Colour of Transition Elements

Edited By Shivani Poonia | Updated on Jul 02, 2025 07:32 PM IST

Transition metal ions form colours in an aqueous solution; this is due to the adsorption of visible light radiation, which promotes an electron from one d-orbital to another. The ions of d-orbital transition elements absorb a specific wavelength of radiation and reflect the rest, giving the solution colour. Transition metal complexes have a wide range of hues in different solvents.

This Story also Contains

Colour Of Transition Elements
Some Solved Examples
Conclusion

Colour of Transition Elements | Transition Metal Properties

Coloured complexes are also formed by elements in a lanthanide class. Lanthanides are sometimes known as transition metals or simply as transition metals subclass. The colourful complexes, and on the other hand, are caused by 4f electron transition. When transitioning to a transition metal complex, the hues of lanthanide complexes are less affected by the nature of their ligands. A coloured solution is produced when visible light of a certain energy level is not absorbed. When transition metal compounds are dissolved in water, they take on a wide range of vibrant colours.

When an electron from a lower energy d orbital is excited to a higher energy d orbital, the energy of excitation corresponds to the frequency of light absorbed. This frequency generally lies in the visible region. The colour observed corresponds to the complementary colour of the light absorbed. The frequency of the light absorbed is determined by the nature of the ligand. In aqueous solutions where water molecules are the ligands, the colours of the ions are observed.

Some Solved Examples

Q.1 Which one of the following when dissolved in water gives the coloured solution in nitrogen atmosphere?

(1) Cu₂Cl₂

(2) ZnCl₂

(3) CuCl₂

(4)AgCl

Solution:

As we learned -

The given metal cations are

Cu⁺,Zn₂⁺,Cu₂⁺ and Ag⁺

Out of these, only Cu₂⁺ contains unpaired electrons and can show colour.

The other ions (Cu⁺, Ag⁺, and Zn₂⁺) have d10 configuration and do not show color.

Hence, the answer is the option (3).

Q.2 The set having ions which are coloured and paramagnetic both is -

(1) Cu²⁺,Cr³⁺,Sc⁺

(2) Cu²⁺,Zn²⁺,Mn⁴⁺

(3) Sc³⁺,V⁵⁺,Ti⁴⁺

(4) Ni²⁺,Mn⁷⁺,Hg²⁺

Solution:

As we learned -

The set of ions which are coloured and paramagnetic is

Cu²⁺(3d⁹),Cr³⁺(3d³) and Sc⁺4(s¹3d¹)

Species having d⁰or d¹⁰ configuration are diamagnetic, such as

Zn²⁺(3d¹⁰),Sc³⁺(3d⁰),Ti⁴⁺(3d⁰),V⁵⁺(3d⁰) and Mn⁷⁺(3d⁰)

Hence, the correct answer is Option (1)

Q.3 When XO₂is fused with an alkali metal hydroxide in the presence of an oxidizing agent such as KNO₃, a dark green product is formed which disproportionates in an acidic solution to afford a dark purple solution. X is :

1)Ti

2)V

3)Cr

4) (correct)Mn

Solution

As we have learnt,

The given reaction sequence is a method of preparation of KMnO₄

The reactions mentioned are given below

MnO₂+KOH→K₂MnO₄ dark green → solution Acidic KMnO₄ dark purple

Hence, the answer is an option (4).

Conclusion

Transition metals are elements with half-filled d-orbitals. The periodic table places this group of elements near the centre (between s-block and p-block elements). These elements have a proclivity for various oxidation states, the formation of complex compounds, toughness, and high density, and lanthanoid contraction can be seen in them. Colourful ions, complexes, and compounds are formed by transition metals.

Frequently Asked Questions (FAQs)

1. Why do transition elements exhibit color?

Transition elements exhibit color due to d-d electronic transitions. When light hits a transition metal compound, electrons in the partially filled d-orbitals can absorb specific wavelengths of visible light, promoting them to higher energy levels. The remaining wavelengths are reflected or transmitted, giving rise to the observed color.

2. What is the relationship between color and complementary color in transition metal complexes?

The color observed in a transition metal complex is the complementary color of the light absorbed. For example, if a complex absorbs orange light, it will appear blue. This is because the observed color is composed of all the wavelengths of light that are not absorbed by the complex.

3. How does the oxidation state of a transition metal affect its color?

The oxidation state of a transition metal significantly influences its color. Higher oxidation states generally result in more intense colors due to increased splitting of d-orbitals. This affects the energy required for d-d transitions, altering the wavelengths of light absorbed and thus the observed color.

