Chemistry Reference
In-Depth Information
[M(L
a
)
x
(CF
3
SO
3
)
y
](CF
3
SO
3
)
z
+ y(L
b
)
poorly-coordinating
non-aqueous solvent
[M(L
a
)
x
(L
b
)
y
](CF
3
SO
3
)
y+z
EXAMPLE:
R
F
3
C
2+
N
(6.20)
O
O
S
3+
(CF
3
SO
3
-
)
3
(CF
3
SO
3
-
)
2
O
-
Co
N
N
H
3
N
NH
3
H
3
N
NH
3
sulfolane
+
Co
H
3
N
NH
3
H
3
N
NH
3
N
NH
3
NH
3
R
Another approach is to employ the effect of heat on a solid complex. It has been known
for a long time that metal complexes, if heated strongly, undergo decomposition reactions
that eventually take them through to usually simple salts or oxides. Generally, ligands are
lost in a series of steps, related in part to their volatility, and this can be probed using a
technique called thermal analysis, which effectively amounts to following weight change
with a sensitive balance during heating of a small sample. Simple neutral ligands often
depart the coordination sphere as molecular species over a reasonably small and well-
defined characteristic temperature range, so that heating to a controlled temperature can
allow controlled conversion to occur. The simplest examples are the hydrated salts of metal
ions, such as [M(OH
2
)
n
](SO
4
), which on heating lose water to form anhydrous M(SO
4
),
usually with a distinctive colour change (such as from blue to nearly colourless, as seen
for copper ion). For complexes containing water as one of several ligands, its loss tends to
occur ahead of other ligands such as amines, allowing partial change of the coordination
sphere. The metal centre involved still seeks to retain its original coordination geometry,
so that loss of water is usually associated with replacement in the coordination sphere by
an involatile anion of the original salt, such as in (6.21).
[Co(NH
3
)
5
(OH
2
)]Cl
3
[CoCl(NH
3
)
5
]Cl
2
+ H
2
O
heat the solid;
110
o
C, hrs
3+
2+
(Cl
-
)
2
OH
2
Co
Cl
-
Co
(Cl
-
)
3
H
3
N
NH
3
H
3
N
NH
3
(6.21)
H
3
N
NH
3
H
3
N
NH
3
NH
3
NH
3
red-pink
purple
This approach permits the insertion of a wide range of stable anions apart from chloride
into the coordination sphere, simply by commencing with them present as the counter-ion.
At higher temperature, amine ligands can be lost in the same manner that water is lost, and
may occur in a stepwise process that permits isolation of useful intermediate complexes.
Eventually, at sufficiently high temperature, all ligands are lost, as in (6.22).
2+
H
3
N
NH
3
Cl
-
H
3
N
anhydrous
platinum(II)
chloride
(Cl
-
)
2
Pt
Pt
H
3
N
NH
3
Cl
-
NH
3
[Pt(NH
3
)
4
]Cl
2
[PtCl
2
(NH
3
)
2
] + 2 NH
3
PtCl
2
+ 2 NH
3
heat the solid;
250
o
C, hrs
(6.22)
extend heating;
500
o
C, hrs
white
yellow
This reaction of Pt(II) is general for a range of coordinated amines apart from ammonia,
and appears to yield exclusively the
trans
geometric isomer (which is called, because of
this exclusivity, a
stereospecific
reaction). Even chelated diamines can be substituted, as
they are inevitably more volatile than any anions present; thus chelated 1,2-ethanediamine
can be replaced by two chloride ions in [Co(en)
3
]Cl
3
to form
cis
-[CoCl
2
(en)
2
]Cl, and by
