CHE2621 Assignment 1 solutions 2026
0793226427
UNIVERSITY OF SOUTH AFRICA
Department of Chemistry
CHE2621 Assignment 1 solutions 2026
CHE2621 – Inorganic Chemistry II (Practical)
Assessment 01 – Practical Report
COMPLEX FORMATION:
SPECTROPHOTOMETRIC STUDY OF THE SUBSTITUTION OF WATER
BY EN IN NICKEL(II) COMPLEXES
SECTION B: APPENDIX – COMPULSORY REFERENCE PAPER
Reference Paper Included as Required:
Hill, Z.D. and MacCarthy, P. (1986). Novel approach to Job's method.
Journal of Chemical Education, 63(2), pp. 162–167.
All papers Attached at the bottom of this assignment
Year Module – 2026
Module: CHE2621
Lecturer: Mr KC Tapala
, CHE2621 Assignment 1 solutions 2026
0793226427
SECTION A: THE PRACTICAL REPORT
1. INTRODUCTION
The formation of coordination complexes between transition metal ions and ligands frequently
coincides with the appearance of characteristic colours in solution. This colour arises because
d-block metal ions absorb visible light due to electronic transitions within their d-orbitals, a
phenomenon governed by Crystal Field Theory (CFT). The specific wavelength (and thus
colour) absorbed depends on the nature and number of ligands coordinated to the metal ion,
making spectrophotometry a powerful tool for identifying complex species in solution.
This experiment investigates the complex formation between nickel(II) ions (Ni²⁺) and the
bidentate ligand 1,2-diaminoethane (ethylenediamine, en) in aqueous solution. The Ni²⁺ ion,
with a d⁸ electron configuration, forms a series of octahedral complexes by progressive
substitution of coordinated water molecules by en ligands. The three possible complexes are:
[Ni(H₂O)₄(en)]²⁺ (n = 1), [Ni(H₂O)₂(en)₂]²⁺ (n = 2), and [Ni(en)₃]²⁺ (n = 3).
The aim of this experiment is to identify which of these Ni(II)-en complexes are present in
solution by applying Job's Method of Continuous Variations. This spectrophotometric method
systematically varies the mole fraction of the ligand (en) relative to the metal ion (Ni²⁺), while
keeping the total concentration constant, and monitors the resulting absorbance. The mole
ratio at which maximum corrected absorbance occurs reveals the stoichiometry of the complex
formed.
Absorbance measurements were recorded at four wavelengths (520, 550, 580, and 640 nm)
to ensure that the identification of complexes is wavelength-independent, thereby confirming
the presence of specific species in solution.
2. THEORY
2.1 Crystal Field Theory and Colour in d-Block Complexes
Transition metal compounds owe their characteristic colours to the absorption of
electromagnetic radiation in the visible region of the spectrum (approximately 400–700 nm).
In an octahedral crystal field, the five degenerate d-orbitals of a metal ion split into two sets:
the lower-energy t₂g set (dxy, dxz, dyz) and the higher-energy eg set (dx²−y², dz²). The energy
difference between these two sets is the octahedral crystal field splitting parameter, Δo.
When a photon of light with energy equal to Δo is absorbed, an electron is promoted from the
t₂g to the eg level. The wavelength (λ) of this absorbed light is related to the energy by:
Δo = hc/λ
where h is Planck's constant, c is the speed of light, and λ is the wavelength. The
complementary colour to the absorbed light is what is observed. The magnitude of Δo depends
on the nature of the ligands, as described by the spectrochemical series, and on the metal ion
involved.
, CHE2621 Assignment 1 solutions 2026
0793226427
2.2 Beer-Lambert Law
The relationship between the absorbance of a solution and the concentration of the absorbing
species is described by the Beer-Lambert Law:
A = log(I₀/It) = εcl
where A is the absorbance (optical density, D), I₀ is the intensity of incident light, It is the
intensity of transmitted light, ε is the molar extinction coefficient (L·mol⁻¹·cm⁻¹), c is the molar
concentration (mol·L⁻¹), and l is the path length of the cell (cm). This law underpins the
quantitative use of spectrophotometry in this experiment.
2.3 Job's Method of Continuous Variations
Job's Method allows the stoichiometry of a complex in solution to be determined
spectrophotometrically. Equimolar solutions of the metal (M) and ligand (L) — both at
concentration Z — are mixed in varying proportions such that:
[Ni²⁺] + [en] = Z (total concentration is constant)
The mole fraction of the ligand is x, and the mole fraction of the metal is (1 − x). The measured
absorbance, Dmeasured, is the sum of contributions from all species in solution. In the
absence of complex formation, the theoretical absorbance Dtheoretical would be:
Dtheoretical = (εM·Z(1−x) + εL·Z·x)·l
Since ethylenediamine (en) is colourless and does not absorb at the wavelengths used (εL =
0), the theoretical absorbance simplifies to:
Dtheoretical = (1 − x)·DM
where DM is the optical density of the pure Ni²⁺ solution (at x = 0). The corrected absorbance
Y is defined as:
Y = Dmeasured − (1 − x)·DM
When Y is plotted against the mole fraction of en (x), a maximum in the curve occurs at the
mole fraction Xmax at which complex formation is most pronounced. The composition of the
complex is then determined from Xmax using:
n = Xmax / (1 − Xmax)
where n is the number of en ligands in the complex [Ni(H₂O)₆₋₂ₙ(en)ₙ]²⁺. Measurements at
multiple wavelengths allow independent verification of the complex stoichiometry and can
reveal the presence of more than one complex species.
