Question

Which of the following compounds is not colored yellow? (a) \(\mathrm{Zn}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (b) \(\mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right]\) (c) \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{AS}\left(\mathrm{Mo}_{3} \mathrm{O}_{10}\right)_{4}\right]\) (d) \(\mathrm{BaCrO}_{4}\)

Short answer

(c) \( \left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{AS}\left(\mathrm{Mo}_{3}\mathrm{O}_{10}\right)_{4}\right] \) is not colored yellow.

Step-by-step solution

Analyzing the Color of Zn2[Fe(CN)6]

The compound \( \mathrm{Zn}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \) refers to zinc hexacyanoferrate(II). Transition metal complexes like this can possess colors due to electronic transitions, but zinc compounds typically do not impart strong color. However, iron could be responsible for the color here. Zinc hexacyanoferrate can appear pale blue to colorless, not yellow.

Analyzing the Color of K3[Co(NO2)6]

The compound \( \mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right] \) is potassium hexanitrocobaltate(III). This is a cobalt complex where Co is surrounded by nitrite ligands. Such complexes often display various colors, with yellow being a common color due to ligand-to-metal charge transfer transitions.

Analyzing the Color of (NH4)3[AS(Mo3O10)4]

The compound \( \left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{AS}\left(\mathrm{Mo}_{3}\mathrm{O}_{10}\right)_{4}\right] \) contains ammonium, arsenic, and molybdenum oxides. Molybdenum complexes can vary in color, often resulting in complex ions that are not typical colors like yellow. These are likely to be darker, depending on the exact structure and oxidation states of molybdenum.

Analyzing the Color of BaCrO4

The compound \( \mathrm{BaCrO}_{4} \) is barium chromate. Chromate ions (CrO4) are typically yellow due to d-d transitions and charge transfer bands. This color is associated with the chromium present in the oxidation state +6.

Comparing Analyzed Colors

Based on the above analysis, the compound \( \left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{AS}\left(\mathrm{Mo}_{3}\mathrm{O}_{10}\right)_{4}\right] \) is most likely not yellow. The others contain either chromate or nitrite-linked complexes, which are commonly yellow.

Key concepts

Transition Metal Complexes

Transition metal complexes are fascinating structures where a central transition metal atom is bonded to surrounding molecules or ions known as ligands. These complexes often exhibit a variety of colors.
Transition metals can engage in the formation of complex ions due to their ability to hold electrons in d orbitals. The d orbitals can split into subsets, called d-orbitals’ splitting, in the presence of ligands.

Depending on the nature and strength of the ligands, this splitting results in different energy levels, allowing for the movement or transition of electrons between these levels. This electron movement within the d orbitals can absorb part of the visible spectrum light, and the color observed is the complementary color of what is absorbed.

• These complexes are often involved in processes like catalysis and electronic materials.
• Electronic transitions between the d orbital levels are responsible for the colors observed in these complexes.
• The specific color seen depends on the metal involved and the type of ligands attached.

Understanding these complexes provides insight into a wide range of chemical and physical processes.

Ligand-to-Metal Charge Transfer

Ligand-to-metal charge transfer (LMCT) occurs when electrons move from a ligand to the transition metal ion within a complex. This process is crucial for explaining the vibrant colors seen in various compounds.
This electron movement usually involves ligands that can easily donate electrons, such as oxygen or nitrogen-coordinated ligands.
When an electron moves from the ligand to a partially filled or empty orbital of the metal, it leads to an excited state that can absorb light in the visible spectrum.

• LMCT is common in compounds with highly electronegative ligands such as nitro groups or oxyanions.
• This transfer contrasts with metal-to-ligand charge transfer (MLCT), where the electron moves from the metal to the ligand.
• The wavelengths of light absorbed during LMCT correspond to specific colors seen in complexes.

Overall, LMCT can be a significant source of color in transition metal complexes. It adds to the richness and variety of colors observed, alongside d-d transitions.

Color of Chromium Compounds

Chromium compounds are renowned for their vivid colors, which arise due to the presence of chromium in various oxidation states. Chromium can exist in several oxidation states, but the +6 state is particularly significant in coloration.
In the +6 oxidation state, as seen in chromate ions (CrO_4^{2-}), the yellow color is primarily due to a charge transfer band. This band results from electron transitions between molecular orbitals.

• The chromate ion, due to its tetrahedral shape, allows efficient electron transitions visible in color formation.
• Such transitions result in the bright yellow color typical of compounds like barium chromate (BaCrO_4).
• Other oxidation states of chromium, such as +3 or +2, can lead to green or green-blue colors, respectively.

Understanding the color of chromium compounds involves exploring the electronic structure and the environment of the chromium atoms. The interplay of oxidation states and ligand coordination impacts the colors that we observe.