Chemistry Reference
In-Depth Information
Whereas reactions of metal ions like Cu(II), Co(II), Mn(II) and Zn(II) are fast (these being
labile
complexes), metals ions like Ni(II) react somewhat slower, and Pt(II) much slower.
Metals like Co(III), Rh(III), Cr(III) and Pt(II) are
inert
, and to make their reactions occur
in a reasonable timeframe, it is often necessary to heat the reaction mixture. This is helpful
because reactions typically double in rate for approximately every five degrees rise in
temperature. An example is the inert red-purple [RhCl
6
]
3
−
ion, which reacts when boiled
in aqueous solution with the chelating ligand oxalate (C
2
O
4
2
−
) in approximately two hours
to form yellow [Rh(C
2
O
4
)
3
]
3
−
, whereas reaction at room temperature does not occur over
reasonable time periods (6.7).
K
3
[RhCl
6
] + 3 Na
2
(ox
2-
) K
3
[Rh(ox)
3
]
O
3-
heat;
hours
O
+ 6 NaCl
O
Rh
O
OO
O
(K
+
)
3
3-
Cl
Rh
(6.7)
O
Cl
Cl
O
O
(K
+
)
3
O
Cl
Cl
Cl
O
Most examples such as the ones above have involved total ligand exchange to introduce
just a single new type of ligand, driven usually by use of excess incoming ligand. This
does not mean that partial substitution cannot occur. Often, partial substitution is driven
by large differences in the rate of substitution of different ligands on the reacting complex
along with differing thermodynamic stability in products. For example, in (6.8) the reaction
effectively stops following substitution of the coordinated chloride ions, even with excess
oxalate ion, because the two 1,2-ethanediamine chelate ligands are strongly bound and not
readily substituted even by another chelating ligand.
cis
-[CoCl
2
(en)
2
]Cl + Na
2
(ox
2-
) [Co(en)
2
(ox)]Cl + 2 Nacl
+
+
NH
2
Co
NH
2
Co
O
H
2
N
O
H
2
N
Cl
(Cl
-
)
(6.8)
Cl
-
H
2
N
O
H
2
N
Cl
O
NH
2
NH
2
We have already seen an example earlier with the formation of [PtCl
2
(NH
3
)
2
], driven in
this case by low solubility of this neutral species allowing it to crystallize out of the reaction
mixture rather than continue reaction to form ionic [Pt(NH
3
)
4
]
2
+
. Another way to achieve
partial substitution is through, use of a stoichiometric amount of a reagent. For example,
reaction (6.9) may occur, where only one of two available coordinated chloride ions are
substituted because of the availability of only one molar equivalent of added cyanide anion.
cis
-[CoCl
2
(en)
2
]Cl + Na(CN) [CoCl(CN)(en)
2
]Cl + NaCl
one molar
equivalent only
+
+
NH
2
Co
NH
2
Co
H
2
N
Cl
H
2
N
Cl
Cl
-
(6.9)
Cl
-
H
2
N
Cl
H
2
N
CN
NH
2
NH
2
However, partial substitution is compromised in many cases by the formation of lesser
amounts of species with both greater and lower levels of substitution than the target complex.
This is a particularly common outcome with labile systems; for example, addition of two
molar equivalents of ammonia to Cu
2
+
aq
will not lead to only [Cu(NH
3
)
2
]
2
+
aq
, but also
