Alkene and Alkyne Complexes

Introduction

Alkenes and alkynes are among the most important ligands in catalysis, because hydrogenation, polymerisation, hydroformylation, Heck coupling and Wacker oxidation all begin with a C=C or C≡C bond binding to a metal. Unlike CO, these ligands have no lone pair; they bind side-on through their π electrons. The Dewar–Chatt–Duncanson (DCD) model, a close relative of synergic CO bonding, explains how they bind, how the bound ligand changes and why its reactivity can be either enhanced or reversed by coordination.

Core explanation

Two components. In the DCD model the filled C=C π orbital donates electron density into an empty metal orbital of σ symmetry pointing at the midpoint of the double bond. At the same time a filled metal d orbital of matching symmetry back-donates into the empty C=C π orbital. Both components remove bonding electron density from, or add antibonding density to, the C=C bond, so coordination lengthens it.

Two limiting descriptions. When back-donation is weak, the alkene remains close to a planar π ligand: the C=C bond lengthens only slightly, and the substituents stay roughly coplanar. In Zeise's anion [PtCl₃(C₂H₄)]⁻ the C=C bond is about 1.37 Å, compared with 1.34 Å in free ethene. When back-donation is strong, the complex approaches a metallacyclopropane : the C–C bond approaches single-bond length (about 1.43 Å in Pt(PPh₃)₂(C₂H₄)), the carbons become more sp³-like and the substituents bend back away from the metal. Electron counting is unaffected by this choice for a neutral alkene treated as an L ligand, but the oxidation-state assignment differs if the metallacycle is viewed as an X₂ ligand.

What pushes a complex towards each limit. Electron-rich, low-oxidation-state metals with donor ligands (Pt(0), Ni(0)) back-donate strongly. Electron-withdrawing substituents on the alkene (tetrafluoroethene, tetracyanoethene) lower π and also favour the metallacyclopropane form. Cationic or high-oxidation-state metals (Pd(II), Pt(II), Ag(I)) back-donate weakly, leaving a π complex dominated by donation.

Consequences for reactivity. A weakly back-bonded alkene has lost electron density to the metal, so it becomes electrophilic and can be attacked by nucleophiles such as water, alcohols or amines — the key step in Pd(II)-catalysed Wacker oxidation. A strongly back-bonded alkene carries extra density and is instead nucleophilic or prone to insertion chemistry. Coordination can therefore reverse the normal behaviour of an alkene, which in organic chemistry reacts mainly with electrophiles.

Alkynes. Alkynes bind similarly but have a second, orthogonal π/π pair. They can act as two-electron donors or, when the metal has an empty orbital of suitable symmetry, as four-electron donors using the second π set. Coordinated alkynes bend: in Pt(PPh₃)₂(PhC≡CPh) the C–C–Ph angles are near 140°, and the C≡C stretch drops by several hundred wavenumbers.

Step-by-step reasoning

1. Identify the metal's oxidation state, charge and ancillary ligands. 2. Judge its back-donating ability: electron-rich metals back-donate more. 3. Consider alkene substituents: electron-withdrawing groups lower π and increase back-donation. 4. Predict C–C bond length, substituent bending and whether the ligand is activated towards nucleophiles.

Visual explanation

Draw the metal below an ethene molecule. Show the π orbital as a filled sausage-shaped lobe donating downward, and a metal d orbital with alternating phases pushing density up into the two π lobes. Then draw the metallacyclopropane beside it, with hydrogen atoms folded back like the ribs of a half-closed umbrella.

Real-world analogy

Think of a handshake that can range from a light touch to a firm grip. With a light touch (weak back-donation) both hands keep their shape; with a firm grip (strong back-donation) the hands deform around each other. Real complexes fall anywhere along that range.

Real-world example

Silver(I) ions bind alkenes weakly and reversibly through mainly σ-type donation. This is exploited in argentation chromatography and in proposed membrane processes for separating ethene from ethane, since only the alkene can coordinate to silver and be released again by a small change in conditions.

Why?

Why does back-donation lengthen the C=C bond so much more than donation alone? Donation removes bonding electrons from π, but back-donation adds electrons to π , which is antibonding. Filling an antibonding orbital lowers bond order roughly as much as emptying a bonding one, so the two effects add together.

Common misconception

"A metal–alkene complex is just an alkene sitting on a metal." The bound alkene is a changed molecule: its C=C bond is longer, its carbons rehybridise towards sp³ to varying degrees, and its reactivity can switch from nucleophilic to electrophilic.

Worked example

Question: Which complex should show the longer coordinated C–C bond: [PtCl₃(C₂H₄)]⁻ or Pt(PPh₃)₂(C₂F₄)? Explain.

Reasoning: In Pt(PPh₃)₂(C₂F₄) the metal is Pt(0), electron-rich, with donor phosphines, and the fluorinated alkene has a low-lying π . Both factors maximise back-donation. In the Pt(II) anion, back-donation is modest.

Answer: Pt(PPh₃)₂(C₂F₄), which is close to the metallacyclopropane limit.

Quick check

1. Which orbital of the alkene receives electron density from the metal in the Dewar–Chatt–Duncanson model? Answer: The empty C=C π orbital, which is antibonding between the two carbon atoms.

Exam focus

Draw both DCD components with correct orbital phases. Link metal electron richness to C–C lengthening, substituent bending and reduced ν(C=C). Explain that electron-poor metals activate alkenes towards nucleophilic attack.

Advanced insight

Bound alkenes can rotate about the metal–alkene axis; barriers measured by variable-temperature NMR reflect the loss of back-donation overlap during rotation. Prochiral alkenes bind through one of two enantiotopic faces, and distinguishing those faces with chiral ligands is the foundation of asymmetric hydrogenation.

Summary

Alkenes and alkynes bind side-on through σ-type donation from π and back-donation into π . Weak back-donation gives a slightly lengthened, electrophilic π ligand; strong back-donation gives a metallacyclopropane with near-single C–C bonds and bent-back substituents. Alkynes can donate two or four electrons. Metal electron density thus tunes both structure and reactivity.

Practice questions

1. Why is the C=C bond in Zeise's anion only slightly longer than in free ethene? Answer: Pt(II) in an anion with chloride ligands is relatively electron-poor, so back-donation into π is modest. 2. What structural feature shows that a bound alkene is near the metallacyclopropane limit? Answer: A C–C bond approaching single-bond length and substituents bent back away from the metal, indicating sp³-like carbons. 3. Why can water attack an alkene bound to Pd(II) but not free ethene under mild conditions? Answer: Coordination to the electron-poor metal withdraws π density, making the alkene electrophilic and susceptible to nucleophilic attack. 4. How can an alkyne act as a four-electron donor? Answer: It uses its second, perpendicular π orbital to donate to another empty metal orbital as well as the first π orbital.