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Experiment 6
Preparation of an Inorganic Cobalt Complex: Co(NH
n
Cl
Discussion
Werner coordination complexes are compounds formed between transition metal ions and a variety
of organic and inorganic ions or neutral molecules. The elucidation of the details of the structure of
these complexes is of current interest, although Werner correctly predicted the geometry of many of
them more than fifty years ago.
A well-known complex ion is the tetraammine copper(II) ion in which four ammonia molecules are
arranged (coordinated) about a central cupric ion at the corners of a square.
Cu
H
N:
H
N: :NH
:NH
or Cu
H
N:
H
N: :NH
:NH
(dashes indicate geometry only,
not bonds)
The nature of the bonds between the central atom and the coordinating groups (termed ligands)
varies with the nature of the species involved. In some complexes the bonding may be described as
largely covalent in character, whereas in others electron sharing is relatively unimportant and the
interaction between the central atom and the ligands is essentially electrostatic in nature.
Octahedral Complexes
In these, six ligands (L) are arranged about the central atom (M) at the apices of an octahedron.
The resultant charge on the complex depends on the nature of M and the ligand L.
M
L
L
L
L
L
L
or M
L
L
L
L
L
L
(dashes indicate geometry only,
not bonds)
If M is trivalent cobalt (Co
3+ ) and L is ammonia (NH 3
), the complex ion will be tripositively
charged. An example is, then, hexammine cobalt(III) chloride - [Co(NH 3
6
]
3+ 3Cl
where the three
chloride ions are not part of the complex but simply independent ionic species such as present in
potassium chloride, calcium chloride, etc.
One or more of the ammonia ligands may be replaced by chloride ions to yield complex
compounds such as:
C o
NH
NH
NH
Cl
2Cl
H
N
H
N
Cl
Cl
Cl
NH
NH
C o
H
N
H
N
chloropentammine cobalt(III) dichlorotetraammine cobalt(III)
chloride chloride
etc.
The chloride ions attached to the central metal atom do not ionize readily, and thus the complex
compound behaves accordingly. For example, chloropentaammine cobalt(III) chloride is a 2:
electrolyte similar to calcium chloride (Ca
2+ 2Cl
), only two-thirds of the chlorine being outside the
complex attached to it by ionic bonds.
In this lab experiment you will synthesize a cobalt coordination complex of the general formula
CoCl 3
(NH
3
x
, with x=3-6. In experiment 7 to 9 you will perform four experiments to determine the
unknown x. With any luck, all of your data will point to the same structure.
i) Preparation of Compound X
Procedure (Adapted from Inorganic Syntheses , Volume 9, p. 160, 1967)
Fit a 250-mL Erlenmeyer flask with a cork stopper. Measure 20 mL of concentrated ammonia
from the dispenser in the hood into your flask. Ammonia is an irritating and pungent gas. Avoid
breathing it or dropping it on skin. Always replace stoppers of dispensing bottles immediately.
Stopper the Erlenmeyer flask before bringing it out from the hood in order to contain the ammonia.
Weigh out 3.0 g of ammonium chloride (NH 4
Cl) to the nearest 0.1 g. Weigh into a square piece of
weighing paper whose edges have been folded up sharply. Working in the hood, dissolve the
approximately 3.0 g of ammonium chloride in the ammonia solution with a little gentle swirling of
the loosely stoppered flask.
Weigh accurately by difference about 5 g of granulated cobalt(II) chloride hexahydrate
(Co(H 2
O)
6
Cl 2
). Proceed as follows:
- Obtain a clean, dry, stoppered weighing bottle from your equipment. If bottle is not clean ,
wash with tap water and cleanser, rinse with distilled water, and dry in oven at 110˚C for about
a half-hour. When drying weighing bottles (empty or containing a solid), place cover in a
tilted position on top of bottle to permit water vapor to escape. Place bottle and cover within a
250-mL beaker covered with a ribbed watch glass. (This prevents foreign material from
dropping into bottle and permits water vapor to escape.) Be sure to identify bottle and beaker
by marking the ground-glass area with a lead pencil; do not use a wax marking pencil. After
drying period, remove bottle with tongs and place in covered desiccator to cool.
- Place about 5 g of the cobalt reagent in the weighing bottle. To assist you in judging the
appropriate volume, look at the exhibit.
- Weigh the bottle and contents. Record weight of bottle and contents in your laboratory
notebook.
