Among the ligands NH3, en, CN- and CO, the correct order of their increasing field strength, is
(a)

Q: Why is CO a stronger ligand than CN- even though CN- has a negative charge?

While $\text{CN}^-$ is a good $\sigma$-donor due to its negative charge, $\text{CO}$ is a much better $\pi$-acceptor. The empty $\pi^*$ antibonding orbitals of $\text{CO}$ are lower in energy and better aligned to accept electron density back from the metal's $d$-orbitals. This stronger synergic bonding makes $\text{CO}$ the strongest ligand in the spectrochemical series.

Q: Why does 'en' have a stronger field than NH3?

Ethylenediamine ($\text{en}$) is a bidentate ligand, meaning it forms a ring structure with the central metal ion. This is called the chelate effect. Chelating ligands generally cause a slightly larger crystal field splitting than their monodentate counterparts (like $\text{NH}_3$) with the same donor atoms.

Q: Do I need to memorize the entire spectrochemical series for JEE?

While memorizing the exact position of every single ligand isn't strictly necessary, you absolutely must know the general trend based on donor atoms (Halogen $<$ O $<$ N $<$ C) and the relative positions of common ligands like $\text{H}_2\text{O}$, $\text{NH}_3$, $\text{CN}^-$, and $\text{CO}$.

Q: What exactly makes a ligand a 'strong field' ligand?

A strong field ligand is one that causes a large energy difference ($\Delta_o$) between the split $t_{2g}$ and $e_g$ $d$-orbitals. This usually happens when the ligand can participate in $\pi$-backbonding, pulling it closer to the metal and increasing the electrostatic interaction.

Q: Are all carbon donors strong field ligands?

Generally, yes. Ligands that coordinate through a carbon atom (like $\text{CO}$, $\text{CN}^-$, and alkyl groups) are typically strong field ligands because carbon is less electronegative than nitrogen or oxygen, making it a better electron donor, and they often have empty orbitals suitable for $\pi$-backbonding.

Question

Among the ligands $	ext{NH}_3$, $	ext{en}$, $	ext{CN}^-$ and $	ext{CO}$, the correct order of their increasing field strength, is

Options:

(A) $	ext{CO} < 	ext{NH}_3 < 	ext{en} < 	ext{CN}^-$

(B) $	ext{NH}_3 < 	ext{en} < 	ext{CN}^- < 	ext{CO}$

(C) $	ext{CN}^- < 	ext{NH}_3 < 	ext{CO} < 	ext{en}$

(D) $	ext{en} < 	ext{CN}^- < 	ext{NH}_3 < 	ext{CO}$

Sigma Percentile · Accepted Answer

### The Power of Ligands: Decoding the Spectrochemical Series

In the fascinating world of Coordination Chemistry, not all ligands are created equal. When ligands approach a central metal ion, they cause the five degenerate $d$-orbitals of the metal to split into different energy levels. The energy difference between these split levels is known as the **Crystal Field Splitting Energy ($\Delta_o$)**.

The **spectrochemical series** is an experimentally derived list that ranks ligands based on their ability to cause this splitting. But memorizing a long list of chemical formulas can be tedious. Instead, let's understand the underlying logic!

### The Donor Atom Trend

A brilliant shortcut to mastering the spectrochemical series is to look at the atom actually donating the electron pair to the metal. As a general rule of thumb, the field strength increases in the following order:

**Halogens $<$ Oxygen donors $<$ Nitrogen donors $<$ Carbon donors**

Halogens are highly electronegative and hold onto their electrons tightly, making them weak donors (weak field ligands). On the other end of the spectrum, carbon is less electronegative and more willing to share its electrons, making carbon-based ligands incredibly strong.

### Analyzing the Nitrogen Donors

In our problem, we have two nitrogen donors: ammonia ($	ext{NH}_3$) and ethylenediamine ($	ext{en}$).

While both donate through nitrogen, $	ext{en}$ is a **bidentate ligand**, meaning it attaches to the metal at two points, forming a ring structure. This is known as the **chelate effect**, which provides extra stability and causes a slightly larger splitting of the $d$-orbitals compared to the simple, monodentate ammonia molecule.

Therefore, we can confidently say: **$	ext{NH}_3 < 	ext{en}$**.

### The Kings of the Series: Carbon Donors

Next, we look at our carbon donors: the cyanide ion ($	ext{CN}^-$) and carbon monoxide ($	ext{CO}$). These ligands are the heavyweights of the spectrochemical series.

Why are they so strong? It's all about **synergic bonding** (or $\pi$-backbonding). Not only do these ligands donate a lone pair to the metal (forming a $\sigma$-bond), but they also have empty $\pi^*$ antibonding orbitals that can accept electron density *back* from the metal's filled $d$-orbitals.

Between the two, **$	ext{CO}$ is a superior $\pi$-acceptor** compared to $	ext{CN}^-$. This intense two-way electron sharing pulls the ligands closer to the metal, causing a massive splitting of the $d$-orbitals.

Therefore, the order is: **$	ext{CN}^- < 	ext{CO}$**.

### The Final Verdict

By combining our logical deductions, we arrive at the final, correct order of increasing field strength:

**$	ext{NH}_3 < 	ext{en} < 	ext{CN}^- < 	ext{CO}$**

Understanding the *why* behind the spectrochemical series not only helps you solve questions like this in seconds but also deepens your appreciation for the elegant dance of electrons in coordination complexes!

Animated Solution for Chemistry - Coordination Compounds: Among the ligands $NH_{3}$ , $en$ , $CN^{-}$ and $CO$ , the correct order of their increasing field strength, is

Select Answer:

Visualized Solution

$The Spectrochemical Series$

$Comparing N-donors$

$Comparing C-donors$

$Final Order$

The Sigma Insight: Bonding and Crystal field

The Power of Ligands

The Donor Atom Trend

Analyzing the Nitrogen Donors

The Kings of the Series

The Final Verdict

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Animated Solution for Chemistry - Coordination Compounds: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Among the ligands NH3​, en, CN− and CO, the correct order of their increasing field strength, is