be the lowest-energy pathway in the reverse direction.
Let's illustrate this with an example of a generic organometallic complex, LnM(A)(B) undergoing reductive elimination of AB:
In the example shown above, the activation energy, f in the forward direction is 20 kcal/mol. If we wish to do the reverse reaction, oxidative addition of AB to LnM, then we have to surmount a higher barrier with a r of 30 kcal/mol. This does not mean that the reverse reaction does not or can not occur, only that it is thermodynamically disfavored.
In other cases, the relative energies of the products and reactants might make the forward activation barrier higher in energy or even equal to that of the reverse reaction:
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f in the forward direction is 20 kcal/mol. If we wish to do the reverse reaction, oxidative addition of AB to LnM, then we have to surmount a higher barrier with a 
