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1. 1
Is there a correlation between the nature of a ligand (𝐂𝐥−
, 𝐍𝐇 𝟑, 𝐇 𝟐 𝐎, and 𝑵𝑯 𝟑) and wavelength of light
absorbed by transition metal complexes measured using a colorimeter?
Personal Engagement
Since childhood I have been curious to know how things get their identical colors and recently in a very
interesting lecture in my chemistry class I learned that transition metals have more than one color, I was
fascinated how a transition metal could change color once introduced to different ligands. This sparked my
curiosity to know what effect does changing the ligand has on the color of the transition metal complex formed.
This led me to my investigation on what is the correlation between a ligand and the wavelength of light
absorbed by a transition metal complex.
Background information
The IUPAC definition of transition element is that a transition element is an element that has an atom with an
incomplete d-sublevel or that gives rise to cations with an incomplete d-sublevel. These refer to elements in
groups 3-11 for example such as copper, zinc, vanadium etc. A transition metal complex (for example
[Ni(NH₃)₆]²⁺) can be made by coordinate bonding of metal ion to ligands. A ligand is a molecule or an ion
which has a lone pair of electrons that forms coordinate bonds with transition metal ions to form a complex.
Most of the transition metal complexes are colored; the reason behind the color is the electronic transition
between levels whose spacing correlates with the wavelength available in the visible light. At the point when a
metal ion forms a complex with a ligand, there is repulsion between the ligands and the electrons in the d-orbital
of the metal ion as a result there is d-d orbital splitting, where d orbital are spilt into groups where some have
high energy and some have lower energy.
The color wheel can used to identify the color of light transmitted which is the complementary color of the light
absorbed. For example, the reason behind the color blue color of copper (II) sulfate is that when a white light is
passed through copper (II)sulfate, the copper (II) sulfate absorbs the light in red region of the spectrum and all
other wavelengths of light pass through resulting in the pale blue color we see. A colorimeter can be used to
measure the wavelength absorbed a transition metal complex. A colorimeter is a light-sensitive device used for
measuring the transmittance and absorbance of light passing through a liquid sample. The device measures the
intensity or concentration of the color that develops upon introducing a specific reagent into a solution. To find
out the wavelength absorbed, look at the wavelength at which the absorbance value is 1.00, this will be the
wavelength absorbed.
The diagram is taken from: (n.d.). Retrieved from
https://www.chemguide.co.uk/inorganic/complexions/colour.html.

2. 2
Research Question
Is there a correlation between the nature of a ligand (Cl−
, NH3, H2O, and OH¯) and wavelength of light
absorbed by transition metal complexes measured using a colorimeter when volume and temperature is kept
constant?
Hypothesis
I predict that the wavelength of light absorbed will be the longest for Cl−
ligand and wavelength of light
absorbed will be shortest for NH3 ligand. I have predicted this because the amount of light splitting is the
smallest for Cl−
and largest for NH3. The greater the splitting the more the energy is required to promote an
electron from a lower group of orbitals to a higher one therefore higher energy correlates to a shorter
wavelength.
Scientific Support
I have based my hypothesis on a research paper entitled “Absorption spectra of coordination compounds”
written by Ryutaro Tsuchida in 1938, which observed that the light splitting when Cl−
𝑖𝑜𝑛 used was the shortest
and longest when NH3 ligand was used. This is due to the electronic transition between levels whose spacing
correlates with the wavelength available in the visible light. Hence, I hope to get similar results in my
experiment. Another research I have referred to for NH3+ ligand and Cu2+ ion is entitled “Studies on absorption
spectra” written by the scientist Jannick Bjerrum.
Variables
Independent variable: Nature of ligands
Independent Variable Description Reason
Nature of ligand Four different ligands will be used
(Cl−
, NH3, H2O, and 𝑁𝐻3) to make
four different transition metal
complexes using the same transition
metal ion. (𝐶𝑢2+
, 𝐶𝑜3+
, 𝐹𝑒2+
, Fe3+,
Cr2+ )
I want to see what effect changing the
ligand has on the color of the
transition metal complex formed by
the transition metal.
Dependent variable: The wavelength absorbed
Dependent Variable Description Reason
The wavelength absorbed The wavelength absorbed each
transition metal complex will
be found out using a
colorimeter.
I want to see what effect changing the ligand
has on the color of the transition metal
complex formed by the transition metal.
Controlled variables:
Controlled Variable How it will be controlled Why will it be controlled
The concentration of Same concentration of 0.5 The concentration has an effect on the

