Quantum mechanical calculations, using the G2 procedure, have yielded optimized geometries and total energies for several singlet and triplet isomers of NaC 3 H and NaC 3 + 2 . The recommended sodium cation affinities (SCAs) of singlet H 2 CCC and c-C 3 H 2 are, respectively, 135.4 and 147.1 kJmol - 1 ; these values are significantly higher than the SCAs of smaller nucleophiles such as NH 3 , H 2 O and HCN. Proton affinities determined by G2 theory for various NaC 3 H isomers are also high (for example, PA(c-C 3 HNa) = 1121 kJmol - 1 ), although not so high as that of NaNH 2 for which we determine PA(NaNH 2 ) = 1163 kJmol - 1 . Differences in the trends observed for sodium cation affinities and proton affinities reflect the different types of bonding involved in the sodium-containing ions. Several of the neutral and protonated NaC 3 H isomers possess ''non-classical'' or bridged structures, an observation which we attribute to the largely electrostatic nature of the sodium-carbon bond in these species. We discuss, briefly, the prospects for formation of NaC 3 H within astrophysical environments.