JEE Mains Chemistry · Coordination Compounds

Metal Carbonyls, Stability and Applications

Carbon monoxide binds metals by σ donation and π back-donation, which is why carbonyls hold metals in the zero oxidation state; stability constants measure how firmly a complex holds its ligands; and a short list of complexes matters in biology, medicine, industry and analysis.

All Coordination Compounds notes JEE Mains Chemistry strategy

Why this matters

Twenty-one PYQs, fifteen of them multiple choice, and two from 2026. Nine are about metal carbonyls: synergic bonding, structures and bridging CO groups; six use stability constants or the chelate effect; six are match-the-list questions on the uses of complexes and catalysts.

Concept 1 of 3: Synergic bonding and structures of metal carbonyls

CO gives its carbon lone pair to an empty metal orbital (a σ bond). The metal, rich in electrons, gives electrons back from a filled d orbital into the empty π* orbital of CO (a π bond). Each half strengthens the other, so the bonding is called synergic. The metal–carbon bond gets stronger, and the C–O bond gets weaker because π* is antibonding.

Definition

σ bond: lone pair on C → vacant metal orbital. π bond: filled metal d orbital → vacant π* of CO.
Result: M–C bond strengthened; C–O bond weakened (its stretching frequency falls).
Low oxidation states are stabilised by π-ACCEPTOR ligands such as CO, which take electron density off an electron-rich metal. CO is neutral, so the metal in $\mathrm{Ni(CO)_4}$ and $\mathrm{Fe(CO)_5}$ is in the zero oxidation state.
Bridging CO links two metals through one carbon; terminal CO binds one metal.

Carbonyl	Shape at each metal	Bridging CO	Metal–metal bonds
$\mathrm{Ni(CO)_4}$	Tetrahedral	0	0
$\mathrm{Fe(CO)_5}$	Trigonal bipyramidal	0	0
$\mathrm{Cr(CO)_6}$ , $\mathrm{W(CO)_6}$	Octahedral	0	0
$\mathrm{Mn_2(CO)_{10}}$	Octahedral (five CO and one Mn–Mn bond)	0	1 Mn–Mn Decacarbonyldimanganese(0) has ten terminal CO groups and no bridge.
$\mathrm{Co_2(CO)_8}$	Two Co(CO)₃ units joined by two CO bridges	2 (with 6 terminal)	1 Co–Co

Only the dicobalt carbonyl here has bridging CO groups.

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The same idea in a real exam question:

JEE Mains · 2026 · 5 Apr 2026 Shift 1 · Q36Moderate

Example 1 · Coordination Compounds · Metal Carbonyls, Stability and Applications

The correct statements about metal carbonyls are (A) The metal-carbon bonds in metals carbonyls possess both

\sigma

and

\pi

-character. (B) Due to synergic bonding interactions between metals and CO ligand the metal-carbon bond becomes weak. (C) The metal-carbon

\sigma

bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of metal. (D) The metal-carbon

\pi

bond is formed by the donation of electrons from filled d-orbital of metal into vacant

\pi^{*}

orbital of CO. Choose the correct answer from the options given below:

Synergic bonding strengthens the metal–carbon bond

Back-donation adds a π bond to the σ bond, so the M–C bond becomes STRONGER. It is the C–O bond that weakens, because electrons enter the antibonding π* orbital of CO.

π-acceptors, not π-donors, stabilise low oxidation states

A metal in the zero oxidation state is electron rich and needs ligands that take electrons away. A reason saying low oxidation states need π-DONOR ligands is false.

Drill 8 more on synergic bonding and structures of metal carbonyls

Concept 2 of 3: Stability constants and the chelate effect

A stability constant is the equilibrium constant for making the complex from the free metal ion and ligands. A huge value means almost no free metal ion is left once enough ligand is present. That is why a complexed metal can refuse to precipitate, and why a precipitate can dissolve in a ligand. Chelating ligands form more stable complexes than the same number of separate donor atoms.