3. 3
compound. molar will be used throughout
the experiment for each
transition metal ion.
absorbance value.
Temperature The air conditioner will be
switched on and will be set to
23℃ throughout the
experiment.
The temperature has an effect on the rate of
reaction so it will be controlled
The volume of ligand and
transition metal ion.
The same volume will be used
throughout the experiment
3ml transition metal ion and
2ml ligand
To keep it a fair test.
The charge on the transition
metal ion.
Transition metals ion will the
charge of only 2+ will be used
in this investigation.
The charge on the transition metal ion has
an effect on the value of the wavelength
absorbed.
The thickness and quality of
the glass of the test tube used
in the colorimeter
The same kind of test tubes
will be used throughout the
experiment
The thickness and quality of the test the
have an effect on the readings of the
colorimeter.
Colorimeter The same colorimeter will be
used throughout the
experiment
The uncertainty will change with the
colorimeter which will make the results
unreliable.
Materials
Materials Specification
Copper sulfate solution 0.5 molar concentration
Cobalt(III)chloride 0.5 molar concentration
Iron (II) chloride 0.5 molar concentration
Ammonia 25% concentration
Hydrochloric acid 35% concentration
Sodium hydroxide 1 molar concentration
Distilled water -
Colorimeter Uncertainty: ±10 nm
Measuring cylinder Range [0-10ml], Uncertainty: ±0.5ml
dropper -
Magnetic Stirrer -
Risk Assessment
Safety considerations: Always wear your lab coat, gloves and goggles while working the laboratory as some
these chemicals such as hydrochloric acid are corrosive they can burn your skin and cause you severe damage.
Ammonia is dangerous it can cause skin burn and eye damage and is harmful if inhaled so always wear your
nose mask while working with ammonia.
Ethical considerations: There were no ethical considerations to be taken in account
Environmental considerations: There were no environmental considerations to be taken in account.

4. 4
Experimental procedure
Making 0.5 molar copper (II) sulfate solution
1. 8 grams of copper (II) sulfate was weighed using an electronic weighing scale
2. 100ml of distilled water was measured out in a beaker using a measuring cylinder
3. 8g copper (II) sulfate was added to the 100ml water
4. Stirred the solution until the copper (II) sulfate completely dissolved, using a magnetic stirrer.
Preparation of 0.5 molar Iron (II)sulfate solution
1. 7.6 grams of iron (II) sulfate was weighed using an electronic weighing scale
2. 100ml distilled water was measured out in a beaker using a measuring cylinder
3. 7.6g iron (II) sulfate was added to the 100ml water
4. Stirred the solution until the iron (II) sulfate completely dissolved,using a magnetic stirrer.
Preparation of 0.5 molar cobalt (II) chloride solution
1. 6.5 grams of cobalt (II) chloride was weighed using an electronic weighing scale
2. 100ml distilled water was measured in a beaker using a measuring cylinder
3. 6.5g cobalt (II) chloride was added to the 100ml water
4. Stirred the solution until the cobalt (II) chloride completely dissolved, using a magnetic stirrer.
Method
1) 0.5 molar solution was made (copper(II) sulfate , Iron (II) sulfate or Cobalt (II)chloride)
2) 2 ml of any one ligand (Cl−
, NH3, H2O, and 𝑁𝐻3)) was added using a dropper in a test tube containing
3ml of transition metal ion.( copper(II) sulfate , Iron (II) sulfate or Cobalt (II)chloride)
3) Then the wavelength absorbed was found out using a colorimeter.
4) Colorimeter was calibrated before every trial.
5) Repeated step 1-4 using the other 3 ligands and the other 2 transition metal ions
6) Taken 3 trails for each ligand to make the results more reliable

5. 5
Experimental setup
Transition
metal ion
with
different
ligands
Colorimeter
Transition
metal
complexes

