Catalytic cross-dimerization is one of the powerful synthetic methods to produce linear molecules with high atom and step economy. Because this process involves a carbon–carbon-bond-forming reaction, enantioselective reactions have also been achieved. The most well-reviewed area in this field is probably hydrovinylation using ethylene, but the linear cross-dimerizations using substituted alkenes and alkynes have also been extensively developed. Not only do the products vary depending on these substrates, but they mostly differ from hydrovinylation in the mechanism. This Perspective presents a comprehensive summary on the basis of these substrates, including their brief historical background, mechanism, applications to the synthesis of biologically active compounds or contributions to the total synthesis, and the state-of-the-art advancements. The controlling factors in the chemo- and regioselectivities are also discussed.

Reduction of [FeCl₂(depe)₂] (depe = Et₂PCH₂CH₂PEt₂) by sodium–naphthalene under nitrogen gave a dinitrogen complex of iron(0), [Fe(N₂)(depe)₂] 1, in 72% yield. The crystal structure of 1 shows that the dinitrogen ligand bonds in an end-on fashion in a trigonal-bipyramidal geometry. Protonolysis of 1 by HCl gave trans-[FeH(Cl)(depe)₂] with quantitative evolution of molecular nitrogen and then [FeCl₂(depe)₂] with molecular hydrogen. The dinitrogen ligand in 1 can be replaced by small molecules such as CO, CS₂ or H₂ to give [Fe(CO)(depe)₂], [Fe(CS₂)(depe)₂] and cis-[FeH₂(depe)₂] respectively. The molecular structure of [Fe(CO)(depe)₂] has been established by X-ray analysis.