Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following metal complex, does the central metal atom have zero Oxidation state? (a) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{SO}_{4}\) (b) \(\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (c) \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) (d) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\)

Step-by-Step Solution

Answer

(c) \( \left[\mathrm{Ni}(\mathrm{CO})_{4}\right] \) has the central metal with zero oxidation state.

Step 1: Identify Oxidation State Formula

The oxidation state of a metal in a complex can be determined using the formula: \( \text{Oxidation State of Metal} = \text{Charge of Complex} - \sum \text{Charges of Ligands} \). This involves knowing the charges of common ligands and the overall charge of the complex.

Step 2: Evaluate Option (a)

For \( \left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{SO}_{4} \), the ammonia ligand (\( \mathrm{NH}_3 \)) is neutral, and \( \mathrm{SO}_4 \) has a -2 charge. So, the overall complex is neutral, and the charge on copper (Cu) will be +2 to balance the sulfate's -2 charge.

Step 3: Evaluate Option (b)

In \( \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \), each cyanide ion (\( \mathrm{CN}^- \)) has a charge of -1. With 6 cyanides, the total negative charge is -6. Since there are 3 potassium ions (\( \mathrm{K}^+ \)), contributing +3, the overall complex charge is -3. Therefore, the oxidation state of iron (Fe) needs to be +3 to make the overall charge -3 [(+3) + (-6)].

Step 4: Evaluate Option (c)

In \( \left[\mathrm{Ni}(\mathrm{CO})_{4}\right] \), \( \mathrm{CO} \) (carbon monoxide) is a neutral ligand. Therefore, the nickel complex is neutral overall, implying the oxidation state of nickel is 0.

Step 5: Evaluate Option (d)

For \( \left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right] \), ammonia (\( \mathrm{NH}_3 \)) is neutral, and each chloride ion (\( \mathrm{Cl}^- \)) has a charge of -1. Thus, with two chlorides, the total negative charge is -2. The platinum (Pt) must have a +2 charge to make the entire complex neutral.

Step 6: Identify Zero Oxidation State Complex

From the evaluation, option (c) \( \left[\mathrm{Ni}(\mathrm{CO})_{4}\right] \) has a central metal atom (Ni) with an oxidation state of 0, as all the carbon monoxide ligands are neutral.

Key Concepts

Understanding Metal Complexes

Metal complexes are fascinating structures in chemistry. They consist of a central metal atom or ion surrounded by molecules or ions known as ligands. These complexes play a significant role in various fields, including bioinorganic chemistry, catalysis, and material sciences.
Most metal complexes are formed by transition metals, known for their diverse oxidation states and ability to form colored compounds. In these complexes, ligands donate electron pairs, forming coordinate bonds with the central metal.

• Central Metal Atom or Ion: Usually a transition metal. Important for its ability to adopt different oxidation states.
• Ligands: They wrap around the metal atom and stabilize the complex. They can be neutral or charged.

Metal complexes can vary greatly in terms of geometry, stability, and the number of bonds formed, also known as coordination number. Recognizing these aspects can help in predicting their stability and reactivity in different chemical environments.

What are Ligands?

In metal complexes, ligands play a crucial role by surrounding the central metal. They coordinate with the metal through lone pairs of electrons, forming strong bonds.

• Neutral Ligands: These do not carry a charge. Example: Ammonia (NH_3), Carbon monoxide (CO).
• Anionic Ligands: These carry a negative charge, like chloride (Cl^-) or cyanide (CN^-).

Ligands can vary in their denticity, which refers to the number of donor atoms they use to bind to the metal. Monodentate ligands use one donor atom, while polydentate ligands use multiple. This distinction is important in understanding the structure and stability of the metal complex.
The type of ligand and its charge is essential in determining the overall charge and oxidation state of the metal within the complex.

Calculating the Charge

Determining the charge of a metal complex involves understanding the charges associated with ligands and the central metal atom. To calculate the oxidation state of the metal, the following formula is used:
Oxidation State of Metal = Charge of Complex - Sum of Charges of Ligands
This formula helps identify the charge on the metal atom, making it easier to predict the behavior of the complex.

• Neutral Ligands: Contribute zero to the sum of charges.
• Charged Ligands: Add their charge value to the overall sum.

For example, when considering [Ni(CO)_4], the carbon monoxide ligands are neutral, making the overall charge of the complex neutral too. Thus, the oxidation state for nickel here is zero. On the contrary, in a complex like K_3[Fe(CN)_6], charged cyanide ligands greatly influence the metal's oxidation state calculation, resulting in an Fe oxidation state of +3.

The Role of Transition Metals

Transition metals are the backbone of metal complexes. They are located in the central block of the periodic table and showcase unique properties, prominently including variable oxidation states. This ability allows them to form complex structures with a range of ligands.
Properties of Transition Metals:

• Variable Oxidation States: Enables the formation of multiple types of complexes.
• Colorful Compounds: Due to d-d electron transitions, contributing to their application in pigments.
• Magnetic Properties: Due to unpaired d-electrons in certain oxidation states.

These metals, like Ni in [Ni(CO)_4], can be found with different oxidation states, which is crucial while calculating the charge of the complexes. Their flexibility in coordination numbers and geometry is a major reason for their widespread utility in catalysis and material technology.