Experiment 15: Synthesis of a Coordination
Compound and Its Chemical Analysis
and
Experiment 17: Colorimetric or
Spectrophotometric Analysis
Background:Coordination compounds or complexes are an exciting branch of chemistry and
area where much research is focused today. There are several types of coordination com-
pounds. One type of coordination compound consists of a cation bonded to a complex ion, of-
ten highly colored, i.e., K 2 [Ni(CN) 4 ], potassium tetracyanonickelate(II). The complex ion, the
section in square brackets, consists of a centrally located metallic ion, surrounded by ligands.
Ligands may be either polar molecules or simple anions. The bonding within this complex ion
is through coordinate covalent bonds. Another type of coordination compound forms when nei-
ther the ligands nor the central atom has a charge, i.e., [Cr(CO) 6 ], hexacarbonylchromium (0),
or when the central atom does have a charge, i.e. [Ni(H 2 O) 2 (NH 3 ) 4 ](NO 3 ) 2 , tetraamminedi-
aquanickel(II) nitrate.
The rate of reaction involving the formation of complex ions is often very rapid. Given this
fact, the ion that is formed is usually the most stable, consistent with the principles of equilib-
rium and subject to shift. Labile reactions are reactions that involve very fast reversible com-
plex ion formation. However, complex ions may not be labile, that is they do not exchange
ligands rapidly and are referred to as nonlabile. These complex ions exchange ligands slowly in
substitution reactions and may be favored more kinetically than thermodynamically. Adding a
catalyst and thereby changing the rate of formation of the product(s) may change the nature of
the complex ion produced. An easy method to determine whether a complex ion is labile or not
is to note whether a color change occurs in a solution containing the ion when a good complex-
ing ligand is added.
Chemists are involved in both synthesis, that is creating compounds, and analysis, determing
the nature of compounds. In this lab, a coordination compound containing Ni2+, NH 3 and Cl–
will be synthesized. Once it has been synthesized, the next step will be to analyze the coordina-
tion compound to determine its exact formula.
When the compound that is synthesized is irradiated with white light, some of the light at par-
ticular wavelengths will be selectively absorbed. To determine the wavelength(s) of light ab-
sorbed, we can expose a solution of the compound to varying wavelengths of monochromatic
light and record the responses. If a particular wavelength of light is not absorbed, the intensity
of the light directed at the solution (Io) will match the intensity of the light transmitted by the
solution (It). For wavelengths of light absorbed, the intensity will be measurably less intense,
therefore, It<Io.
%%T=IIot# 100Part III: AP Chemistry Laboratory Experiments