Ab initio molecular orbital calculations were employed to re-examine a number of structural and energetic aspects of both LiC2H2 and NaC2H2 adducts. Geometries were optimized using UMP2/6-311++G(d, p) wavefunctions, while relative energies were uniformly estimated at the PUMP4/6- 311++G(d, p)+ZPE level. The QCISD(T) method and 6-311++G(2df, 2pd) basis set were also considered. Each metal-acetylene has a cis-bent bridged geometry and a2B2 ground electronic state. The2A’ and2A1 forms exist as shallow minima only for lithium-acetylene. Metal-acetylenes have2B2 ground states. In each system, both isomers have similar energies and are separated from each other by a substantial energy barrier. While lithium-acetylene is observable, sodium-acetylene is not stable with respect to fragmentation. Conversion of sodium plus acetylene to sodium-vinylidene, without initial formation of a sodium-acetylene intermediate, is shown to be possible through a ‘neutral pathway’. This process is, however, endothermic, not spontaneous, does not involve any preliminary charge transfer from sodium to acetylene and is favoured over the excitation of either sodium or acetylene. Both metal-acetylene and metal-vinylidene isomers can be distinguished by13C electron spin resonance spectrometry. © 1992 Taylor & Francis Ltd.