Introduction

Transition metals can produce complexes or ions with a wide range of colors. The colors depend on the element and on whether or not it is dissolved in water or another solvent. Because they show the makeup of the sample, the colors are helpful in qualitative research. An element is considered to be a transition element if it can give rise to stable ions with incompletely filled d-orbitals. According to this theory, not all elements in group d of the periodic table are transition elements. A typical transition metal can exist in a wide variety of oxidation states because it lacks a partially filled d-orbital.

Transition elements

Elements near the center of the periodic table with occupied d orbitals are sometimes referred to as transition elements, however, they should be called d block elements instead. An element is said to be transitional if it produces one or more stable ions with only partly full d orbitals. Zn with the electronic configuration [Ar]3d104s2 is not a transition element by any definition. Its 3D orbital structure is complete. Ionization removes the 4s electrons, leaving the 3d shell full.

Why do we see different colors among transition elements?

Coloration is common in complexes that contain transition elements but not in those that do not. This demonstrates that the unoccupied d-orbitals contribute to color creation. Keep in mind that the d-orbitals of transition metals are only half full.

Since transition elements have unoccupied d-orbitals, the resulting complexes, and solutions tend to be a rainbow of hues. Since the d-orbitals are degenerate, the ions lack inherent color. They share similar spectral signals and energies, which is another way of saying they are similar. As they bond with other molecules, transition metal ions take on new colors in the process of forming complexes and compounds. Complexes are formed when one or more negatively or neutrally charged ligands bind to a transition metal. The ligand alters the d-orbital geometry. The energy levels of some d-orbitals increase while those of others decrease. This causes an energetic hoover. The (wavelength) of the absorbed photon is set by the magnitude of the energy gap.

Colors of transition metal ions

Unsuppressed light waves go through a structure. Some light can be reflected by a molecule as well. The complexes' apparent colors result from a trifecta of reflection, absorption, and transmission. For instance, a red-light-absorbing electron might be stimulated to a higher energy state. As the non-absorbed light is the color we perceive, the color would be blue-green.

 Why aren't all transition elements colored?

The occurrence of color is not a certain consequence of oxidation. Transition metal ions with zero or ten d-electrons create a colorless solution.

The fact that not all of the components in the group are transition elements is another reason why not all of them display colours. Not all elements in the d block are transition elements since they do not all have partially filled d orbitals. Since Zn2+ has a totally filled d-orbital configuration and Sc3+ has entirely empty d-orbitals, they do not meet the criteria for becoming transition elements.

Factors influencing  the color of a complex of transition elements

Each complex will have a unique metal ion at its core and will have various settings. There are several factors that influence the color of these metal complexes:

• Different ligands have varying effects on the energies of the core ion's d-orbitals. Certain ligands with strong electrical fields produce a significant energy gap. When the d orbitals break out into two groups, there is a massive discrepancy in energy. The gaps are less pronounced in others because their fields are weaker.

• The degree of d-orbital splitting increases with increasing oxidation state. Variations in oxidation state alter the color of the received light and, by extension, the color of the light that reaches your eyes.

• The color shifts when the coordination number of elements changes since octahedral ions are more easily divisible than tetrahedral ions.

Different Colors of Transition Complexes

The colors of transition metal complexes change dramatically between solvent types. In a complex, the color is set by the ligand. Fe2+ is barely visible as a faint green in the water, but it precipitates out as a bright green in concentrated NaOH/carbonate/NH3. Co2+ produces a pink-colored solution in water, but it precipitates in three distinct colors when combined with NaOH, NH3, and carbonate: blue-green, straw, and pink.

The lanthanide elements can also produce colored complexes. Lanthanides are a category of transition metals that are also known as inner transition metals. Nevertheless, 4f electron transitions are responsible for the colorful compounds. The colors of lanthanide complexes are less sensitive to ligand type than those of transition metal complexes.

Summary

It may be deduced that when atoms and molecules absorb light at the correct frequency, the electrons are stimulated to relatively high-energy orbitals. Absorption of visible light is common in coordination compounds due to the energy gap between their d orbitals. A free ion of a transition element has degenerate d-orbitals (all have identical energy.) Yet, the d-orbitals of transition elements become non-degenerate as they interact with the electron cloud of the ligands in coordination compounds (not all having identical energy.)

Frequently Asked Questions

1. Why does the violet color of KMnO4 vanish (become colorless) when heated?

It has a violet hue because it is an octahedral compound that gives out light in the violet spectrum. In the absence of a ligand, the splitting of d-orbitals does not occur, and the substance becomes colorless as a result of dehydration caused by heating.

2. What is the reason for color in chromate ions?

Ans. Most transition metals show color due to d-d transition. CrO42- is salt, and its color does not result from a d-d transition.

3. What is the effect of multiple valencies of transition metals?

It's because of the ligand's chemical composition. Based on the ligand it interacts with the same charge on a metal ion and might yield a distinct color.