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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.。
基元反应活化能
有机合成反应的活化能是影响反应速率的一个重要因素，有机合成反应的基元反应激活能是指光、热、电和前驱体给予基元反应的额外能量，使反应中的物质达到活性点，从而使反应变得更快，有机化学中商用反应活化能分为舍伍德定律（费米动力学）和 hans-meiser表达式（统计动力学）。
舍伍德定律关于活化能的概念是：在激活反应基元之前，物质必须获得的激活能量等于反应的机械难度乘以一个常量，即活化能。
舍伍德定律是激活反应基元前提下的动力学表示，反应的机械难度取决于基元的原子的结合及其之间的距离，这些因素定义了反应所要求的激活能量。
hans-meiser表达式提出，在统计动力学中，基元反应的激活能=在低温控制下，反应所要求的激活能是物质跃迁消耗的能量，它和物质状态有关，表示反应物质正反应末端由熵变化所决定的能量大小。
基元反应的激活能主要影响反应的活性和速度，激活能越小，反应速度就越快，反之，反应活性就越大，因此，精确控制反应活化能对于有机合成反应是十分重要的。
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.。