6. 6
Raw Data
Transition metal
complex
Color Observed Wavelength Absorbed (nm)
(±10nm)
Trail 1 Trail 2 Trial 3 Trail 1 Trail 2 Trail 3
[Cu(H2O)4]2+ Light
Blue
Light
Blue
Light Blue 580 580 580
[Cu(NH3)4]2+ Dark
Blue
Dark
Blue
Dark Blue 550 545 550
[Cu(H2O4)(OH)2] Blue Blue Blue 595 595 595
[Cu(Cl)4]2- Green Green Green 630 630 630
[Co(H2O6)]2+ Red Red Red 510 510 520
[Co(H2O)4(OH)2] Dark
Blue
Dark
Blue
Dark Blue 600 600 600
[Co(NH3)6]2+ Dirty
brown
Dirty
brown
Dirty brown 500 500 500
[CoCl4] 2- Green Green Green 620 620 620
[Fe(H2O)6]2+ Pale
Green
Pale
Green
Pale Green 610 610 610
[Fe(OH)2(H2O)4] Dirty
Green
Dirty
Green
Dirty Green 605 605 605
[Fe(H2O) 4(OH)2] Dirty
Green
Dirty
Green
Dirty Green 620 620 620
[FeCl2] Dark
Green
Dark
Green
Dark Green 660 660 660
[Cr(H2O)6]3+ Dark
green
Dark
green
Dark green 500 500 500
[Cr(OH)3(H2O)3] Dirty
Dark
green
Dirty
Dark
green
Dirty Dark green 430 430 430
[CrCl4]– dark
green
dark
green
dark green 700 700 700
[Cr(NH3)6]3+ Dirty
dark
green
Dirty
dark
green
Dirty dark green 480 480 480
[Fe(H2O)6]3+ Brown Brown Brown 470 470 470
[FeCl4]- yellow yellow Yellow 530 530 530
[Fe]-NH3 brown brown Brown 480 480 480
[Fe]- OH- Dirty
brown
Dirty
brown
Dirty brown 420 420 420

7. 7
Processed Data
Transition metal
complex
Color Observed Wavelength Absorbed (nm)
(±10nm)
Avg. wavelength
absorbed (nm)
Trail 1 Trail 2 Trial 3 Trail 1 Trail
2
Trail 3
[Cu(H2O)4]2+ Light
Blue
Light
Blue
Light
Blue
580 580 580 580
[Cu(NH3)4]2+ Dark
Blue
Dark
Blue
Dark
Blue
550 545 550 548
[Cu(H2O4)(OH)2] Blue Blue Blue 595 595 595 595
[Cu(Cl)4]2- Green Green Green 630 630 630 630
[Co(H2O6)]2+ Red Red Red 510 510 520 513
[Co(H2O)4(OH)2] Dark
Blue
Dark
Blue
Dark
Blue
600 600 600 600
[Co(NH3)6]2+ Dirty
brown
Dirty
brown
Dirty
brown
500 500 500 500
[CoCl4] 2- Green Green Green 620 620 620 620
[Fe(H2O)6]2+ Pale
Green
Pale
Green
Pale
Green
610 610 610 610
[Fe(OH)2(H2O)4] Dirty
Green
Dirty
Green
Dirty
Green
605 605 605 605
[Fe(H2O) 4(OH)2] Dirty
Green
Dirty
Green
Dirty
Green
620 620 620 620
[FeCl2] Dark
Green
Dark
Green
Dark
Green
660 660 660 660
[Cr(H2O)6]3+ Dark
green
Dark
green
Dark
green
500 500 500 500
[Cr(OH)3(H2O)3] Dirty
Dark
green
Dirty
Dark
green
Dirty
Dark
green
430 430 430 480
[CrCl4]– dark
green
dark
green
dark
green
700 700 700 700
[Cr(NH3)6]3+ Dirty
dark
green
Dirty
dark
green
Dirty
dark
green
480 480 480 430
[Fe(H2O)6]3+ Brown Brown Brown 470 470 470 470
[FeCl4]- yellow yellow Yellow 530 530 530 530
[Fe]-NH3 brown brown Brown 480 480 480 480
[Fe]- OH- Dirty
brown
Dirty
brown
Dirty
brown
420 420 420 420