Definition

$\mathrm{M + nL \rightleftharpoons ML_n}$ : $\beta_n = \dfrac{[\mathrm{ML}_n]}{[\mathrm{M}][\mathrm{L}]^n}$ ; the overall constant is the product of the stepwise ones, $\beta_n = K_1K_2\cdots K_n$ .
Overall dissociation (instability) constant = $1/\beta_n$ .
With excess ligand, $[\mathrm{M}] = \dfrac{[\mathrm{ML}_n]}{\beta_n[\mathrm{L}]^n}$ , where [L] is the FREE ligand left after complexation.
Chelate effect: $\mathrm{[Co(en)_3]^{2+} > [Co(en)_2(NH_3)_2]^{2+} > [Co(en)(NH_3)_4]^{2+} > [Co(NH_3)_6]^{2+}}$ .
AgCl dissolves in ammonia as $\mathrm{[Ag(NH_3)_2]Cl}$ . Cu²⁺ with excess CN⁻ gives $\mathrm{[Cu(CN)_4]^{3-}}$ , and $\mathrm{H_2S}$ then precipitates no sulphide. Prussian blue, $\mathrm{K_3[Co(NO_2)_6]}$ and ammonium arsenomolybdate are insoluble; basic iron(III) acetate is soluble.

Overall stability constant

\beta_n = \frac{[\mathrm{ML}_n]}{[\mathrm{M}][\mathrm{L}]^n} = K_1K_2\cdots K_n \qquad K_{\text{diss}} = \frac{1}{\beta_n}

Worked example

In a solution, 0.010 M zinc is present almost entirely as

\mathrm{[Zn(NH_3)_4]^{2+}}

and the free ammonia is 0.10 M. With

\beta_4 = 1.0 \times 10^{9}

, find the concentration of free

\mathrm{Zn^{2+}}

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The same idea in a real exam question:

JEE Mains · 2021 · Paper 20 · Q59Moderate

Example 2 · Coordination Compounds · Metal Carbonyls, Stability and Applications

The number of moles of

NH_{3}

, that must be added to

2\text{ }L

0.80MAgNO_{3}

in order to reduce the concentration of

Ag^{+}

ions to

5.0 \times10^{- 8}M\left( K_{\text{formation~}} \right.

for

\left. \ \left\lbrack Ag\left( NH_{3} \right)_{2} \right\rbrack^{+}= 1.0 \times10^{8} \right)

is ____ . (Nearest integer) [Assume no volume change on adding

NH_{3}

]

Use the FREE ligand, not the total added

The ligand bound in the complex is not free. If 1.0 mol of metal ion takes 2 mol of ligand into the complex, subtract those 2 mol from the total before putting [L] into

\beta

Chelation raises stability at the same metal and charge

Replacing two

\mathrm{NH_3}

by one en keeps the donor atoms the same but adds a ring, and each step raises the stability:

\mathrm{[Co(en)_3]^{2+}}

is the most stable of the ammine–en series.

Drill 5 more on stability constants and the chelate effect

Concept 3 of 3: Complexes in biology, medicine, industry and analysis

These questions are pure matching: a pigment, drug, catalyst or reagent with its metal or its use. The list is short, and the trap is always a swapped pair, such as chlorophyll with cobalt instead of magnesium.

Definition

Biology: chlorophyll (Mg), haemoglobin (Fe), vitamin B₁₂, cyanocobalamin (Co).
Medicine: cisplatin, cis- $\mathrm{[Pt(NH_3)_2Cl_2]}$ , inhibits tumour growth; EDTA removes lead in lead poisoning; D-penicillamine removes excess copper.
Industry: Wilkinson's catalyst $\mathrm{[RhCl(PPh_3)_3]}$ hydrogenates alkenes; Ziegler–Natta catalyst ( $\mathrm{TiCl_4}$ with $\mathrm{Al(C_2H_5)_3}$ ) polymerises alkenes; Grubbs catalyst (Ru) for alkene metathesis.
Analysis and processes: EDTA titration for water hardness (Ca²⁺, Mg²⁺); dmg for Ni²⁺; hypo dissolves unexposed AgBr in photography as $\mathrm{[Ag(S_2O_3)_2]^{3-}}$ ; cyanide leaches silver and gold as $\mathrm{[Ag(CN)_2]^-}$ and $\mathrm{[Au(CN)_2]^-}$ .

Substance	Metal	Role
Chlorophyll	Mg	Photosynthetic pigment
Haemoglobin	Fe	Oxygen carrier in blood
Vitamin B₁₂ (cyanocobalamin)	Co	Anti-pernicious-anaemia factor
Cisplatin	Pt	Anticancer drug
Wilkinson's catalyst	Rh	Hydrogenation of alkenes
Ziegler–Natta catalyst	Ti (with Al)	Polymerisation of alkenes
Grubbs catalyst	Ru	Alkene metathesis
$\mathrm{[Ag(S_2O_3)_2]^{3-}}$ (from hypo)	Ag	Fixing in black-and-white photography Photography uses the thiosulphate complex, not $\mathrm{[Ag(CN)_2]^-}$ .
$\mathrm{[Ag(CN)_2]^-}$ , $\mathrm{[Au(CN)_2]^-}$	Ag, Au	Extraction by cyanide leaching; electroplating
EDTA	Ca, Mg (and Pb)	Water-hardness titration; treatment of lead poisoning
D-Penicillamine	Cu	Chelating drug for excess copper

Most match lists pair a biological or catalytic name with its metal.