0793226427
UNIVERSITY OF SOUTH AFRICA
Department of Chemistry
CHE2621 Assignment 1 solutions 2026
CHE2621 – Inorganic Chemistry II (Practical)
Assessment 01 – Practical Report
COMPLEX FORMATION:
SPECTROPHOTOMETRIC STUDY OF THE SUBSTITUTION OF WATER
BY EN IN NICKEL(II) COMPLEXES
SECTION B: APPENDIX – COMPULSORY REFERENCE PAPER
Reference Paper Included as Required:
Hill, Z.D. and MacCarthy, P. (1986). Novel approach to Job's method.
Journal of Chemical Education, 63(2), pp. 162–167.
All papers Attached at the bottom of this assignment
Year Module – 2026
Module: CHE2621
Lecturer: Mr KC Tapala
, CHE2621 Assignment 1 solutions 2026
0793226427
SECTION A: THE PRACTICAL REPORT
1. INTRODUCTION
The formation of coordination complexes between transition metal ions and ligands frequently
coincides with the appearance of characteristic colours in solution. This colour arises because
d-block metal ions absorb visible light due to electronic transitions within their d-orbitals, a
phenomenon governed by Crystal Field Theory (CFT). The specific wavelength (and thus
colour) absorbed depends on the nature and number of ligands coordinated to the metal ion,
making spectrophotometry a powerful tool for identifying complex species in solution.
This experiment investigates the complex formation between nickel(II) ions (Ni²⁺) and the
bidentate ligand 1,2-diaminoethane (ethylenediamine, en) in aqueous solution. The Ni²⁺ ion,
with a d⁸ electron configuration, forms a series of octahedral complexes by progressive
substitution of coordinated water molecules by en ligands. The three possible complexes are:
[Ni(H₂O)₄(en)]²⁺ (n = 1), [Ni(H₂O)₂(en)₂]²⁺ (n = 2), and [Ni(en)₃]²⁺ (n = 3).
The aim of this experiment is to identify which of these Ni(II)-en complexes are present in
solution by applying Job's Method of Continuous Variations. This spectrophotometric method
systematically varies the mole fraction of the ligand (en) relative to the metal ion (Ni²⁺), while
keeping the total concentration constant, and monitors the resulting absorbance. The mole
ratio at which maximum corrected absorbance occurs reveals the stoichiometry of the complex
formed.
Absorbance measurements were recorded at four wavelengths (520, 550, 580, and 640 nm)
to ensure that the identification of complexes is wavelength-independent, thereby confirming
the presence of specific species in solution.
2. THEORY
2.1 Crystal Field Theory and Colour in d-Block Complexes
Transition metal compounds owe their characteristic colours to the absorption of
electromagnetic radiation in the visible region of the spectrum (approximately 400–700 nm).
In an octahedral crystal field, the five degenerate d-orbitals of a metal ion split into two sets:
the lower-energy t₂g set (dxy, dxz, dyz) and the higher-energy eg set (dx²−y², dz²). The energy
difference between these two sets is the octahedral crystal field splitting parameter, Δo.
When a photon of light with energy equal to Δo is absorbed, an electron is promoted from the
t₂g to the eg level. The wavelength (λ) of this absorbed light is related to the energy by:
Δo = hc/λ
where h is Planck's constant, c is the speed of light, and λ is the wavelength. The
complementary colour to the absorbed light is what is observed. The magnitude of Δo depends
on the nature of the ligands, as described by the spectrochemical series, and on the metal ion
involved.
, CHE2621 Assignment 1 solutions 2026
0793226427
2.2 Beer-Lambert Law
The relationship between the absorbance of a solution and the concentration of the absorbing
species is described by the Beer-Lambert Law:
A = log(I₀/It) = εcl
where A is the absorbance (optical density, D), I₀ is the intensity of incident light, It is the
intensity of transmitted light, ε is the molar extinction coefficient (L·mol⁻¹·cm⁻¹), c is the molar
concentration (mol·L⁻¹), and l is the path length of the cell (cm). This law underpins the
quantitative use of spectrophotometry in this experiment.
2.3 Job's Method of Continuous Variations
Job's Method allows the stoichiometry of a complex in solution to be determined
spectrophotometrically. Equimolar solutions of the metal (M) and ligand (L) — both at
concentration Z — are mixed in varying proportions such that:
[Ni²⁺] + [en] = Z (total concentration is constant)
The mole fraction of the ligand is x, and the mole fraction of the metal is (1 − x). The measured
absorbance, Dmeasured, is the sum of contributions from all species in solution. In the
absence of complex formation, the theoretical absorbance Dtheoretical would be:
Dtheoretical = (εM·Z(1−x) + εL·Z·x)·l
Since ethylenediamine (en) is colourless and does not absorb at the wavelengths used (εL =
0), the theoretical absorbance simplifies to:
Dtheoretical = (1 − x)·DM
where DM is the optical density of the pure Ni²⁺ solution (at x = 0). The corrected absorbance
Y is defined as:
Y = Dmeasured − (1 − x)·DM
When Y is plotted against the mole fraction of en (x), a maximum in the curve occurs at the
mole fraction Xmax at which complex formation is most pronounced. The composition of the
complex is then determined from Xmax using:
n = Xmax / (1 − Xmax)
where n is the number of en ligands in the complex [Ni(H₂O)₆₋₂ₙ(en)ₙ]²⁺. Measurements at
multiple wavelengths allow independent verification of the complex stoichiometry and can
reveal the presence of more than one complex species.
