Upon treatment with methyl-lithium followed by HBF₄·OEt₂ a carbon monoxide ligand of the µ-alkylidene complex [Ru₂(CO)₂(µ-CO)(µ-CMe₂)(η-C₅H₅)₂](1) is converted into µ-ethylidyne, giving [Ru₂(CO)₂(µ-CMe)(µ-CMe₂)(η-C₅H₅)₂]⁺(2). This is deprotonated readily by water to form the µ-vinylidene complex [Ru₂(CO)₂(µ-CCH₂)(µ-CMe₂)(η-C₅H₅)₂](3), which quantitatively regenerates (2) with HBF₄·OEt₂. Addition of NaBH₄ to (2) results in hydride attack on µ-CMe to yield the di-µ-alkylidene complex [Ru₂(CO)₂(µ-CHMe)(µ-CMe₂)(η-C₅H₅)₂](4) as cis and trans isomers. The structure of the trans isomer has been established by X-ray diffraction. Crystals are triclinic, space group P, with Z= 2 in a unit cell for which a= 8.474(2), b= 7.802(3), c= 12.989(5)Å, α= 99.42(3), β= 96.96(3), and γ= 107.73(3)°. The structure was solved by heavy-atom methods and refined to R 0.026 (R′ 0.031) for 4 092 independent intensities. A ruthenium–ruthenium single bond of 2.701(1)Å is symmetrically bridged by ethylidene [mean Ru–C 2.079(3)] and isopropylidene [mean Ru–C 2.107(3)Å] ligands to form an approximately planar Ru₂C₂ ring with a non-bonding Me₂C··CHMe distance of 3.20 Å. Upon thermolysis the alkylidenes link to evolve Me₂CCHMe, Me₂CHCHCH₂, and Et(Me)CCH₂. The absence of C₄ and C₆ hydrocarbons indicates that the alkylidene coupling occurs intramolecularly, and the electronic and stereochemical requirements of this process are discussed. Unlike mono-µ-alkylidene complexes, [Ru₂(CO)₂(µ-CO)(µ-CR₂)(η-C₅H₅)₂], the cis and trans forms. of (4) do not interconvert thermally below 145 °C, but u.v. irradiation effects a slow trans to cis isomerisation. U.v. irradiation of (4) in the presence of dimethyl acetylenedicarboxylate promotes ethylidene–alkyne linking to form [Ru₂(CO)(µ-CMe₂){µ-C(CO₂Me)C(CO₂Me)CHMe}(η-C₅H₅)₂], but with ethyne both of the alkylidenes are lost and the ruthenium–ruthenium double-bonded complex [Ru₂(µ-CO)(µ-C₂H₂)(η-C₅H₅)₂] is produced.