4. What is crystal field theory and how does it explain the color of transition metal complexes?

Crystal field theory explains the splitting of d-orbitals in transition metal complexes due to the presence of ligands. This splitting determines the energy difference between d-orbitals, which in turn affects the wavelengths of light absorbed. The theory helps predict and explain the colors observed in different transition metal complexes based on ligand strength and geometry.

5. Why are some transition metal compounds colorless?

Some transition metal compounds are colorless because they either have a fully filled or empty d-orbital configuration. In these cases, d-d transitions are not possible, so no visible light is absorbed. Examples include Zn2+ and Sc3+ compounds, which have completely filled and empty d-orbitals, respectively.

6. How does the spectrochemical series relate to the color of transition metal complexes?

The spectrochemical series ranks ligands based on their ability to cause d-orbital splitting. Ligands higher in the series cause greater splitting, resulting in higher energy transitions and often more intense colors. This series helps predict how different ligands will affect the color of transition metal complexes.

7. What is meant by "high-spin" and "low-spin" complexes, and how do they affect color?

High-spin complexes have electrons occupying all d-orbitals before pairing, while low-spin complexes have electrons paired in lower energy orbitals. Low-spin complexes generally have larger d-orbital splitting, leading to higher energy transitions and often more intense colors compared to high-spin complexes of the same metal.

8. Why do transition metal ions often change color when different ligands are added?

Different ligands cause varying degrees of d-orbital splitting, changing the energy required for d-d transitions. This alters the wavelengths of light absorbed and consequently the observed color. For example, [Cu(H2O)6]2+ is light blue, but [Cu(NH3)4]2+ is deep blue due to the stronger field ligand NH3.

9. How does the geometry of a complex affect its color?

The geometry of a complex influences the arrangement and splitting of d-orbitals. Different geometries (e.g., octahedral, tetrahedral, square planar) result in distinct patterns of d-orbital splitting, leading to varied absorption spectra and colors. For instance, tetrahedral complexes often have weaker field splitting and different colors compared to octahedral complexes of the same metal and ligands.

10. What is the difference between d-d transitions and charge transfer transitions in terms of color intensity?

d-d transitions involve electron movement within the d-orbitals of a single metal ion and typically result in weaker, less intense colors. Charge transfer transitions involve electron movement between the metal and ligand orbitals, leading to much more intense colors. Charge transfer transitions are often responsible for the vivid colors seen in some transition metal compounds.

11. How does Jahn-Teller distortion affect the color of transition metal complexes?

Jahn-Teller distortion occurs in certain d-orbital configurations (e.g., d9 for Cu2+) and causes further splitting of the d-orbitals. This additional splitting can lead to changes in the absorption spectrum and, consequently, the observed color. For example, it contributes to the blue color of many copper(II) compounds.

12. Why do lanthanides generally have paler colors compared to transition metals?

Lanthanides generally have paler colors because their f-f transitions are shielded by outer electrons, making them less affected by the environment. These transitions are also forbidden by selection rules, resulting in weak absorption and thus paler colors compared to the d-d transitions in transition metals.

13. How does temperature affect the color of transition metal complexes?

Temperature can affect the color of transition metal complexes by influencing the population of different electronic states. Higher temperatures can lead to broader absorption bands due to increased vibrational energy, potentially resulting in slight color changes. In some cases, temperature changes can also cause shifts between high-spin and low-spin states, leading to more dramatic color changes.

14. What is meant by "intervalence charge transfer" and how does it contribute to color?

Intervalence charge transfer occurs when an electron moves between metal ions of different oxidation states in a mixed-valence compound. This type of transition often results in intense colors, as seen in minerals like blue sapphire (Fe2+ to Fe3+ transfer) or the intense blue of Prussian blue ([Fe(CN)6]4- to [Fe(CN)6]3- transfer).

15. How do coordination number and ligand field strength interact to influence color?

Coordination number and ligand field strength both affect d-orbital splitting. Higher coordination numbers generally lead to smaller splitting, while stronger field ligands increase splitting. The interplay between these factors determines the energy of d-d transitions and thus the observed color. For example, a complex with a high coordination number but weak field ligands might have similar splitting to one with a lower coordination number but stronger field ligands.

