Question

When degenerate d-orbitals of an isolated atom/ion are brought under the impact of magnetic field of ligands, the degeneracy is lost. The two newly formed sets of d-orbitals, depending upon nature and magnetic field of ligands are either stabilized or destabilized. The energy difference between the two sets whenever lies in the visible region of the electromagnetic spectrum, then the electronic transition \(\mathrm{t}_{2 \mathrm{~g}} \rightleftharpoons \mathrm{e}_{\mathrm{g}}\) are responsible for colours of the co-ordination compounds Which of the following colour is not due to d-d transition of (a) Yellow colour of CdS. (b) Red colour of blood (c) Orange colour of \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) in acidic medium. (d) Both (a) and (c).

Short answer

CdS and Cr2O7^{2-} colors are not due to d-d transitions.

Step-by-step solution

Understanding the Scenario

This problem asks us to identify which substances' colors are not due to d-d electronic transitions. This is related to the electronic structure of transition metal compounds, where the d-orbitals can split in energy levels due to ligands, causing color changes when electrons transition between these energy levels.

Reviewing d-d Transition

The d-d transition occurs in coordination complexes, like transition metal ions, where electrons jump between split d-orbitals. These transitions absorb specific wavelengths of light, producing characteristic colors related to certain metals.

Analyzing Yellow CdS

Cadmium sulfide (CdS) is a compound where cadmium is not a transition metal, as it lacks d-electrons in its ground state that can undergo d-d transitions. As a result, the yellow color of CdS is typically due to band gap transitions, not d-d transitions.

Analyzing Blood's Red Color

The red color of blood is due to heme groups in hemoglobin, which contain iron, a transition metal. The coordination complex of iron in heme can facilitate d-d transitions; however, the color mainly arises due to conjugated bonds and not d-d transitions.

Analyzing Orange
Cr2O7^{2-} Color

The orange color of dichromate ion (
Cr2O7^{2-}) is due to charge transfer transitions instead of d-d transitions. This involves electron movement between metal and ligand rather than within the d-orbitals.

Identifying the Correct Answer

Given that the colors of CdS and Cr2O7^{2-} do not arise from d-d transitions, they are not due to the electronic transitions usually responsible for color in coordination compounds. Thus, options (a) and (c) are correct.

Key concepts

d-d electronic transitions

In coordination chemistry, d-d electronic transitions are a fascinating phenomenon. These occur when an electron jumps between d-orbitals that have been split into different energy levels due to the presence of ligands around a metal ion. Normally, the d-orbitals in a free metal ion or atom are degenerate, meaning they have the same energy level. However, when ligands approach, they create a magnetic field that affects these d-orbitals, causing their energies to split. The energy difference between these split d-orbitals typically falls within the visible region of the electromagnetic spectrum. When light shines on the coordination complex, the electron absorbs energy and moves from a lower-energy d-orbital to a higher-energy one, a process known as a d-d transition. This absorbed light corresponds to specific wavelengths and is subtracted from white light, producing the perceived color of the compound.

ligand field theory

Ligand field theory is a crucial concept in understanding the behavior of coordination compounds. It explains how ligands affect the energy levels of d-orbitals in a transition metal ion. When ligands surround a central metal ion, they create an electrostatic field that influences the energies of the metal's d-orbitals. This process is different for various ligands and results in a different pattern of splitting. Role of Ligands Ligands are classified based on their ability to split the d-orbital energies. Strong-field ligands, like CN^-, cause a large split in d-orbital energy levels, whereas weak-field ligands, like I^-, cause a smaller split. Splitting Patterns The specific arrangement and type of ligands determine how much the d-orbitals split and the pattern of this splitting. For example, in an octahedral complex, the d-orbitals split into two sets: t_2g (lower energy) and e_g (higher energy). The size of this splitting dictates how easily d-d transitions can occur, influencing the color characteristics of the compound.

color of coordination compounds

The brilliant and diverse colors of coordination compounds can be attributed mainly to d-d electronic transitions and, sometimes, charge transfer. The color observed for a coordination compound is directly related to which wavelengths of light are absorbed by the compound due to electronic transitions. d-d Transitions When electrons transition between split d-orbitals within a transition metal, they absorb certain wavelengths of light. The remaining light is what human eyes perceive as color. For a d-d transition to contribute to color, the energy gap between orbitals must match the energy of visible light. Charge Transfer In some cases, charge transfer can also contribute significantly to the color of coordination compounds. This occurs when an electron transfers from a ligand to the metal or vice versa. This type of transition tends to be more intense than d-d transitions and contributes to more vivid colors. In conclusion, by understanding the interaction between light and coordination compounds, especially in terms of d-d transitions and ligand field effects, we can begin to appreciate the vivid colors of these chemical entities.