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5.5.5 - Colour in Coordination Compounds

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Introduction to Colour in Coordination Compounds

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Teacher
Teacher

Today, we're examining how coordination compounds exhibit colors. Who can tell me what determines the color we see in a compound?

Student 1
Student 1

Is it about the light they absorb?

Teacher
Teacher

Exactly! The color observed is indeed related to the light absorbed. Specifically, the complex absorbs certain wavelengths of visible light and reflects others. This reflection gives us the visible color of the compound.

Student 2
Student 2

But why do different compounds have different colors?

Teacher
Teacher

Great question! The variety in colors stems from electronic transitions between d orbitals that change as we alter the ligands and their arrangement around the metal ion.

Student 3
Student 3

Is that linked to crystal field theory?

Teacher
Teacher

Yes, exactly! Crystal field theory explains how ligands affect the energies of d orbitals, resulting in color changes.

Student 4
Student 4

Can you give us an example?

Teacher
Teacher

Certainly! Let's consider [Ti(H2O)6]3+. It absorbs blue-green light which makes it appear violet. This relationship is crucial in understanding color in coordination compounds.

Crystal Field Theory and Splitting

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Teacher
Teacher

Let's dive deeper into crystal field theory. Who can explain what happens to d orbitals when ligands approach a metal ion?

Student 1
Student 1

The d orbitals split into different energy levels due to electron repulsion?

Teacher
Teacher

Exactly. In octahedral complexes, for instance, the d orbitals split into two groups: t2g and eg. The extent of this splitting affects the color seen.

Student 2
Student 2

What determines the amount of splitting?

Teacher
Teacher

Good point! The type of ligands—whether they are strong field or weak field ligands—play a major role in this. Strong field ligands cause greater splitting.

Student 3
Student 3

Can you explain how that affects colors?

Teacher
Teacher

Sure! Greater splitting means that specific energies are required for electron transitions. Depending on which energies are absorbed, we see different colors.

Student 4
Student 4

Could you summarize how we determine the color?

Teacher
Teacher

Certainly! The color observed is the complementary color of the light absorbed, influenced by the type of ligands and their arrangement around the metal.

Examples of Colour and Chemistry

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Teacher
Teacher

Let's review some examples to solidify our understanding. What color does the complex [Cu(H2O)4] appear?

Student 1
Student 1

I think it’s blue.

Teacher
Teacher

Correct! And why is that?

Student 2
Student 2

Because it absorbs red light.

Teacher
Teacher

Exactly! Let's look at another example: [Ni(H2O)6]2+. What color is this complex?

Student 3
Student 3

It appears green, right?

Teacher
Teacher

That's correct! It absorbs light in the red region, which explains the green color observed.

Student 4
Student 4

What about [CoCl(NH3)5]?

Teacher
Teacher

Excellent inquiry! The color depends on its specific structure and interactions with ligands. Coordination compounds truly exhibit fascinating colors due to these interactions.

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section explores how coordination compounds exhibit a variety of colors resulting from d-d electronic transitions and crystal field splitting.

Standard

Coordination compounds display distinct colors due to the absorption of specific wavelengths of light, which can be explained through crystal field theory and the arrangement of d orbitals. The resulting color of a coordination complex is the complementary color of the absorbed light, influenced by ligands and the geometry of the complex.

Detailed

Colour in Coordination Compounds

Coordination compounds are notable for their rich variety of colors, which arise primarily from electronic transitions of d electrons within the metal ion. When light interacts with these compounds, specific wavelengths are absorbed while others are reflected or transmitted, resulting in the observed color.

According to crystal field theory, the presence of ligands around a central metal ion leads to the splitting of d orbitals into two energy levels—higher and lower energy—depending on the geometry of the complex (e.g., octahedral, tetrahedral).

Key Points:

  • Complementary Colour: The color seen is the complementary color of the absorbed light.
  • Crystal Field Splitting: The arrangement of ligands influences the degree of splitting of d orbitals and thus affects the color observed.
  • Example - [Ti(H2O)6]3+: This complex absorbs blue-green light, resulting in its violet appearance.

In a systematic exploration, various coordination entities like [CoCl(NH3)5] and [Cu(H2O)4] illustrate the relationship between absorbed wavelengths and the colors produced, emphasizing how different ligands can modify the electronic structure of the metal ion, ultimately leading to different observed colors.

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Audio Book

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Tetrahedral vs Octahedral Splitting

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In tetrahedral coordination entity formation, the d orbital splitting is inverted and is smaller as compared to the octahedral field splitting. For the same metal, the same ligands and metal-ligand distances, it can be shown that Dt = (4/9) D0. Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and, therefore, low spin configurations are rarely observed. The ‘g’ subscript is used for the octahedral and square planar complexes which have centre of symmetry. Since tetrahedral complexes lack symmetry, ‘g’ subscript is not used with energy levels.

