Carbonyl Infrared Spectroscopy

Introduction

Infrared spectroscopy is the quickest way to "ask" a metal carbonyl how electron-rich its metal centre is. The C–O stretch absorbs in a region of the spectrum that is almost empty of other bands, it is intense, and its position responds sensitively to how much electron density the metal pushes into CO π orbitals. Building on the synergic σ-donation/π-back-donation model of metal–CO bonding, this page shows how to read ν(CO) values as a practical probe of back-bonding, charge, ligand donor strength and even molecular geometry.

Core explanation

The reference point. Free carbon monoxide absorbs at 2143 cm⁻¹. Almost every terminal metal carbonyl absorbs below this value, typically between about 1850 and 2120 cm⁻¹. The shift to lower wavenumber measures how much the C–O bond has been weakened by back-donation into its π orbitals: the more π population, the lower the bond order and the lower the stretching frequency.

Charge and oxidation state. Adding negative charge to a metal raises the energy of its d orbitals and makes them better donors, so back-donation increases. The isoelectronic d⁶ series is the classic illustration:

Complex Approximate ν(CO) / cm⁻¹ --- --- [Mn(CO)₆]⁺ 2090 Cr(CO)₆ 2000 [V(CO)₆]⁻ 1860

Each unit of negative charge lowers ν(CO) by well over 100 cm⁻¹. Highly reduced carbonylates such as [Fe(CO)₄]²⁻ absorb near 1790 cm⁻¹, while some cationic carbonyls of electron-poor late metals absorb above 2143 cm⁻¹, showing that back-donation there is minimal and σ donation (with electrostatic effects) dominates.

Ancillary ligands. Other ligands compete with CO for the same metal d electrons. Strong σ-donors that are poor π-acceptors (alkylphosphines, amines) make the metal richer and lower the remaining ν(CO) values. Strong π-acceptors (PF₃, extra CO, NO⁺) compete for back-donation and raise ν(CO). This is the basis of the Tolman electronic parameter : the A₁ band of Ni(CO)₃L ranges from about 2056 cm⁻¹ for P(tBu)₃ to about 2111 cm⁻¹ for PF₃.

Coordination mode. A CO that bridges two metals (μ₂) accepts back-donation from both and usually absorbs around 1720–1850 cm⁻¹; a face-capping μ₃-CO absorbs lower still, around 1600–1730 cm⁻¹. Fe₂(CO)₉, for example, shows terminal bands near 2080 and 2030 cm⁻¹ and a bridging band near 1830 cm⁻¹. Overlap between ranges means a band near 1850 cm⁻¹ needs other evidence before assignment.

Number of bands. The CO stretches of a complex couple into symmetry-defined combinations, and only some are infrared active. Octahedral M(CO)₆ shows one strong band; M(CO)₅L shows three; trans -M(CO)₄L₂ shows essentially one; cis -M(CO)₄L₂ shows four; fac -M(CO)₃L₃ shows two and mer -M(CO)₃L₃ shows three. Counting bands can therefore distinguish isomers.

Formulae

For a harmonic oscillator, ν̃ = (1 / 2πc) √(k / μ), where ν̃ is the wavenumber, k the bond force constant, μ the reduced mass and c the speed of light. Back-donation lowers k, so ν̃ falls. For ¹²C¹⁶O, μ ≈ 6.86 atomic mass units; ¹³CO substitution raises μ and lowers ν̃ by roughly 2%.

Step-by-step reasoning

1. Confirm the bands you compare are terminal CO stretches in comparable geometries. 2. Note the overall charge and formal oxidation state: more negative charge or lower oxidation state means more back-donation. 3. Assess the other ligands: good σ-donors lower ν(CO); π-acceptors raise it. 4. Predict the order of ν(CO), then check the number of bands against the expected symmetry.

Visual explanation

Sketch a spectrum axis from 2200 to 1600 cm⁻¹. Mark free CO at 2143, a cationic carbonyl near 2090, a neutral one near 2000, an anionic one near 1860, then a shaded bridging window at 1720–1850 and a μ₃ window lower still. Moving right along the axis corresponds to increasing back-donation into CO π .

