Coordination
chemistry is the branch of chemistry which explores the properties and
interactions of complex structures. A complex structure is a chemical
unit (species) which consists of a centrally located metal ion surrounded by a
specific number of polar molecules and/or anions called ligands. Each
ligand donates a pair of electrons to the metal forming a coordinate covalent
bond. Ligands vary in strength depending on their relative tendencies to
dissociate from the metal ion in solution. A strong ligand is tightly bound to
the metal and, therefore, is difficult to remove. A weak ligand is one which
is easily removed from the complex structure. The ligands involved in this
experiment are nitrite ion (NO2-) and ethylenediamine
(NH2CH2CH2NH2, abbreviated "en"),
both of which are strong ligands.
The total number of bonds formed between the central metal ions and the
attached ligands is called the coordination number of the
compound1. The coordination number is also equal to the number of
electron pairs involved in the bonding between the central metal ion and the
surrounding ligands. It follows that in complex structures all bonds between
the metal cation and its ligands are single bonds.
Complex structures can be cations, anions or non-ionic compounds. The complex
structure to be analyzed in this experiment is
dinitrobis(ethylenediamine)cobalt (III) nitrate. A neutral compound containing
at least one complex structure is called a coordination compound. If the
complex structure itself is neutral (i.e.
[Co(NH3)4Cl2], then the complex structure is
also the coordination compound. If the complex structure has an overall
positive or negative charge (i.e.
[Cu(NH3)6]+2), it is referred to as a complex
ion. Neutral coordination compounds can be formed by bonding complex ions to
counter ions, which attach themselves to the polar complex structure but are
not considered part of it. Counter ions are regular ions, (Cl-,
NO3-,etc.) which are present in solution, and they bond
with complex structures through electrostatic attraction.
To illustrate the interactions between complex ions and counter ions, consider
the complex structure hexamine [Cu(NH3)6]+2.
Hexammine copper(II) itself is a complex cation, exhibiting an overall charge
of +2. To form a neutral compound with hexamine copper(II), a species with
charge -2 must be attached7. In solution
[Cu(NH3)6]+2 will bond with two Cl-
anions to obtain the overall neutral charge necessary to form a coordination
compound. The two Cl- anions which bond with the complex structure
are counter ions, and the overall species is the coordination compound
[Cu(NH3)6]Cl2. In summation, the neutrality
of coordination compounds is due to a balance of charges, either within the
complex structure itself or between complex ions and counter ions. The
[Co(en)2(NO2)2]NO3 to be analyzed
in this experiment is a coordination compound formed by joining a counter ion
(NO3-) with the cobalt complex.
Molecules or ions having the same chemical composition but different structures
are called isomers. Isomerism is generally observed only in complexes
which are relatively stable and slow to react. Several different types of
isomers exist: the two most important of which are geometrical and optical.
Optical isomerism is exhibited by two isomers which are mirror images of one
another and have identical physical and chemical properties. The only
difference between them being the angle at which they reflect polarized
light3.
[Co(en)2(NO2)2]NO3 exhibits
geometrical isomerism; it exists in both the cis form, in which similar
ligands are oriented next to one another, and the trans form, in which
similar ligands are opposite each other. Geometrical isomerism is also
referred to as cis-trans isomerism because ligand orientation determines
structural geometry.
The structural isomers of the cobalt complex
[Co(en)2(NO2)2] are shown in the following
diagrams:
|
|
trans-[Co(en)2(NO2)2] |
cis- [Co(en)2(NO2)2] |
The structural differences between the cis- and trans- isomers cause them to exhibit different chemical characteristics despite their identical chemical composition.
In
the first part of this project, the trans and cis isomer of
dinitrobis(ethylenediamine)cobalt (III) nitrate were reacted with hydrochloric
acid, hydrobromic acid, and hydroiodic acid. The chloride, bromide and iodide
anions were reacted to replace the nitrite anions and consequently changed the
ligand field polarizabilty of the trans- and cis- cobalt coordination complex.
The other part of the project investigates the change in the ligand field of
the cis and trans when reacted with different anions in an eighty-percent
methanol solvent system. The anions tested, bromide (Br--),
thiocyanate (SCN-), and acetate (AC-) are known to
replace chloride ligands in [Co(en)2Cl2]+.
First, it was determined whether or not these anions would replace the nitrite
ligands of cis-dinitrobis(ethylenediamine)cobalt (III) nitrate in methanol as
they did the chloride ligand. Then, the same anions were reacted with the
trans-isomer to see if both the cis- and the trans-isomers reacted similarly
with the anions.
