AbstractThe potential energy surface for the addition of the hydride ion to hydrogen isocynide reported previously was re‐examined. An improved energy surface was explored at the MP2/6–31 + + G** level while relative energies were estimated at the MP4SDTQ/6–311 + + G** level and corrected for zero‐point vibrational contributions. The calculated results show the existence of transition structures for both cis and trans‐additions of H− to the carbon end of HNC even though the corresponding barrier heights are small. These transition structures did not exist at a lower level of theory (HF/4–31G) employed previously.The cis addition is calculated to be favoured over the trans addition by 10 kcal mol−1. The overall addition is thus strongly stereoselective, if not purely stereospecific. This result confirms our previous analysis of the stereoelectronic effect in the addition–elimination process involving imidoyl anions as intermediates. Recent experimental work on the addition of OH− to ArNC is discussed.