8. 8
Sample Calculation
Average =
Sum of values
No.of values
=
550 +545 +550
3
=548.333333
=548(to nearest 1)
Graphs: showing processed data
Graph 1: Wavelength absorbed for Cu2+ ion with different ligands
(Wavelength up to 3 significant figures)
Graph 2: Wavelength absorbed for Co2+ ion with different ligands
(Wavelength up to 3 significant figures)
0
100
200
300
400
500
600
700
Cu²⁺
Wavelengthabsorbed(nm)
Tranition metal ion
NH₃
OH⁻
H₂O
Cl¯
0
100
200
300
400
500
600
700
Co²⁺
Wavelengthabsorbed(nm)
Transition metal ion
NH₃
OH⁻
H₂O
Cl¯

9. 9
Graph 3; Wavelength absorbed for Fe2+ ion with different ligands
(Wavelength up to 3 significant figures)
Graph 4: Wavelength absorbed for Cr2+ ion with different ligands
(Wavelength up to 3 significant figures)
Graph 5: Wavelength absorbed for Fe3+ ion with different ligands
(Wavelength up to 3 significant figures)
0
100
200
300
400
500
600
700
800
Fe²⁺
Wavelengthabsorbed(nm)
Transition metal ion
NH₃
OH⁻
H₂O
Cl¯
0
100
200
300
400
500
600
700
800
Cr²⁺
Wavelengthabsorbed(nm)
Transition metal ion
NH₃
OH⁻
H₂O
Cl¯
0
100
200
300
400
500
600
700
800
Feᶟ⁺
Wavelengthabsorbed(nm)
Transition metal ion
NH₃
OH⁻
H₂O
Cl¯

10. 10
Graph 6: Compares all the data
(Wavelength up to 3 significant figures)
Data analysis:
After looking at graphs 1 to 6, a trend is repeated in every graph that the wavelength absorbed for Cl−
ligand is
the longest, then the wavelength absorbed H2O is a little shorter than the wavelength Cl−
ligand, then the
wavelength absorbed OH¯ is a little shorter than the wavelength H₂O ligand. Lastly the shortest wavelength is
absorbed by NH3 ligand. For example, in graph 1 when the transition metal ion Cu2+ was used. The wavelength
absorbed when the NH3 ligand was used was 548nm, the wavelength absorbed when the OH¯ ligand was
580nm, The wavelength absorbed when the H2O ligand was 595nm and The wavelength absorbed when the Cl−
ligand was 630nm, from this data its clearly visible that the longest wavelength was absorbed by the Cl−
ligand
and the shortest by the NH3 ligand. Furthermore in the processed data there wasn’t much difference in the value
obtained between each trail for example: when H2O ligand was used with the transition metal ligand Cu2+
[Cu(H2O)4]2+ the value for all three trails was same (580nm). Another observation that was made is that the
average wavelength absorbed for the transition metal complexes for each transition metal ion with different
ligands was within the range of 100nm, for example the smallest wavelength absorbed for Cu2+ was 548nm and
the longest wavelength absorbed was 630nm so it has a range of 82nm, but there is an exception, Cr2+, the
smallest wavelength absorbed was 480nm but the longest wavelength absorbed is 700nm so it has a range of
320nm.
Conclusion
It can be concluded that results support my hypothesis (I have predicted that wavelength of light absorbed will
be the longest for Cl−
ligand and wavelength of light absorbed will be shortest for NH3 ligand. I have predicted
this because the amount of light splitting is the smallest for Cl−
and largest for NH3. The greater the splitting
the more the energy is required to promote an electron from a lower group of orbitals to a higher one. Therefore,
higher energy correlates to a shorter wavelength as it’s clearly visible in graph 1 the wavelength absorbed
for the Cl−
is the longest as compared to the other three ligands for each transition metal ion. For example, for
Cu2+ ion the wavelength absorbed by the transition metal complex formed using Cl−
ligand absorbed the
wavelength of the length 630 nm which was the longest wavelength absorbed compared to any other transition
metal formed using the other 3 ligands (NH3, H2O and OH¯) Furthermore graph 1 depicts that the shortest
wavelength absorbed was by NH3 for each transition metal ion used in the experiment. For example, for Cu2+
ion the wavelength absorbed by the transition metal complex formed using NH3 ligand absorbed the wavelength
0
100
200
300
400
500
600
700
800
Cu²⁺ Co²⁺ Fe²⁺ Cr²⁺ Feᶟ⁺
Wavelengthabsorbed(nm)
Transition metal ion
Cl¯
NH₃
OH¯
H₂O

