Erek Dyskant
Chemistry 10
Dr. Mary Roth
Monday Lab Section
Determination of Donor Atom Preference in Cobalt(III) Complexes
This experiment determined Cobalt (III)'s preference between several donor atoms. It
was conducted by dissolving a sample of each complex in water, measuring the absorption
spectrum with a scanning spectrophotometer, aging the solution for a week in a dark space, and
re-measuring the spectrum. For simplicity of data analysis, we will use the maximum
absorption wavelength to gauge a change in the spectrum. If the maximum wavelength in the
spectrum changed, this indicates that ligand exchange occurred. If the maximum absorption
wavelength remained the same, then that determines that ligands were exchanged or rearranged*
Results from spectrophotometric analysis.
The maximum wavelengths from both measurements are shown in Table 1. In summary,
[Co(NH3)6]Cl (ammonia compound), and Co(NH3)5H2O]Cl3 (H2O compound) had no significant
change in their maximum absorption over the two weeks, suggesting that there was not a change
of structure in either compound.
We determined that the ligand in [Co(NH3)5(ONO)]Cl2 (ONO compound) was ONO, as
the maximum absorption wavelength was 494 nm, which is close to the H2O compound's
absorption of 491 nm, strongly suggesting that they share the same donor atom type, Oxygen.
After one week, the maximum absorption of the ONO compound fell to 458 nm. As the
original wavelength is close to that of an H2O ligand, this change could not have resulted from
H2O replacing the ONO ligand. As there were no other ligand candidates present, this change is
due to ONO rearranging itself to form NO2.
*A possible exception is an exchange from the ONO ligand to the H2O ligand as both
complexes have the same absorption spectrum, however this experiment determined that
ONO exchanges with NO2.
The [Co(NH3)5Cl]Cl2 (chlorine complex) changed from 533 nm to 512 nm over the week.
This shows that there was some exchange with the H2O ligand, however that the exchange was
not complete. The solution reached equilibrium with an absorption half way between the
maximum absorption frequency of the H2O complex and that of the pure Chlorine Complex.
The [Co(NH3)5Br]Br2 (bromine complex)'s absorption went down from 550 nm to 494
nm. The final value is very close to the wavelength of the H2O complexes, indicating that the
bromine ligand undergoes almost complete exchange with H2O.
Donor Atom Preference
Table 2 summarizes Cobalt (III)'s relative donor atom preference, as determined through
the experimental results. Oxygen is preferred over Bromine and Chlorine, as ligands with those
donor atoms were replaced by some H2O, which has Oxygen as its donor atom. Chlorine is
preferred over bromine as Chlorine only undergoes partial exchange with H2O. Since NO2
replaced ONO, Nitrogen is preferred over Oxygen, which is confirmed by the NH3 ligands
remaining stable in all of the solutions.
Conclusion
Whenever a donor atom was exchanged, it was always exchanged with an atom that
yielded higher energy, or lower frequency, absorption. Bromine changed with H2O, changing
from 550 to 494 nm absorption. Chlorine changed partially with H2O. As the Chlorine has a
smaller difference in absorption, it doesn't have enough of a difference to justify exchanging all
the ligands, and reaches equilibrium short of a complete exchange. ONO exchanges with NO2,
yielding a 36 nm decrease in absorption. In the H2O compound, H2O does not exchange with the
Chlorine counter-ion, which would have resulted in an absorption energy decrease. Similarly,
NH3 did not exchange with any of the other atoms when given the opportunity. As it has an
absorption of 474 nm, it would have resulted in an energy decrease in all cases except for NO2,
however there was a limited quantity of NO2 available.
To test this hypothesis, I would place each of these compounds in solutions with excess
ligands present, and determine whether exchange occurs. If ligand exchange only occurs where
there is an increase in absorption energy then my hypothesis would be supported. For example,
if the H2O complex were placed in an environment with excess NO2, ligand exchange would
support my hypothesis. If [Co(NH3)6]Cl exchanged its ligands with excess Bromine than the
hypothesis would be disproved.
Maximum Absorption Wavelengths of Several Cobalt(III) Complexes
Initial Complex dissolved Initial Maximum Maximum
after Complexes present after
one week
one week
[Co(NH3)6]Cl
474 nm
475 nm
[Co(NH3)6]Cl
[Co(NH3)5H2O]Cl3
491 nm
492 nm
[Co(NH3)5H2O]Cl3
[Co(NH3)5ONO]Cl2
494 nm
458 nm
[Co(NH3)5NO2]Cl2
[Co(NH3)5Cl]Cl2
533 nm
512 nm
[Co(NH3)5Cl]Cl2 and
[Co(NH3)5H2O]Cl3
[Co(NH3)5Br]Br2
550 nm
494 nm
[Co(NH3)5H2O]Br3
Table 1: Maximum absorption wavelengths of several Cobalt (III) complexes as measured with
spectrophotometry and the complexes present during each of the measurements. The initial
value was measured immediately after dissolution in water, and the second was measured after
one week in solution. The complexes present after one week were determined as described in
the text.
Relative Preference of Donor Atoms in Cobalt (III) as Determined Experimentally
Most Preferred
Nitrogen
Oxygen
Chlorine
Least Preferred
Bromine
Table 2: Cobalt (III)'s donor atom preference, in decreasing order.