16. Why do some transition metal compounds change color when dissolved in different solvents?

Solvent effects can alter the ligand field strength and geometry around the metal ion. Different solvents may replace or modify the existing ligands, changing the d-orbital splitting and thus the color. For example, anhydrous copper(II) sulfate is colorless, but turns blue when dissolved in water due to the formation of [Cu(H2O)6]2+ complex.

17. How does the concept of "nephelauxetic effect" relate to the color of transition metal complexes?

The nephelauxetic effect refers to the expansion of d-orbitals in transition metal complexes due to increased covalent bonding with ligands. This effect reduces the interelectronic repulsion, leading to smaller energy gaps between d-orbitals. As a result, complexes exhibiting a strong nephelauxetic effect often absorb light at lower energies, shifting their color towards the red end of the spectrum.

18. Why do some transition metal compounds appear to have different colors when viewed in reflected light versus transmitted light?

This phenomenon, known as dichroism, occurs due to differential absorption of light in different orientations of the crystal. In reflected light, we see the color of light that is reflected from the surface. In transmitted light, we see the color of light that passes through the sample. The difference arises from the complex's structure and how it interacts with light in different directions.

19. How does the presence of unpaired electrons in a transition metal complex relate to its magnetic properties and color?

The number of unpaired electrons in a transition metal complex affects both its magnetic properties and color. Complexes with more unpaired electrons (high-spin) are typically paramagnetic and often have paler colors due to smaller d-orbital splitting. Complexes with fewer unpaired electrons (low-spin) are usually less paramagnetic or diamagnetic and often have more intense colors due to larger d-orbital splitting.

20. What is the relationship between the strength of the crystal field and the position of a transition metal in the periodic table?

Generally, the strength of the crystal field increases as you move across the periodic table from left to right within a transition series. This is due to the increasing effective nuclear charge, which leads to stronger metal-ligand interactions. Consequently, elements on the right side of the transition series (e.g., Ni, Cu) often form stronger field complexes with more intense colors compared to those on the left (e.g., Ti, V).

21. How do π-acceptor ligands influence the color of transition metal complexes?

π-acceptor ligands, such as CO or CN-, can accept electrons from filled d-orbitals of the metal into their empty π* orbitals. This additional bonding increases the d-orbital splitting, often leading to higher energy transitions and more intense colors. For example, [Fe(CN)6]4- is pale yellow, while [Fe(H2O)6]2+ is pale green due to the stronger field created by the CN- ligands.

22. Why do some transition metal compounds exhibit thermochromism?

Thermochromism in transition metal compounds occurs when temperature changes cause alterations in the complex's structure, hydration, or spin state. These changes affect the d-orbital splitting or the nature of the transitions, resulting in color changes. For example, [Co(H2O)6]Cl2 changes from pink to blue when heated due to a change in coordination geometry.

23. How does the concept of "hole formalism" help in understanding the colors of d9 and d1 complexes?

The hole formalism considers d9 configurations as a single hole in a filled d10 shell, and d1 as a single electron in an empty d-shell. This approach simplifies the analysis of electronic transitions in these systems. It explains why Cu2+ (d9) and Ti3+ (d1) complexes often have similar colors, as they represent complementary situations in terms of electron/hole transitions within the d-orbitals.

24. What is the significance of Laporte selection rule in determining the intensity of color in transition metal complexes?

The Laporte selection rule states that transitions between orbitals of the same parity (e.g., d-d transitions) are forbidden in centrosymmetric molecules. This rule explains why d-d transitions often result in weak colors. However, the rule can be relaxed due to vibrational coupling or lack of a center of symmetry, allowing these transitions to occur with low intensity. Understanding this rule helps explain why some complexes have pale colors while others are more intensely colored.

25. How do actinides differ from transition metals in terms of their color properties?

Actinides, like lanthanides, involve f-orbital transitions rather than d-orbital transitions. However, actinides can exhibit more intense colors than lanthanides due to greater f-orbital extension and increased interaction with the environment. Additionally, many actinides can access multiple oxidation states more readily than lanthanides, leading to a wider range of possible colors and more complex spectra.

26. Why do some transition metal complexes exhibit color changes when exposed to light?

Some transition metal complexes exhibit photochromism, changing color when exposed to light. This occurs when light energy causes structural changes in the complex, such as ligand dissociation or isomerization. These structural changes alter the d-orbital splitting or the nature of the electronic transitions, resulting in a color change. For example, some spiropyran-metal complexes change color reversibly upon UV light exposure.

