The geometries of the metallacyclopropenes c-[MX 2C2H2] (M = C, Si, Ge, Sn; X= H, F) are predicted by using the 3-21G(d) basis set and SCF wave functions. The nature of the ring bonding is investigated via analysis of the total electron density and is found to have little or no 11'-complex character. As a further probe of the electronic structure, bent bond lengths and intrinsic vibrational frequencies are computed. The classical barrier heights and the thermodynamics of the reaction MX 2 + HC==CH-- c-[MX 2C2H2] are predicted by using MP2/3-21G(d) energies.