Real-world analogy

A carbonyl band behaves like a tyre-pressure gauge attached to the metal. You cannot see the air inside the tyre directly, but the gauge reading tells you how much is there. Likewise, ν(CO) reads out the electron richness of the metal without measuring it directly.

Real-world example

Chemists designing phosphine ligands for catalysis compare donor strength by making the corresponding Ni(CO)₃L or, more safely, rhodium or iridium carbonyl complexes such as trans -RhCl(CO)L₂ and recording ν(CO). In-situ infrared cells also monitor carbonyl intermediates during rhodium-catalysed hydroformylation and iridium-catalysed methanol carbonylation, revealing which species dominate under operating conditions.

Why?

Why does negative charge lower the frequency so strongly? Extra electron density raises metal d-orbital energies closer to the CO π level, improving energy matching and overlap. More electron density enters an orbital that is antibonding between C and O, reducing the force constant of the bond, and a weaker spring vibrates more slowly.

Common misconception

"A lower ν(CO) means a weaker metal–CO bond." Usually the opposite is true: the back-donation that weakens C–O strengthens M–C. A low C–O frequency generally signals a strongly bound, strongly back-bonded carbonyl, not one that is about to dissociate.

Worked example

Question: Rank Cr(CO)₆, Cr(CO)₅(PMe₃) and Cr(CO)₅(PF₃) by their highest-energy A₁ ν(CO) band.

Reasoning: PMe₃ is a strong σ-donor and weak π-acceptor, so it enriches chromium and increases back-donation to the remaining CO ligands. PF₃ is a strong π-acceptor that competes with CO for d-electron density, reducing back-donation to CO. CO itself is intermediate.

Answer: Cr(CO)₅(PF₃) > Cr(CO)₆ > Cr(CO)₅(PMe₃).

Quick check

1. Why does [V(CO)₆]⁻ absorb at a much lower ν(CO) than the isoelectronic Cr(CO)₆? Answer: The negative charge makes the vanadium centre more electron-rich, so it back-donates more into CO π orbitals and weakens the C–O bond.

Exam focus

Always anchor comparisons to free CO at 2143 cm⁻¹, state that back-donation populates π , lowering C–O bond order and ν(CO), and restrict comparisons to similar geometries. Be ready to use band counts to distinguish cis / trans or fac / mer isomers.

Advanced insight

Frequencies above 2143 cm⁻¹ in "non-classical" carbonyls such as [Ir(CO)₆]³⁺ arise partly because the positive metal field polarises the CO σ framework, strengthening the C–O bond even without back-donation. Computational studies show that ν(CO) is a net electronic probe, so it cannot cleanly separate σ-donation from π-acceptance of an ancillary ligand.

Summary

ν(CO) is a sensitive, convenient probe of metal–CO back-donation. Relative to free CO at 2143 cm⁻¹, stretching frequencies fall as metal charge becomes more negative, as ancillary ligands become better donors and as CO bridges more metals. The number of IR-active bands reflects molecular symmetry and helps assign isomers. Comparisons are most reliable for similar terminal carbonyls.

Practice questions

1. Predict which has the higher ν(CO): [Co(CO)₄]⁻ or Ni(CO)₄. Explain. Answer: Ni(CO)₄, because the neutral nickel centre is less electron-rich than the anionic cobalt centre and back-donates less to CO. 2. A dinuclear carbonyl shows bands at 2060, 2020 and 1835 cm⁻¹. What does the 1835 cm⁻¹ band suggest? Answer: A bridging μ₂-CO ligand, whose C–O bond is weakened by back-donation from two metals. 3. How many IR-active CO stretching bands are expected for trans -Mo(CO)₄(PPh₃)₂ and for cis -Mo(CO)₄(PPh₃)₂? Answer: Essentially one for the trans isomer and four for the cis isomer. 4. Replacing CO in Ni(CO)₄ by P(tBu)₃ lowers the remaining ν(CO) values. Why? Answer: P(tBu)₃ is a strong σ-donor and weak π-acceptor, so nickel becomes more electron-rich and back-donates more to the remaining CO ligands.