27. How does the presence of multiple metal centers in a complex affect its color properties?

Complexes with multiple metal centers can exhibit more complex color properties due to potential interactions between the metal ions. These interactions can lead to new types of electronic transitions, such as metal-to-metal charge transfer, in addition to the usual d-d transitions. This can result in unique or more intense colors. For example, the intense blue color of Prussian blue arises from intervalence charge transfer between iron ions in different oxidation states.

28. What is the relationship between the color of a transition metal complex and its ability to act as a catalyst?

The color of a transition metal complex and its catalytic activity are both related to its electronic structure, particularly the d-orbital configuration. Complexes with partially filled d-orbitals often exhibit both color and catalytic activity. The ability to change oxidation states, which is crucial for many catalytic processes, also influences color. However, there isn't a direct correlation; a complex's color doesn't necessarily indicate its catalytic ability, as other factors like ligand lability and coordination geometry also play crucial roles in catalysis.

29. How do lanthanide contraction and the spectrochemical series interact to influence the colors of transition metal complexes across a period?

Lanthanide contraction causes a gradual decrease in atomic and ionic radii across the lanthanide series, affecting the properties of subsequent transition metals. This contraction leads to stronger metal-ligand interactions for the later transition metals in a period. Combined with the spectrochemical series, which ranks ligands based on their field strength, this effect can result in more intense colors for complexes of later transition metals with the same ligands, due to increased d-orbital splitting.

30. Why do some transition metal complexes exhibit different colors in solid state versus in solution?

The color difference between solid and solution states can arise from several factors:

31. How does spin-orbit coupling affect the color of transition metal complexes, particularly for heavier elements?

Spin-orbit coupling becomes more significant for heavier transition elements, particularly those in the second and third row. This coupling can lead to additional splitting of energy levels, resulting in more complex absorption spectra and potentially different colors compared to lighter analogues. For instance, this effect contributes to the differences in color between some platinum and palladium complexes despite their similar electronic configurations.

32. What is the role of HOMO-LUMO gap in determining the color of organometallic transition metal complexes?

In organometallic transition metal complexes, the HOMO-LUMO (Highest Occupied Molecular Orbital - Lowest Unoccupied Molecular Orbital) gap often corresponds to the energy of light absorbed. A smaller HOMO-LUMO gap results in absorption of lower energy (longer wavelength) light, shifting the color towards red. Conversely, a larger gap leads to absorption of higher energy light, shifting the color towards blue. The size of this gap is influenced by factors such as the metal's oxidation state, the nature of the organic ligands, and the complex's geometry.

33. How do inner-sphere and outer-sphere electron transfer mechanisms relate to the color changes observed in redox reactions of transition metal complexes?

Inner-sphere electron transfer involves the sharing of a ligand between the oxidized and reduced species, often resulting in a gradual color change as the electron transfer proceeds. Outer-sphere electron transfer occurs without ligand sharing and can lead to more abrupt color changes. Both mechanisms can contribute to the colorful nature of many redox reactions involving transition metals. Understanding these mechanisms helps explain the diverse color changes observed in redox titrations and other chemical processes involving transition metal complexes.

34. Why do some transition metal complexes exhibit vapochromism, and how is this property useful?

Vapochromism is the change in color of a compound upon exposure to vapor. In transition metal complexes, this often occurs due to the incorporation of volatile organic compounds (VOCs) into the crystal structure, altering the metal's coordination environment or the packing of the complex. This property is useful for creating sensors for detecting VOCs or other gases. The color change provides a visible indicator of the presence of specific vapors, making vapochromic complexes valuable in environmental monitoring and safety applications.

35. How does the concept of "optical electronegativity" relate to the color of transition metal oxides?

Optical electronegativity is a measure of an atom's ability to attract electrons in a chemical bond, as reflected in its optical properties. In transition metal oxides, the difference in optical electronegativity between the metal and oxygen influences the degree of covalency in the metal-oxygen bond. This, in turn, affects the energy of charge transfer transitions between metal and oxygen orbitals, which often dominate the color of these compounds. A larger difference typically results in more ionic bonding and higher energy transitions, leading to paler or more blue-shifted colors.

36. What is the significance of the "spectrochemical series" in predicting the magnetic properties and colors of transition metal complexes?

The spectrochemical series ranks ligands based on their ability to cause d-orbital splitting in metal complexes. Ligands higher in the series cause greater splitting, which can lead to: