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

(c) \(\left[\mathrm{Ni}\left(\mathrm{NH}_3\right)_6\right]^{2+}\) is an outer orbital complex.

Step 1: Understand the Question

We need to find which of the given complexes is an outer orbital complex. An outer orbital complex involves the use of the outer d-orbitals in bonding rather than the inner d-orbitals.

Step 2: Analyze Valence Configuration of Cobalt

Consider \(\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}\). Cobalt in this complex is in the +3 oxidation state, so it has the electronic configuration \([\mathrm{Ar}] \, 3d^6\). NH3 is a weak field ligand and will not cause pairing.

Step 3: Identify the Type of Hybridization

Since NH3 is a weak field ligand, it will use 4s, 4p, and 4d orbitals for hybridization, resulting in an outer orbital (sp^3d^2) configuration.

Step 4: Analyze Valence Configuration of Iron

Consider \(\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}\). Iron is in the +2 oxidation state, so its electronic configuration is \([\mathrm{Ar}] \, 3d^6\). CN^- is a strong field ligand, leading to 3d orbital pairing and resulting in an inner d²sp³ hybridization.

Step 5: Analyze Valence Configuration of Nickel

Consider \(\left[\mathrm{Ni}\left(\mathrm{NH}_3\right)_6\right]^{2+}\). Nickel is in the +2 oxidation state, having the configuration \([\mathrm{Ar}] \, 3d^8\). NH3 is a weak field, using outer 4s, 4p, and 4d orbitals resulting in sp^3d^2 hybridization, which is an outer orbital complex.

Step 6: Conclusion from Hybridizations

By comparing all steps, \([\mathrm{Ni}\left(\mathrm{NH}_3\right)_6\right]^{2+}\) uses sp^3d^2 and is an outer orbital complex, whereas others use inner orbitals or have strong field ligands causing inner orbital usage.

Key Concepts

Chemical Bonding

Chemical bonding is the force that holds atoms or ions together in a molecule or compound. It is a fundamental concept in chemistry that determines how atoms combine, react, and transform to form new substances. In coordination chemistry, chemical bonding involves the interaction between a central metal atom and surrounding ligands. The type of chemical bond can vary, such as:

• Ionic Bonds: Formed between metals and non-metals where electrons are transferred from one atom to another.
• Covalent Bonds: Involves sharing of electrons between two atoms, typically non-metal atoms.
• Coordinate Bonds: A covalent bond in which both electrons come from the same atom, often seen in coordination complexes.

In the context of outer orbital complexes, the focus is on how ligands bond with metal ions through coordinate covalent bonds, leading to specific configurations and properties.

Coordination Chemistry

Coordination chemistry is the study of compounds composed of a central metal atom bonded to surrounding molecules or ions, known as ligands. The resulting complex can exhibit unique geometric and electronic configurations, influencing its properties and reactivity. In coordination complexes:

• Central Metal Ion: Usually a transition metal, providing empty orbitals for interactions.
• Ligands: Molecules or ions that donate electron pairs to the metal atom, forming coordinate bonds.
• Coordination Number: The total number of coordinate bonds formed between the central metal and ligands.

The interaction between the metal and ligands can lead to variations in bonding, such as using either inner or outer orbitals for hybridization. This determines whether the complex is an inner or outer orbital complex, impacting chemical behavior and stability.

Hybridization

Hybridization is the process where atomic orbitals mix to form new, identical orbitals that can then overlap to form bonds. It's a key concept to explain the geometry and bonding properties in molecules, especially in coordination chemistry. For coordination complexes:

• Inner Orbital Complexes: Involve inner d orbitals, resulting in low-spin states due to pairing of electrons.
• Outer Orbital Complexes: Utilize outer orbitals, such as 4d in the case of transition metals, which are associated with high-spin states and weaker field ligands like NH extsubscript{3}.
• Hybrid Types: Common hybridizations include sp extsuperscript{3}, d extsuperscript{2}sp extsuperscript{3}, and sp extsuperscript{3}d extsuperscript{2}, affecting the resulting geometry such as octahedral, tetrahedral, and square planar.

Understanding hybridization helps identify the type of complex, as seen with Nickel's sp extsuperscript{3}d extsuperscript{2} in an outer orbital complex.

d-Orbitals

d-Orbitals are a set of five orbitals found in the electron shells of transition metals and are critical in determining their chemical properties and bonding potential. In coordination chemistry, d-orbitals play different roles depending on whether an inner or outer orbital complex is formed. Key points about d-orbitals:

• Configurations: In transition metals, d-orbitals can participate in bonding by either remaining unpaired or by pairing up based on the ligand's strength and field.
• Inner vs Outer: Inner orbital complexes use lower energy inner d-orbitals, while outer orbital complexes engage higher energy outer d-orbitals, affecting the arrangement and hybridization scheme.
• Role in Magnetism: The pairing of electrons in d-orbitals plays a crucial role in determining whether a complex is paramagnetic or diamagnetic.

In the example of Nickel's complex, its outer orbital involvement with sp extsuperscript{3}d extsuperscript{2} hybridization involves using higher energy d-orbitals, resulting in an outer orbital complex.