11. 11
of the length 548 nm which was the shortest wavelength absorbed as compared to the transition metal complex
formed by copper using the other 3 ligands.
It can be seen in the graph 1 that there is a particular trend followed in the graph the longest wavelength
absorbed for each transition metal complex was made using the Cl−
ligand, then a wavelength shorter than that
was absorbed by transition metal complex made using OH¯ ligand and a wavelength shorter than this was
observed by H2O in all cases and the shortest wavelength was absorbed by the ligand NH3 in all cases. In
conclusion there is a correlation between the nature of ligand and the wavelength absorbed by the transition
metal complex and the results obtained in my experiment match the results gained in the research paper entitled
“Absorption spectra of coordination compounds” written by Ryutaro Tsuchida in 1938. Another research
journal I referred to entitled “On the absorbtion spectra of hexamimnecobalt (III) and related complexes. II.
Theoretical splitting and shifting of the first and second band due to substitution of ligands”, author: Hideo
Yamatera in 1957 had similar a similar trend to the trend obtained in my experiment. Hence we can say that the
results gained in my investigation and reliable and accurate and can be used to draw a conclusion.
Cl−
OH¯
H2O
NH3
Evaluation:
Hypothesis
The hypothesis stating that the wavelength of light absorbed will be the longest for Cl−
ligand and wavelength
of light absorbed will be shortest for NH3 ligand has proven to be correct as per the results obtained in my
investigation.
Experimental procedure
The most ideal method was designed for the experiment to gain the best results. The colorimeter was calibrated
and the cuvette was wiped with a tissue after every trail for precise and accurate readings. A range of ligands
and transition metal ions were explored, this made the results more reliable. A separate dropper for each
chemical was used to ensure they didn’t mix with each other. Highly concentrated solutions for example 25%
ammonia was used for a faster rate of reaction. The volume was of the transition metal ligand was kept constant
throughout the experiment to gain reliable results. Human error such as parallax error was reduced by taking
upper meniscus reading for colored solutions and lower meniscus reading for colorless solutions.
Materials
The uncertainty could be lowered and precision could be increased by using more precise devices such as
weighing scale and colorimeter, increasing the number of trails taken could reduce random errors and provide
Smallest splitting
Largest Splitting
Longest Wavelength Absorbed
Shortest Wavelength Absorbed

12. 12
more reliable data. However, the data collected in my investigation is reliable as there is very low uncertainty
and a lot of variables were controlled and the results gained are backed up by scientific support.
Weaknesses and improvements
The experiment had a number of weaknesses; firstly, the colorimeter used wasn’t very precise and had an
uncertainty of ±10 nm, this could be improved by taking more no. of trails or using a digital colorimeter which
is more precise and accurate. There could be some amount of solution left in the measuring cylinder so this
varied among each trail and could cause possible error in the results; this could be improved by taking more no.
of trails or carrying out the experiment on a bigger scale. There is a chance of parallax error as not all reading
was taken on eye level, this could cause a minute change in the volume of the ligand used hence affecting the
results, this could be improved by taking all the reading on eye level. The time between adding the ligand to the
transition metal ion and then putting the test tube in the colorimeter wasn’t constant; this could probably affect
the results as the reaction time between each trail wasn’t constant.
Possible Extensions
This investigation could possibly be further extended by using more no. of ligands and using more other
transition metals as this could give me more reliable and accurate results. This investigation could also be
further extended by using transition metals with varying charge as charge of a transition metal also has an effect
on the color of the transition metal complex formed. These extensions could not be made in this investigation
due to limited resources and due to the restrictions on the length of the I.A. In the future if I ever get a chance
to extend my investigation I would very much do it.
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