Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of following complex ions will be coloured in aqueous state? (a) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\) (b) \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) (c) \(\left[\mathrm{Sc}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) (d) Both (b) and (c)

Step-by-Step Solution

Answer

(b) \(\left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+}\) will be colored.

Step 1: Recall the theory of color in complex ions

The color of complex ions is due to d-d electron transitions. For color to appear, there must be unpaired electrons in the d-orbitals. If all d orbitals are fully filled or empty, the complex ion will not display color.

Step 2: Evaluate the electron configuration of Ni in option (a)

In \([\mathrm{Ni}(\mathrm{CN})_{4}]^{2-}\), Ni is in the +2 oxidation state. The electron configuration of Ni is \[\text{Ni: } [\text{Ar}] 3d^8 4s^2\]. In +2 state: \[ 3d^8 \]. With \(\mathrm{CN}^-\) as a strong field ligand, the electrons pair up, leaving no unpaired electrons. Thus, it won't show color.

Step 3: Evaluate the electron configuration of Ni in option (b)

In \[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}]^{2+}\], similarly, Ni is in the +2 oxidation state, giving the electron configuration of \[ 3d^8 \]. \(\mathrm{H}_{2}\mathrm{O}\) is a weak field ligand, leading to not all electrons pairing up. There are unpaired electrons, so this complex ion will show color.

Step 4: Evaluate the electron configuration of Sc in option (c)

In \[\mathrm{Sc}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}]^{3+}\], Sc is in the +3 oxidation state. The electron configuration of Sc is \[\text{Sc: } [\text{Ar}] 3d^1 4s^2\]. In +3 state: \[ 3d^0 \]. With no d-electrons, there cannot be color shown by this complex ion.

Step 5: Determine the correct answer based on analysis

Based on the evaluations: (a) has no unpaired electrons due to strong field ligand; (b) has unpaired electrons because of weak field ligand; (c) has completely empty d-orbital. Therefore, only (b) will be colored.

Key Concepts

Complex Ions

Complex ions are fascinating chemical species. These ions are composed of a central metal atom or ion bonded to surrounding molecules or ions, called ligands. These ligands can be neutral molecules, like water, or charged ions, such as cyanide. Complex ions often exhibit unique properties due to the interactions between the central metal and its ligands, leading to diverse chemical behaviors.

• The central metal ion in a complex is typically a transition metal, known for having partially filled d-orbitals.
• Ligands donate pairs of electrons to the metal ion, forming coordinate covalent bonds, enriching the chemistry of these complexes.
• The number of coordinate bonds a metal can support is called its coordination number, influencing the structure and stability of the complex.

Understanding complex ions is crucial for explaining their color, magnetism, and reactivity.

Electron Configuration

Electron configuration refers to the distribution of electrons in an atom or ion across different orbitals. For complex ions, understanding electron configuration helps in predicting their properties, including their color. Electrons fill orbitals following the Aufbau principle, Pauli Exclusion Principle, and Hund's Rule. Transition metals often have unique configurations due to their d-orbitals.

• The electron configuration of transition metals changes with oxidation state, affecting their chemical behavior significantly.
• For example, in the case of nickel, the neutral atom configures as \[\text{Ni: } [\text{Ar}] 3d^8 4s^2\big]\]. In a "+2" oxidation state, the configuration becomes \[[3d^8\big]\].
• Ligand strength also affects electron configurations; strong field ligands pair up d-electrons, while weak field ligands allow for unpaired electrons.

Recognizing how ligands impact electron configurations helps in predicting whether a complex ion will be colored.

d-d Transitions

d-d transitions are central to the field of coordination chemistry when it comes to understanding the colors of complex ions. Within transition metals, electrons can be elevated from a lower d-orbital to a higher d-orbital, absorbing energy in the form of visible light. These d-d transitions depend on the electron configuration and the crystal field environment provided by the ligands.

• d-d transitions occur when electrons absorb photons and move between split d-orbitals within a transition metal ion.
• The presence of unpaired electrons in the d-orbitals is essential for these transitions, thereby affecting a complex's color.
• The difference in energy between these d-orbitals (known as crystal field splitting) determines the specific wavelength of light absorbed, influencing the observed color of the compound.

The concept is vital for identifying why certain substances appear colored while others do not.

Color Theory in Chemistry

Color theory in chemistry explains how substances absorb and reflect different wavelengths of light, causing them to appear colored to human eyes. For coordination compounds, colors arise primarily due to electronic transitions like d-d transitions, influenced by the metal ion's oxidation state and its surrounding ligands.

• Complex ions can exhibit a wide range of colors depending on the type of metal, its oxidation state, and the nature of the ligands.
• The color observed is complementary to the color of light absorbed; for instance, if a compound absorbs red light, it might appear green.
• The types of ligands influence the extent of d-orbital splitting, determining the wavelengths of light absorbed.

Understanding the basics of color theory in chemistry allows us to predict and interpret the colors of various complex ions.