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Metal carbonyls look odd at first: a neutral metal atom, often in oxidation state zero, holding several neutral CO molecules tightly. The explanation, synergic bonding, is a short NCERT section that turns up in board and JEE questions. This post explains the two halves of the bond, walks through the five structures NCERT shows, works three short problems and lists the mistakes to avoid.
Where is this in the syllabus?
Section 5.6, "Bonding in Metal Carbonyls", is in the NCERT Class 12 chapter Coordination Compounds. The JEE (Advanced) 2026 brochure names "Metal carbonyls" in its coordination compounds list. The JEE (Main) 2026 syllabus covers coordination compounds with "Valence bond approach and basic ideas of Crystal field theory". Check the syllabus for your exam year.
Which structures does NCERT show?
NCERT describes homoleptic carbonyls, which have only CO ligands:
Ni(CO)₄, tetracarbonylnickel(0): tetrahedral.
Fe(CO)₅, pentacarbonyliron(0): trigonal bipyramidal.
Cr(CO)₆, hexacarbonylchromium(0): octahedral.
Mn₂(CO)₁₀: two square pyramidal Mn(CO)₅ units joined by a Mn–Mn bond.
Co₂(CO)₈: a Co–Co bond bridged by two CO groups.
The (0) in each name is the metal's oxidation state. CO is neutral, so the metal carries no charge.
What is synergic bonding?
NCERT says the metal-carbon bond "possess[es] both σ and π character":
Sigma part: the lone pair on the carbon of CO goes into a vacant orbital on the metal. CO acts as a donor.
Pi part: a pair of electrons from a filled d orbital of the metal goes into the vacant antibonding π* orbital of CO. This is back-bonding.
NCERT adds that this metal-to-ligand bonding "creates a synergic effect which strengthens the bond between CO and the metal". The two parts help each other. Sigma donation puts more electron density on the metal, so it has more to give back. Back-donation takes density away from the metal again, which lets it accept more from carbon.
What evidence shows back-bonding happens?
Electrons placed in CO's π* orbital weaken the C–O bond. A weaker bond vibrates at a lower wavenumber in the infrared. NIST's constants for carbon monoxide put the vibration of free CO near 2143 cm⁻¹ (ωe − 2ωexe = 2169.8 − 2 × 13.3). A 2016 review in The Analyst reports that transition metal carbonyls absorb strongly in the 2200–1800 cm⁻¹ region. A 2016 computational study in Chemical Science by Bistoni and colleagues found that π back-donation "is found to correlate tightly" with the change in CO bond distance and the shift in CO stretching frequency, while sigma donation was "largely uncorrelated" with them. If wavenumbers are new to you, see our guide to wavelength, frequency and wavenumber calculations.
Worked example 1: why is Ni(CO)₄ diamagnetic?
NCERT intext question 5.6 asks why [NiCl₄]²⁻ is paramagnetic while Ni(CO)₄ is diamagnetic, though both are tetrahedral. Using the valence bond approach:
Ni(0) is 3d⁸4s². In Ni(CO)₄ the 3d electrons pair up to give 3d¹⁰, leaving 4s and 4p empty.
Four CO ligands use sp³ hybrid orbitals: tetrahedral, with no unpaired electrons.
In [NiCl₄]²⁻, nickel is Ni²⁺, 3d⁸. Chloride does not force pairing, so two electrons stay unpaired.
Worked example 2: Cr(CO)₆ and Fe(CO)₅
Cr(0) is 3d⁵4s¹. Pairing gives 3d⁶ in three d orbitals, leaving two 3d, one 4s and three 4p orbitals for d²sp³: octahedral, diamagnetic.
Fe(0) is 3d⁶4s². Pairing gives 3d⁸, leaving one 3d, the 4s and three 4p orbitals for dsp³: trigonal bipyramidal, diamagnetic.
Both match the shapes NCERT gives.
Worked example 3: why are manganese and cobalt carbonyls dimers?
Count valence electrons on the neutral metal: Mn (3d⁵4s²) has 7 and Co (3d⁷4s²) has 9, both odd. A single Mn(CO)₅ or Co(CO)₄ unit would have an unpaired electron. Joining two units with a metal-metal bond pairs them up, which fits NCERT's Mn–Mn and Co–Co structures. Chromium, iron and nickel have even counts (6, 8 and 10) and form single-metal carbonyls.
Common mistakes and how to avoid them
Bonding through oxygen. The lone pair donated comes from carbon.
A positive oxidation state. All five NCERT examples are metal(0).
Saying back-bonding strengthens C–O. It weakens C–O and strengthens M–C.
Back-donation into a bonding orbital. It goes into CO's empty antibonding π* orbital.
Mixing up the directions. Sigma is ligand to metal; pi is metal to ligand.
A practice routine
Draw all five NCERT carbonyl structures from memory and label each geometry.
Write the sigma and pi parts of the bond in one sentence each, with the direction of electron flow.
Answer NCERT intext question 5.6 in full, then repeat the valence bond reasoning for Cr(CO)₆ and Fe(CO)₅.
Revise naming with our guide to coordination compounds: naming, isomerism and crystal field basics, then mix carbonyl questions into a coordination test.
Orbital pictures like back-donation are easier to follow when you can rebuild them step by step. Fermi's Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can go back to d orbitals or antibonding orbitals when one link is missing.
Today, write the synergic bonding explanation in three sentences without notes, then compare it with NCERT section 5.6.
