Coordination compounds exhibit unique visible colors. The reason behind this is the crystal field splitting of d-orbitals of the central metal atom. The electrons present in the ground state absorb light in visible range and jump into a higher energy level and the complementary colors appear. This phenomenon is known as d-to-d electronic transition.
Principle of d-d transition
It works on the principle of quantization of energy. The energy needed for the ground state electrons to jump into the excited state is fixed. When white light falls on the metal complexes, some of the discrete colors are absorbed and others are transmitted. The transmitted colors are the complementary colors of absorbed light.
How does it happen?
In order to understand the process of d-d transition, let us consider a complex with 5 degenerated orbitals. When ligands approach these orbitals, they split up into two sets. The set at higher energy is eg and the other one at lower energy is the t2g set. So, the electrons in t2g absorb the required energy and jump from the lower energy orbital to the higher energy set. This transition is called d-d transition. It is the origin of the color of metal complexes. d-d transitions are observed only when the d-orbitals are partially filled.
For example, Cu2+ ion in copper sulfate solution has an electronic configuration of [Ar] 3d9. Nine electrons are distributed in five 3d degenerate orbitals. Water forms complex with copper ion as a ligand. This ligand causes repulsion on already present d electrons and ends up splitting it into two sets.
The water and copper tetrahedral complex has intense color because they have unsymmetrical lobes. Therefore, d-d transitions are allowed.
Here the difference in the energy between the nondegenerate d-orbitals ΔE is known as crystal field splitting energy. It depends upon many factors. One of the which is the type of ligand present in the complex.
Similarly, cobalt complexes have electronic spectra with lambda max at 660 nm. The UV-VIS spectroscopy graph is given below;
The energy absorbed by the electron to jump corresponds to the crystal field splitting, so, the greater the energy difference, the higher energy colors would be absorbed.
Relationship between splitting energy and wavelength
ΔE = hv = hc/λ
- h = Plancks constant = 6.63 x 1034Js
- c = speed of light in a vacuum = 3.00 x 108 m/s
- λ= wavelength, in m
Laporte’s selection rule
According to the Laporte rule, transitions among the same orbital g → g are forbidden. This is due to the symmetry. The electron is hesitant to jump to the same symmetrical lobe.
Spin selection rule
- The transition between the same spin states is allowed. So, if the spin remains intact during the transition, the transition will be allowed; ΔS = 0
- The transition between different multiplicities is forbidden; ΔS ≠ 0
Hence, the transition between 3T1g to any other state is allowed.
When d-d transition is possible?
It is possible if non-degenerate d orbitals are partially filled. According to the Laporte selection rule, if the symmetry is absent, the transition is allowed.
What is d-d transition in the case of octahedral and tetrahedral complexes?
d-d transition is the transition from the lower energy d orbitals set t2g in octahedral complex and t2 in tetrahedral to higher energy by absorbing light energy. For example, Ti3+ absorbs a wavelength of 500 nm to do the d-d electronic transition.
Why are d5 complexes colorless?
d5 complexes are not colorless, however, they have very low intensity. The reason is the symmetrical distribution of electrons due to the which the crystal field stabilization energy is much lesser.
Why are d-d transitions Laporte forbidden?
Due to symmetrical lobes, electrons are not allowed to jump in the same orientation lobe according to this rule.
How is the d-d transition calculated?
The energy difference can be calculated by using absorb wavelength and the given formula.
ΔE = hv = hc/λ
Why are so many of the d-d transitions in transition metal ions colored?
The energy required for d-d transition is in the visible color range and so we see the complementary colors.
Why can transition metals have multiple charges?
Transition elements exhibit more than one oxidation state. This is so because d orbitals are present in the valence bonds. The empty or partially filled orbitals are responsible for various electronic configurations. For example, copper has two ionic configurations, Cu2+and Cu+.
Why do transition elements form a complex compound?
According to crystal field theory, transition metals form complexes because they interact electrostatically with ligands as point charges.
Why is cadmium not a transition element?
It does not have partially filled d orbitals in any atomic and ionic state.
- Basic Inorganic Chemistry 3rd edition By Cotton and winkilson
- Inorganic Chemistry by Shriver and Atkins 5th edition
- Transition metal colors (courses.lumenlearning.com)