Detailed Explanation

In coordination compounds, the arrangement of ligands around a metal center affects the energy levels of its d orbitals. In octahedral complexes, d orbitals split into two energy levels due to the symmetrical placement of ligands. Tetrahedral complexes have a different splitting pattern where the lower energy orbitals are now higher in energy compared to the octahedral case. This results in a smaller energy difference (Dt) in tetrahedral complexes, making it less likely for electrons to pair up, often leading to high spin states.

Examples & Analogies

Think of a room filled with chairs (ligands) set up for a meeting. In an octahedral arrangement, chairs are placed symmetrically around a central table (the metal). This setup naturally divides the space into 'boring' and 'exciting' areas. In a tetrahedral setup, the chairs are arranged unevenly, making the 'boring' space feel less appealing, leading to people (electrons) staying rather than pairing up with someone else (which would cost energy).

Understanding Colors in Coordination Compounds

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In the previous Unit, we learnt that one of the most distinctive properties of transition metal complexes is their wide range of colours. This means that some of the visible spectrum is being removed from white light as it passes through the sample, so the light that emerges is no longer white. The colour of the complex is complementary to that which is absorbed. The complementary colour is the colour generated from the wavelength left over; if green light is absorbed by the complex, it appears red. Table 5.3 gives the relationship of the different wavelength absorbed and the colour observed.

Detailed Explanation

Transition metal complexes can absorb specific wavelengths of light due to the excitation of electrons between different d orbitals. The remaining wavelengths that are not absorbed are what we perceive as the color of the compound. For instance, if a compound absorbs light in the green range of the visible spectrum, it will appear red, which is the complementary color. This interaction is described by the color wheel, where opposite colors enhance our understanding of how these compounds behave.

Examples & Analogies

Imagine wearing sunglasses that only let red light pass through. You would see everything around you in shades associated with that filter's effect, much like how transition metal dyes absorb some colors of light and reflect others. If you put on green-tinted glasses that filter out reds, you'll see all other colors shifted, which is akin to how color in coordination compounds works when certain wavelengths are absorbed.

Case Study: [Ti(H2O)6]3+ Coloration

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The colour in the coordination compounds can be readily explained in terms of the crystal field theory. Consider, for example, the complex [Ti(H2O)6]3+, which is violet in colour. This is an octahedral complex where the single electron (Ti is a 3d system) in the metal d orbital is in the t2g level in the ground state of the complex. The next higher state available for the electron is the empty eg level. If light corresponding to the energy of blue-green region is absorbed by the complex, it would excite the electron from t2g level to the eg level (t2g eg ® t2g eg). Consequently, the complex appears violet in colour.

Detailed Explanation

In this specific complex, the presence of water ligands creates an octahedral arrangement around the titanium ion. The electron transitions occurring within the split d orbitals cause the complex to absorb specific wavelengths of light, primarily those associated with blue-green. The energy absorbed causes an electron to jump to a higher energy level, changing the way we perceive the color of the complex, making it appear violet.

Examples & Analogies

Consider a prism separating white light into a rainbow. Each colored band corresponds to different wavelengths of light. When a specific band of light is absorbed by a chemical substance, the remainder determines how we see it. Just as different prisms can filter out specific colors, in compounds like [Ti(H2O)6]3+, certain light wavelengths get absorbed for electron transitions, letting us see the color that remains, much like the colors of a sunset filtered through clouds.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Crystal Field Splitting: The separation of d orbitals caused by the electric field produced by surrounding ligands.

  • Complementary Colors: The color we see is complementary to the color absorbed.

  • Ligands: Ions or molecules that can donate electron pairs to a metal ion in a coordination compound.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • [Ti(H2O)6]3+ absorbs blue-green light and appears violet.

  • [Cu(H2O)4] appears blue as it absorbs red light.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • In coordination's light parade, colors shine, absorbed and displayed.

📖 Fascinating Stories

  • Imagine a room filled with colored lights. Each color dances as its complementary color hides in the shadows. This is like our compounds!

🧠 Other Memory Gems

  • Colors Absorbed = Colors Opposed (CA = CO) to remember complementary colors.

🎯 Super Acronyms

CFL - Color From Light for recalling how light determines color.

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Coordination Compound

    Definition:

    A compound formed from the coordination of metal ions with ligands.

  • Term: Crystal Field Theory

    Definition:

    A model explaining the electronic structure of coordination compounds based on the splitting of d orbitals in the presence of ligands.

  • Term: Ligand

    Definition:

    An ion or molecule that binds to a central atom in a coordination compound.

  • Term: dD Transition

    Definition:

    An electronic transition between d orbitals in coordination compounds that causes color effects.

  • Term: Complementary Colour

    Definition:

    The color observed in a compound which is opposite to the color of light absorbed.