Problem 156 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following complexes is an outer orbital complex? (a) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) (b) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4}\) (c) \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) (d) \(\left[\mathrm{Mn}(\mathrm{CN})_{6}\right]^{4}\)

Step-by-Step Solution

Answer

The outer orbital complex is \([\text{Ni}(\text{NH}_3)_6]^{2+}\).

Step 1: Understand Outer Orbital Complex

An outer orbital complex utilizes the outermost d-orbitals (from the n-th shell) for hybridization. These are typically sp³d² hybridized orbitals.

Step 2: Analyze Option (a) \( \left[\text{Co}\left(\text{NH}_{3}\right)_{6}\right]^{3+} \)

Cobalt in this complex is in the +3 oxidation state, leading to an electronic configuration of [Ar] 3d⁶. NH₃ is a weak field ligand, causing the complex to form outer orbital (sp³d²) hybridization instead of inner (d²sp³).

Step 3: Analyze Option (b) \( \left[\text{Fe}( ext{CN})_{6}\right]^{4}\)

Iron here is in the +2 oxidation state, resulting in an electron configuration of [Ar] 3d⁶. CN⁻ is a strong field ligand, thus causing pairing of electrons and d²sp³ hybridization, making it an inner orbital complex.

Step 4: Analyze Option (c) \( \left[\text{Ni}\left(\text{NH}_{3}\right)_{6}\right]^{2+} \)

Nickel in the +2 oxidation state has the configuration [Ar] 3d⁸. With NH₃ as a weak field ligand, the complex forms sp³d² hybridization, indicating an outer orbital complex.

Step 5: Analyze Option (d) \( \left[\text{Mn}( ext{CN})_{6}\right]^{4} \)

Manganese in the +4 oxidation state results in a configuration of [Ar] 3d³. CN⁻ is a strong ligand, leading to d²sp³ hybridization, confirming it as an inner orbital complex.

Step 6: Summarize the Correct Answer

From the analysis, the complex \( \left[\text{Ni}\left(\text{NH}_{3}\right)_{6}\right]^{2+} \) is the only one forming sp³d² hybridization, making it an outer orbital complex.

Key Concepts

Hybridization in Chemistry

Hybridization in chemistry is a vital concept for understanding the bonding and structure of many compounds. It describes how different atomic orbitals mix to form new hybrid orbitals. These hybrids can better explain the electronic geometry and bonding properties of a molecule. For coordination compounds, in particular, hybridization dictates the spatial arrangement of ligands around the central metal atom. This leads to specific shapes and types of orbital overlap.

• In sp³d² hybridization, which is related to outer orbital complexes, the s, three p, and two d orbitals mix. This creates a set of six hybrid orbitals.
• These orbitals are arranged octahedrally, allowing them to accommodate six ligands, commonly seen in many coordination complexes.

Understanding hybridization helps predict the physical properties and reactivity of compounds, making it essential for any budding chemist.

Coordination Compounds

Coordination compounds are complex structures where a central atom, usually a metal, is surrounded by molecules or working ligands. These ligands, which can be neutral or negative ions, donate one or more pairs of electrons to the central atom, forming coordinate covalent bonds.

• The coordination number refers to the number of ligand atoms bonded to the central atom. This number plays a key role in determining the geometry of the complex.
• Ligands can influence the stability and reactivity of the complex, with stronger field ligands often causing electron pairing, which can influence the type of hybridization and whether a compound is an outer or inner orbital complex.
• An outer orbital complex uses orbitals from an outer shell (like 4d or 5d) to form hybrid orbitals, often associated with larger central atoms and weak field ligands.

The understanding of these compounds is vital in various applications, from material science to biochemical processes.

Crystal Field Theory

Crystal Field Theory (CFT) explains how the arrangement of ligands affects the energy of d-orbitals in transition metal complexes, shedding light on their color and magnetic properties. When ligands approach a central metal ion, they can induce an electric field that alters the metal's d-orbital energies.

• In an octahedral complex, such as many coordination compounds, this results in the splitting of the degenerate d-orbitals into two sets - the lower-energy t₂g orbitals and the higher-energy e_g orbitals.
• The field strength of ligands (weak vs. strong) determines the extent of this splitting. Strong field ligands can cause significant splitting and electron pairing, often resulting in low-spin complexes and inner orbital hybridization.
• Conversely, weak field ligands cause less splitting, often resulting in high-spin complexes and outer orbital hybridization (sp³d²).

Through CFT, one can gain insights into the electronic transitions that cause specific spectral properties, which are key to interpreting the behavior of transition metal complexes. This framework is invaluable in the field of inorganic chemistry and aids in the interpretation of various experimental observations.