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Substitution reactions at octahedral
complexes:
acid and base hydrolysis
Acid hydrolysis
• Protonation of a ligand which then dissociates from the metal
See Questions 1-3
Acid hydrolysis
• Protonation of an inert ligand labilises other ligands
N
Electron rich metal
N
R 3P
PR3
Mo
R 3P
PR3
PR3
PR3 stabilised by π back
bonding from electron rich
metal ion
H
H
2+
H
H
2+
H
H
N
N
N
N
N
N
N
N
R3P
PR3
2H+
R3P
Mo
R3P
Mo
PR3
PR3
PR3
R3P
PR3
PR3
MeOH
R3P
PR3
Mo
R3P
PR3
OHMe
-H+
R3P
+
PR3
Mo
R3P
PR3
OMe
Acid hydrolysis
• Protonation of the metal itself leads to labilisation of a ligand
CO
OC
Fe
CO
H+
OC
CO
OC
H
CO
+
CO
13
CO
O13C
CO
CO
Fe
Fe
CO
CO
+
CO
OC
+ CO
-H+
O13C
Fe
H
CO
CO
CO
CO
Base hydrolysis at a transition metal complex is based on the
observation that reactions of the type are accelerated by increasing [OH-]
[ML4(LH)(X)]n+ + OH-  [ML4(LH)(OH)]n+ + XThe experimental rate law is
d [P]
 kobs [ML 4 (LH)(X)][OH - ]
dt
and the saturation behaviour seen with acid-catalysed reactions (Chapter
2) does not occur
Mechanism proposed by Garrick and called the DCB Mechanism
Dissociative
Conjugate Base
n+
[M(NH 3 )5 X]
K

 [M(NH 3 ) 4 (NH 2 )X](n-1)+ + H 2O
+ OH 

k
[M(NH 3 ) 4 (NH 2 )X](n-1)+ 
 [M(NH 3 ) 4 (NH 2 )]n+ + X 
fast
[M(NH 3 ) 4 (NH 2 )]n+ + H 2O 
[M(NH 3 )5 (OH)]n+
d [ P]
 k[M(NH3 ) 4 (NH 2 )X]n 
dt
Now
[M(NH3 )5 X]0n   [M(NH3 )5 X]tn   [M(NH3 ) 4 (NH 2 )X]( n 1) 
(1)
But
[M(NH 3 ) 4 (NH 2 )X](n-1)+
K
[M(NH 3 )5 X]n+
[OH
]
t
(n-1)+
[M(NH
)
(NH
)X]
(n-1)+
3 4
2
[M(NH 3 )5 X]0n+ 

[M(NH
)
(NH
)X]
3
4
2
K [OH - ]


1
(n-1)+
 1 
[M(NH
)
(NH
)X]
3 4
2
- 
K
[OH
]


[M(NH3 )4 (NH 2 )X]
(n-1)+
K [OH- ][M(NH3 )5 X]0n+

1 + K [OH- ]
Since
K [OH - ] << 1
[M(NH3 ) 4 (NH 2 )X](n-1)+  K [OH - ][M(NH3 )5 X]0n+
Substituting into (1)
d [ P]
 kK [OH - ][M(NH 3 )5 X]0n+
dt
 kobs  kK
Base hydrolysis is usually considerably faster than acid hydrolysis
because of stabilisation of the transition state by the conjugate base.
2+
NH3
NH3
Cl
Co
OH
-
NH2-
Cl
Co
NH3
H3N
NH3
H3N
+
NH3
NH3
NH3
-
+
NH3
Cl
NH2Co
NH3
H3N
NH3
Hence a  donor ligand such as OH- or NH2- is required for this
mechanism