Chemistry Reference
In-Depth Information
slow addition to the reaction mixture, with stirring, of a water-miscible nonaqueous
solvent (such as ethanol) in which the product is only sparingly soluble, until the com-
mencement of precipitation;
addition of an excess of a different simple salt of an anion (often in the Na
+
or K
+
form)
that provides a high concentration of a different anion that forms a less soluble complex
salt, allowing its ready crystallization;
evaporation to dryness, appropriate where the product is sufficiently stable and unreacted
species (such as excess ammonia, for example) are removed by evaporation, with possibly
following recrystallization from a nonaqueous solvent in which the product is sparingly
soluble;
sublimation or distillation of the product following removal of solvent, which is not
usually applicable with ionic products, but finds limited use with some neutral complexes.
As an example of the value of anion exchange reactions, the product from reaction of
Ni(SO
4
) with pyridine in water, the complex cation [Ni(py)
4
](SO
4
), is highly soluble.
However, addition of excess sodium nitrite leads to ready precipitation of the much less
soluble [Ni(py)
4
](NO
2
)
2
complex; change of the counter ion alone has occurred (6.4).
[Ni(py)
4
](SO
4
) + Na(NO
2
) [Ni(py)
4
](NO
2
)
2
low solubility in water
high solubility in water
excess;
solution in
water
2+
(NO
2
-
)
2
2+
(SO
4
2-
)
(6.4)
N
N
N
N
N
N
Ni
Ni
ready
crystallization
N
N
Where the product of a reaction is neutral, as occurs when red anionic [PtCl
4
]
2
−
has
two chloride anions replaced by two neutral ammonia ligands to form yellow neutral
[PtCl
2
(NH
3
)
2
], the solubility of the product may be inherently lower than for an ionic
product, leading to its ready and selective precipitation from aqueous solution (6.5).
K
2
[PtCl
4
] + 2 NH
3
[PtCl
2
(NH
3
)
2
] + 2 KCl
low solubility in water
solution in
water
(6.5)
2-
Cl
NH
3
Cl
Cl
(K
+
)
2
Pt
Pt
Cl
NH
3
Cl
Cl
ready crystallization
Reaction (6.5) is also an example where substitution of a ligand other than coordinated
water is occurring, reminding us that ligand substitution does not inherently limit what the
leaving and entering group can be.
Indeed, there is no requirement that only one type of ligand can be replaced in the one
reaction; for example, purple [CoCl
3
(NH
3
)
3
] reacted with 1,2-ethanediamine (en) forms on
heating yellow [Co(en)
3
]
3
+
, where both chloride and ammonia ligands are substituted, the
reaction (6.6) driven by the high stability of the chelate complex formed:
[CoCl
3
(NH
3
)
3
] + 3 (en) [Co(en)
3
]Cl
3
+ 3 NH
3
3+
(Cl
-
)
3
NH
2
Co
NH
3
Co
H
2
N
2
(6.6)
H
3
N
l
H
2
N
NH
2
H
3
N
Cl
NH
2
Cl
