第五章 反应机理

• The rate-determining step can be any of bridge formation, electron transfer, or bridge breaking • It is common for electron transfer to be the ratedetermining step
一般指配体取代反应，偶见金属取代反应 L5M-X + Y L5M-Y + X
X: 离去配体 (leaving ligand) Y: 进入配体 (incoming ligand) L: 不变配体 (unchanged ligand)
八面体配合物的取代反应
1、离解机理（Dissociation, D, SN1） L5M-X = L5M + X（慢） L5M + Y = L5M-Y 速率方程：d[L5M-Y]/dt = k[L5M-X] 速率与Y浓度无关，是对[L5M-X]的一级反应。 2、缔合机理（Association, A, SN2） L5M-X + Y = L5MXY (慢) L5MXY = L5M-Y + X d[L5M-Y]/dt = k[L5M-X][Y] 属于二级反应。 3、交换机理（Interchange, Ia or Id） 介于D机理和A机理两种极端之间
• 内界机理 inner-sphere mechanism
– In some reactions, the electron transfer can only occur by the transfer of a ligand from reducing agent to the oxidizing agent
ligand
Low value: poor bridging ligand
Inner-sphere mechanism
Thiocyanate as the bridging ligand: NCS- can bond through N or S to metal [Co(NH3)5(NCS-S)]2+ + [Cr(H2O)6]2+ 70% [Cr(H2O)5(NCS-N)]2+ 30% [Cr(H2O)5(NCS-S)]2+
summary points:
• If the redox reaction is faster than the ligand substitution, then the reaction has an outer-sphere mechanism • When the reaction involves ligand transfer, the mechanism will be an inner-sphere mechanism • When the products and reactants are labile, it is difficult to determine if inner- or outer-sphere mechanism
The electron transfer must proceed by a mechanism involving transfer between the two complex ions in outersphere contact Electron transfer is by tunnelling / outer sphere mechanism
过渡态
Ea
反应物 H 产物 反应坐标
经验规则
注意：Cr3+, Co3+多数惰性, Cr2+(HS), Co2+, Ni2+ 多数活性
平面四方形配合物的取代反应
An illustration of the importance of solvent on the substitution pathways for square planar reaction centres
N
C
N
N
N N
N C
N
S
and some heterocyclic ligands………….
Heterocyclic bridging ligands:
M
N
NLeabharlann Baidu
M
pyrazine
M
N
N
M
4,4'-bipyridine
Inner-sphere mechanism
The steps are: 1 Bridge formation 2 Electron transfer across the bridge 3 Bridge cleavage
Second order rate constants for:
[Co(NH3)X]2+ + [Cr(H2O)6]2+ + 5[H3O]+ [Co(H2O)6]2+ + [Cr(H2O)5X]2+ + 5[NH4]+
with different bridging X ligands
X FClBrI[N3][OH]H 2O k / dm3 mol-1 s-1 2.5 x 105 Increase for halides 5 6.0 x 10 1.4 x 106 I- is the best bridging 3.0 x 106 3.0 x 105 1.5 x 106 0.1
The Outer Sphere Mechanism
When both the species in the redox reaction are kinetically inert (non-labile ligands), ligand substitution cannot take place on the very short time scale of the redox reaction. Non-labile: e.g. Low-spin d6 metal ion such as Co(III)
Inner-sphere electron transfer
内层电子转移 ( inner –sphere electron transfer)
这类过程的特征是，氧化值发生 变化的原子周围的配位层发生变化. 有代表性的例子是在放射性氯离子 *Cl–存在条件下以 Cr(Ⅱ) 还原 Co(Ⅲ) 的配合物
Henry Taube
The Nobel Prize in Chemistry 1983
"for his work on the mechanisms of electron transfer reactions, especially in metal complexes"
http://www.nobel.se/chemistry/laureates/1983/index.html
Inner-sphere electron transfer
The reaction :
[Co(NH3)5Cl]
2+
+
[Cr(H2O)6]
2+
H3O+ [Cr(H2O)5Cl]2+ + 5[NH4]+
[Co(H2O)6]2+ +
was one of the first (in 1953 by Taube) to be identified as an inner-sphere process. Tracer experiments showed that all Cl in the product had come from the Cl in the reactant - excess unlabelled Cl- present in solution was not taken into the product
从产物分析推测反应机理
[CoAX(en)2]+离解机理的两种中间体
A = NO2-, X = Cl-
A = OH-, X = Cl-
活性(labile)与惰性(inert)配合物
配体可被快速取代的配合物，称为活性配合物；配体取代缓慢的配合物， 称为惰性配合物 活性与惰性是动力学上的概念，不可与稳定性混为一谈。
The intermediate has a bridging Cl ligand.
H3N H3N
NH3 Co Cl Cr H2O NH3
NH3
OH2
4+ OH2 OH2
OH2
The Cl- ion is a good bridging ligand as it has more than one pair of electrons, and so can form bonds to each of the metal centres simultaneously. Other good bridging ligands include SCN-, N2, N3-, and CN-
反位效应顺序
H2O < OH− < F− ≈ RNH2 ≈ py ≈ NH3 < Cl− < Br− < SCN− ≈ I− ≈ NO2− ≈ C6H5− < SC(NH2)2 ≈ CH3− < NO ≈ H− ≈ PR3 < C2H4 ≈ CN− ≈ CO
反 位 效 应 的 应 用一
( )
反 位 效 应 的 应 用二
[PtBrCl(Py)(NH3)] 三种异构体的制备
( )
Py
与电负性有关
5.2 氧化还原反应 (Oxidation/Reduction, Redox)
• 外界机理 outer-sphere mechanism
– The oxidizing and reducing centres can react with or without a change in their coordination spheres – ONLY transfer of electrons
Outer-sphere electron transfer
外层电子转移 (outer – sphere electron trasfer)
这类过程的特点，氧化值 发生变化的原子周围的配位 层大体不变. Fe(Ⅱ) 配合物分 子与 Ir(Ⅳ) 配合物分子彼此接 近到一定程度后电子由 Fe(Ⅱ) 跳至 Ir(Ⅳ)，并伴随 M-L 键 长的微小变化.
Why?
Two different bridge structures form during the reaction:
(H3N)5Co
Break old bond
SCN
Cr(OH2)5
4+
(H3N)5Co
Break old bond
Make new bond 4+
S C N
Cr(OH2)5
Make new bond
第五章 配合物的反应机理
Reaction Mechanisms of Coordination Compounds
5.1 取代反应 Substitution reactions 5.2 氧化还原反应 Redox reactions
异构化 加成/消除 配体反应 (配位催化)
5.1 取代反应(Substitution Reaction)