金属有机第四章基元反应1

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.