Experiment 4A- Page 1
Synthesis of an Iron (III) Oxalate Complex
In this experiment, you will synthesize a coordination compound in which iron (III) coordinates with oxalate ion,
the anion of a diprotic organic acid.
After isolating and purifying the compound, you will analyze it for its composition
of oxalate (Experiment #4B) and iron (Experiment #4C) using two different analytical chemistry techniques: oxidation-
reduction titration and spectrophotometry.
Based on the results of your analyses, you will then be able to calculate the
empirical formula of the compound.
A coordination compound has at least one complex ion (and often more than one).
A complex ion consists of two
a metal ion (usually a transition metal) and ligands. The metal ion typically carries a +2 or +3 charge in an
Such metal ions tend to be electron deficient (i.e., they are Lewis acids).
A ligand is a molecule or anion that can donate an electron pair (i.e., it is a good Lewis base) to the metal ion.
Water and ammonia molecules are examples of neutral species that can act as monodentate ligands, those that donate a
single pair of electrons to the metal ion to form a complex ion.
Each has a lone pair of electrons on the central atom
which can coordinate with the metal ion.
Anions such as chloride or cyanide are also electron-rich and are good
Commonly, 2, 4 or 6 electron pairs from the ligands will coordinate to a metal center depending on the particular
ligand and metal involved.
The number of electron pairs donated to the metal center is known as the metal's
For examples, the complex ions [Ag(NH
, and [Fe(H
have 2, 4 and 6
monodentate ligands, respectively, each ligand donating an electron pair, and coordination numbers of 2 (for Ag
) and 6 (for Fe
A complex may have an overall charge that is positive, negative, or even zero.
Some ligands such as the oxalate anion have two coordination sites (electron-pairs) available for a metal ion, one
at each end of the ion:
The oxalate ion is said to be bidentate because it can bond at two sites around the metal instead of simply one
(thus, Fe (C
has a coordination number of four, not two!).
The general term used to describe ligands which bind
at more than one site (i.e., multidentate) is chelate (from the Greek word for claw).
Oxalate binds quite readily with the
Fe (III) ion.
We will synthesize the iron (III) oxalate complex and precipitate it as the potassium salt with the general
The whole-number values of
will be determined in Experiments #4B & 4C.
In this experiment, you
will synthesize, isolate and purify the K
O salt product.