Metal alkene and alkyne complexes

Dewar–Chatt–Duncanson model

Two synergic components:

1. σ-donation from the filled alkene π orbital into an empty metal orbital (σ symmetry with respect to the M–alkene axis). 2. π-back-donation from a filled metal d orbital into the empty alkene π* orbital.

Both weaken the C=C bond; back-donation does so most strongly because it fills an antibonding orbital.

Two limiting structures

• Weak back-donation (electron-poor metal, e.g. Pt(II) in Zeise's salt \mathrm{K[PtCl_3(\eta^2\text{-}C_2H_4)]}): C=C only slightly lengthened (about 1.37 Å versus 1.34 Å in ethene), substituents nearly planar, alkene behaves as an L-type ligand.
• Strong back-donation (electron-rich, low-valent metal, e.g. Pt(0)(PPh_3)_2(C_2H_4)) or electron-poor alkenes such as C_2F_4 or tetracyanoethene: C–C approaches a single bond, substituents fold back from the metal (carbons tend toward sp^3), and the unit is better described as a metallacyclopropane with two M–C σ bonds and the metal formally oxidised by two.

Evidence comes from C–C bond lengths, the bend-back angle, lower \nu_{\mathrm{C=C}} and upfield ^{13}C and ^1H NMR shifts, and reduced ^1J_{\mathrm{CH}} (closer to sp^3 values) in the metallacyclopropane limit.

Alkynes

Alkynes have two orthogonal π systems. One interacts as in alkenes; the second can donate a further electron pair (π_\perp) to an electron-poor metal, so alkynes act as 2- or 4-electron donors. Coordination bends the substituents back (the C≡C–R angle can fall well below 180°), giving metallacyclopropene character. Alkynes also bridge two metals, as in \mathrm{Co_2(CO)_6(\mu\text{-}RC{\equiv}CR)}, used to protect alkynes and in the Pauson–Khand reaction.

Reactivity changes

• Nucleophilic attack: coordination to an electron-poor cationic metal (Pd(II), Pt(II), Au(I), Fe(II) in \mathrm{[CpFe(CO)_2(alkene)]^+}) reverses alkene polarity, so nucleophiles attack the coordinated alkene, usually from the face anti to the metal. In the Wacker process, water attacks Pd(II)-bound ethene, β-H elimination and tautomerisation give ethanal, and CuCl_2/O_2 reoxidise Pd(0).
• Migratory insertion: 1,2-insertion into M–H or M–C (hydrogenation, polymerisation).
• Rotation: alkene rotation about the M–alkene axis is observed by variable-temperature NMR; its barrier reflects the strength of back-donation.
• Oxidative coupling of two alkenes gives metallacyclopentanes, key in selective ethene trimerisation.

Alkene and alkyne π-complexes are intermediates in many catalytic processes, and current research focuses on controlling their activation.

π-Acid catalysis. Cationic gold(I) and platinum(II) catalysts activate alkynes and allenes toward intramolecular attack by O, N and C nucleophiles, enabling cycloisomerisations that rapidly build ring systems. Gold's strong relativistic effects make it a soft π-acid with weak back-donation, so coordinated alkynes are strongly electrophilic. Debate continues over the role of gold carbenoid versus carbocation character in rearrangements, addressed by isolation of intermediates and computation.

Selective oligomerisation. Chromium catalysts with PNP or related ligands trimerise or tetramerise ethene to 1-hexene or 1-octene through metallacycle growth, where ring size controls the product; this is now used industrially. Understanding the oxidation states (Cr(I)/Cr(III) versus Cr(II)/Cr(IV)) remains an active research question.

Asymmetric catalysis. In Rh- and Ir-catalysed asymmetric hydrogenation, alkene binding is reversible, and the major product often arises from the less stable but more reactive diastereomeric alkene complex (Curtin–Hammett control, shown by Halpern for Rh-diphosphine systems). Iron and cobalt hydrogenation catalysts aim to reproduce this with Earth-abundant metals.

Bonding analysis. Energy decomposition analysis and natural orbitals for chemical valence quantify σ-donor and π-acceptor contributions, placing complexes on the continuum between π-complex and metallacyclopropane. Strained alkenes (cyclopropenes, trans-cyclooctene) and fluoroalkenes bind tightly, which is exploited in C–F activation.

Alkyne metathesis and polymerisation. Molybdenum and tungsten alkylidyne catalysts with silanolate ligands achieve robust alkyne metathesis, used for macrocycles and conjugated materials.