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The same idea in a real exam question:

JEE Mains · 2023 · 13 April 2023 · Q33Moderate

Example 3 · Coordination Compounds · Metal Carbonyls, Stability and Applications

The mismatched combinations are (A) Chlorophyll - Co (B) Water hardness - EDTA (C) Photography -

\left\lbrack Ag(CN)_{2} \right\rbrack

(D) Wilkinson catalyst -

\left\lbrack \left( Ph_{3}P \right)_{3}RhCl \right\rbrack

(E) Chelating ligand - D - Penicillamine Choose the correct answer from the options given below:

Chlorophyll is magnesium, vitamin B₁₂ is cobalt

The two porphyrin-like biological complexes are easily swapped. Chlorophyll holds Mg²⁺; vitamin B₁₂ holds cobalt; haemoglobin holds iron.

EDTA is not an anticancer drug

EDTA and D-penicillamine are chelating agents that remove unwanted metals (lead, copper). Tumour growth is inhibited by platinum complexes such as cisplatin.

Drill 5 more on complexes in biology, medicine, industry and analysis

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Stability constants and the chelate effect
Overall stability constant
$\beta_n = \frac{[\mathrm{ML}_n]}{[\mathrm{M}][\mathrm{L}]^n} = K_1K_2\cdots K_n \qquad K_{\text{diss}} = \frac{1}{\beta_n}$

Reference tables (2)

Synergic bonding and structures of metal carbonyls5 rows

Carbonyl	Shape at each metal	Bridging CO	Metal–metal bonds
$\mathrm{Ni(CO)_4}$	Tetrahedral	0	0
$\mathrm{Fe(CO)_5}$	Trigonal bipyramidal	0	0
$\mathrm{Cr(CO)_6}$ , $\mathrm{W(CO)_6}$	Octahedral	0	0
$\mathrm{Mn_2(CO)_{10}}$	Octahedral (five CO and one Mn–Mn bond)	0	1 Mn–Mn Decacarbonyldimanganese(0) has ten terminal CO groups and no bridge.
$\mathrm{Co_2(CO)_8}$	Two Co(CO)₃ units joined by two CO bridges	2 (with 6 terminal)	1 Co–Co

Only the dicobalt carbonyl here has bridging CO groups.

Complexes in biology, medicine, industry and analysis11 rows

Substance	Metal	Role
Chlorophyll	Mg	Photosynthetic pigment
Haemoglobin	Fe	Oxygen carrier in blood
Vitamin B₁₂ (cyanocobalamin)	Co	Anti-pernicious-anaemia factor
Cisplatin	Pt	Anticancer drug
Wilkinson's catalyst	Rh	Hydrogenation of alkenes
Ziegler–Natta catalyst	Ti (with Al)	Polymerisation of alkenes
Grubbs catalyst	Ru	Alkene metathesis
$\mathrm{[Ag(S_2O_3)_2]^{3-}}$ (from hypo)	Ag	Fixing in black-and-white photography Photography uses the thiosulphate complex, not $\mathrm{[Ag(CN)_2]^-}$ .
$\mathrm{[Ag(CN)_2]^-}$ , $\mathrm{[Au(CN)_2]^-}$	Ag, Au	Extraction by cyanide leaching; electroplating
EDTA	Ca, Mg (and Pb)	Water-hardness titration; treatment of lead poisoning
D-Penicillamine	Cu	Chelating drug for excess copper

Most match lists pair a biological or catalytic name with its metal.

Watch out for (6)

Synergic bonding strengthens the metal–carbon bond
→ Synergic bonding and structures of metal carbonyls
π-acceptors, not π-donors, stabilise low oxidation states
→ Synergic bonding and structures of metal carbonyls
Use the FREE ligand, not the total added
→ Stability constants and the chelate effect
Chelation raises stability at the same metal and charge
→ Stability constants and the chelate effect
Chlorophyll is magnesium, vitamin B₁₂ is cobalt
→ Complexes in biology, medicine, industry and analysis
EDTA is not an anticancer drug
→ Complexes in biology, medicine, industry and analysis

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