Carbonyl, alkene and other organometallic ligands

A contiguous π attachment needs its own label

Hapticity counts contiguous atoms of a ligand attached to the metal and is written η with a superscript count. It is neither ligand charge nor donated electrons. An η⁵-Cp ring has five contiguous carbon contacts, but it is not five independent two-electron localized metal–carbon pair bonds. A single-carbon σ-bound form instead uses κC for explicit donor specification in the adopted IUPAC naming convention.

For selected Cp⁻ attachment models, η⁵, η³ and single-carbon κC modes have neutral-fragment donations 5, 3, 1 and ionic donations 6, 4, 2 respectively. The ligand’s formal charge remains −1 in these chosen descriptions. Changing attachment can reduce donation while preserving composition. A hapticity change does not by itself establish the energy, rate or measured population of a real intermediate.

Both directions of orbital interaction can matter

A qualitative terminal carbonyl model includes ligand donation from a suitable CO orbital toward the metal and backdonation from occupied metal character into CO π* character. These are interacting molecular-orbital contributions, rather than two separate permanently owned electron parcels. Backdonation can weaken the internal C–O interaction within an appropriate comparison, but the extent depends on the entire ligand environment and electronic structure.

An alkene can likewise interact through its π and π* orbitals in a donation/backdonation picture. More π* character can alter the alkene’s internal bonding, while the metal–ligand interaction depends on both contributions. The original C=C drawing remains a useful starting representation; its ideal bond order is not an exact measurement after binding. Formal charge and a scalar donation count do not fully describe the resulting covalency.

Two alkene contacts need not mean two independent donor pairs

For selected [PtCl₃(η²-C₂H₄)]⁻, assign three Cl⁻ and neutral alkene. Pt is formally +2 and d⁸; three chlorido donations of two plus the alkene L donation of two give sixteen. There are four ligand entities and five metal–atom contacts: three Cl atoms and two contiguous carbon atoms.

In this orbital bookkeeping, the η²-alkene occupies one ligand site/function even though it has two atom contacts. Counting contacts as five therefore does not prove trigonal-bipyramidal geometry or five ordinary independent donor pairs. Classical atom-contact counting, orbital ligand functions and a coordination-site drawing need explicit conventions.

Metal–metal bonding also has its own allocation. For the selected single-bond Mn₂(CO)₁₀ model with five terminal CO per Mn, each neutral-fragment metal count is 7+10=17 before the metal–metal share. One electron per metal from the single Mn–Mn bond gives eighteen each. This per-metal ledger is not the whole molecule’s atomic electron total, and bridging CO requires a separate donation convention.

A frequency change is evidence within a vibrational model

For an invented uncoupled harmonic C–O mode with the same reduced mass before and after binding, wavenumber scales as√(k/μ). Stipulate free-model wavenumber 2100 cm⁻¹ and bound-model 2000 cm⁻¹. The force-constant ratio is(2000/2100)²=400/441≈ 0.907029, a decrease of about 9.297%.

These are teaching inputs rather than measured carbonyl bands. Actual modes can be coupled and affected by geometry, other ligands and environment. A red shift can support an electronic comparison with an appropriate model and supporting evidence, but alone it does not uniquely determine oxidation state, backdonation magnitude or an entire molecular structure.

Preserve whether a bound molecule is intact

An intact dihydrogen ligand retains H–H connectivity in its selected molecular description. Two hydrido ligands after a chosen H₂ oxidative addition have a different connectivity and formal-charge assignment. Both descriptions belong to metal–small-molecule chemistry, yet they should not be silently treated as the same intermediate.

The distinction follows evidence for bonding and the adopted model, rather than the mere presence of two hydrogen nuclei. A reaction class can pass through different ligand states; identifying the state requires structural or spectroscopic support. No one depicted ligand mode is a universal route for all metal complexes.

Worked examples

Try it yourself

1. At fixed formal Cp⁻ charge, how many ionic donated electrons are lost in the selected η⁵→η³ attachment change?

   Show the answer and reasoning

   Two.

   Six changes to four; hapticity and donation remain different counts.

2. In the same equal-mass harmonic model, the bound wavenumber is 0.900 times the free value. What is the force-constant ratio?

   Show the answer and reasoning

   0.810.

   Square the frequency ratio under the stipulated uncoupled harmonic and same-reduced-mass assumptions.

3. Why do five atom contacts in the selected Pt alkene complex not establish five independent donor pairs?

   Show the answer and reasoning

   The η²-alkene is one L donor function with two contiguous contacts.

   A contact count and an orbital donor-function count describe different aspects of the attachment.

4. What connectivity distinction separates an intact H₂ ligand from two selected hydride ligands?

   Show the answer and reasoning

   The intact ligand retains an H–H connection; the hydride description separates it into two metal-bound H assignments.

   Identifying the actual mode needs bonding evidence rather than just the total count of H nuclei.

What to carry forward

• Hapticity is an attachment count, not a charge or donation count.
• Keep η and κ donor conventions explicit.
• Donation and backdonation describe interacting orbitals.
• Count metal–metal contributions under a stated method.
• Interpret frequency changes with their vibrational assumptions.