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A ligand substitution can occur either by an associativeor dissociative route.In a dissociative substitution, one of the existing ligandson the metal center has to fall off (ligand dissociation),LnM+ :L'' --------> LnM-L"fastthis opens up a free coordination site to which the newligand can coordinate.S‡is significantly positive, the mechanism is almost certainlydissociative.If the rate does not depend on [L], the mechanism is almost certainlydissociative.axial site.The 14e-three coordinate intermediate is actually almost immediately coordinated by a solvent molecule to produce theConsider the following two substitution reactions.the most stable productSince we are starting with an 18e-complex, we have to proceed by adissociative substitution reaction. The phosphite ligand is the more weaklycoordinated and somewhat bulky due to the t-butyl groups. Thus it is the mostlikely ligand to fall off first.The P(OMe)3ligand has about the same σ-donorability as pyridine, but is a considerably better acceptor ligand, thus completing with the trans* Rate depends on both LnM-L' and L"ΔS‡is significantly negative, the mechanism is almost certainlyAssociative SubstitutionsThese occur first by a ligand addition to the metal complexfollowed by the dissociation of one of the original ligands.typically need to have an unsaturated(17e-or lower) complex in order topropose an associative substitution mechanism.there are very few verified examples of this in theliterature.Since we are starting with an 16e-Ni complex without any stericproblems we can to proceed by an associative substitutionreaction. Once the CO ligand coordinates, the weakest ligandthat should fall off is the THF. Late transition metals have a farPentadienylThe pentadienyl ligand is an acyclic version of Cp that does not have any aromatic stabilization. This has two important effects:Consider the following two reactions:Reaction a) is much faster than reaction b). Discuss why this is so.NitrosylThis can occasionally lead to interesting behavior where the linearIn general the stereochemistry of a square-planarcomplex is retained through the substitution reaction.。
A ligand substitution can occur either by an associative
or dissociative route.
In a dissociative substitution, one of the existing ligands
on the metal center has to fall off (ligand dissociation),
LnM+ :L'' --------> LnM-L"fast
this opens up a free coordination site to which the new
ligand can coordinate.
S‡is significantly positive, the mechanism is almost certainly
dissociative.
If the rate does not depend on [L], the mechanism is almost certainly
dissociative.
axial site.
The 14e-three coordinate intermediate is actually almost immediately coordinated by a solvent molecule to produce the
Consider the following two substitution reactions.
the most stable product
Since we are starting with an 18e-complex, we have to proceed by a
dissociative substitution reaction. The phosphite ligand is the more weakly
coordinated and somewhat bulky due to the t-butyl groups. Thus it is the most
likely ligand to fall off first.
The P(OMe)
3ligand has about the same σ-donor
ability as pyridine, but is a considerably better acceptor ligand, thus completing with the trans
* Rate depends on both LnM-L' and L"
ΔS‡is significantly negative, the mechanism is almost certainly
Associative Substitutions
These occur first by a ligand addition to the metal complex
followed by the dissociation of one of the original ligands.
typically need to have an unsaturated(17e-or lower) complex in order to
propose an associative substitution mechanism.
there are very few verified examples of this in the
literature.
Since we are starting with an 16e-Ni complex without any steric
problems we can to proceed by an associative substitution
reaction. Once the CO ligand coordinates, the weakest ligand
that should fall off is the THF. Late transition metals have a far
Pentadienyl
The pentadienyl ligand is an acyclic version of Cp that does not have any aromatic stabilization. This has two important effects:
Consider the following two reactions:
Reaction a) is much faster than reaction b). Discuss why this is so.
Nitrosyl
This can occasionally lead to interesting behavior where the linear
In general the stereochemistry of a square-planar
complex is retained through the substitution reaction.。
