Problem 59 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The correct statement(s) regarding the binary transition metal carbonyl compounds is (are) (Atomic numbers: \(\mathrm{Fe}=26, \mathrm{Ni}=28\) ) (a) Total number of valence shell electrons at metal centre in \(\mathrm{Fe}(\mathrm{CO})_{5}\) or \(\mathrm{Ni}(\mathrm{CO})_{4}\) is 16 (b) These are predominantly low spin in nature (c) Metal-carbon bond strengthens when the oxidation state of the metal is lowered (d) The carbonyl C-O bond weakens when the oxidation state of the metal is increased

Step-by-Step Solution

Answer

(b), (c), and (d) are correct.

Step 1: Understanding the Valence Electrons in Metal Carbonyls

For binary transition metal carbonyls like \( \text{Fe(CO)}_5 \) and \( \text{Ni(CO)}_4 \): For \( \text{Fe(CO)}_5 \), iron has 8 valence electrons (since it's Group 8) and gains 10 electrons from the 5 CO ligands (each CO provides 2 electrons), totaling 18. For \( \text{Ni(CO)}_4 \), nickel has 10 valence electrons (Group 10) and gains 8 electrons from the 4 CO ligands, summing also to 18.

Step 2: Valence Electron Count

The statement regarding the total number of valence shell electrons in \( \text{Fe(CO)}_5 \) or \( \text{Ni(CO)}_4 \) is incorrect if it proposes they have 16 electrons, as the correct total is 18 electrons for both complexes.

Step 3: Spin States in Metal Carbonyls

Binary metal carbonyls such as \( \text{Fe(CO)}_5 \) and \( \text{Ni(CO)}_4 \) are typically low-spin complexes due to the strong field of CO ligands, which leads to pairings of electrons in lower energy orbitals.

Step 4: Metal-Carbon Bond Strength and Oxidation States

A lower oxidation state of the metal typically implies a greater buildup of electron density back-donated from the CO to the metal, strengthening the metal-carbon bond. Therefore, statement (c) is true.

Step 5: Effect of Oxidation State on C-O Bond

Increasing the oxidation state of a metal reduces back-donation to CO π* orbitals, weakening the C-O bond (increasing bond order). Therefore, statement (d) is true.

Key Concepts

Valence Electrons

In transition metal carbonyl compounds, understanding valence electrons is essential to comprehend their stability and properties. Valence electrons are the electrons present in the outermost shell of an atom, which participate in bonding. In the case of metal carbonyls like \( \text{Fe(CO)}_5 \) and \( \text{Ni(CO)}_4 \), they achieve the stable 18-electron rule.
This rule states that transition metals can reach a stable electronic configuration akin to noble gases when they have a total of 18 electrons. For \( \text{Fe(CO)}_5 \), iron contributes 8 electrons while the 5 CO ligands provide an additional 10 electrons (2 per CO), summing to 18. Similarly, \( \text{Ni(CO)}_4 \) starts with 10 valence electrons from nickel and gains 8 electrons from 4 CO ligands.
Achieving this electron count helps these complexes to be highly stable, and deviations can lead to reactive or unstable configurations.

Low Spin Complexes

Low spin complexes usually occur in the presence of strong field ligands, like carbonyl (CO). In these complexes, the electrons prefer to pair up in lower energy orbitals before occupying higher energy ones.
The strong field strength of CO ligands ensures that compounds such as \( \text{Fe(CO)}_{5} \) and \( \text{Ni(CO)}_{4} \) are low spin. This behavior minimizes the overall energy of the system by reducing electron unpairing and thus stabilizing the complex.
• Strong field ligands like CO cause electrons to fill lower energy orbitals first.
• Leads to paired electrons in lower energy orbitals.
Low spin states often result in fewer unpaired electrons, affecting the magnetic and optical properties of the transition metal complexes.

Metal-Carbon Bond Strength

The strength of the metal-carbon bond in carbonyl complexes is intricately linked to the metal's oxidation state. This concept becomes critical when examining electron sharing between the metal atom and CO ligands.
A lower oxidation state typically allows for greater electron density at the metal center. This density enables better electron back-donation from the metal's d-orbitals into the empty \(\pi^*\) orbitals of CO.
• Metal in lower oxidation states has more electrons to back-donate.
• Stronger back-donation strengthens the metal-carbon (M-C) bond.
This intensification of bond strength is central to maintaining the stability and structural integrity of transition metal carbonyls, particularly under varying environmental conditions.

C-O Bond Weakening

In transition metal carbonyls, the interaction between the metal center and CO ligands can alter the native strength of the C-O bond. When the oxidation state of the metal is increased, the ability to donate electrons back to the CO ligands diminishes.
This reduced electron back-donation leads to a weaker bond within CO, as the π-electron density is decreased, making it more like a free \(C\equiv O\) bond.
• Higher metal oxidation state lessens electron back-donation.
• Results in weaker C-O bonds (higher bond order of \(C\equiv O\)).
Understanding these changes is pertinent when studying the reactivity and behavior of such complexes in chemical reactions or under stress.

Oxidation States

The concept of oxidation states is crucial to understanding the overall electron distribution in transition metal complexes like carbonyls. Oxidation states indicate the hypothetical charge the metal center would have if all bonding electrons were assigned to the most electronegative elements, typically the ligands.
In transition metal carbonyls, varying the oxidation state directly impacts electron availability for bonding both with the CO ligands and within the metal itself.
• Lower oxidation states mean more electron density around the metal.
• Influences bond strengths of M-C and C-O bonds.
Changes in oxidation due to chemical reactions can radically alter the properties and usefulness of these complexes, providing insight into their role in catalysis and industrial applications.