The He I and He II photoelectron spectra are reported and assigned for three closely related molecules, Si(Bu^tNCHCHNBu^t), Si(Bu^tNCH₂CH₂NBu^t) and SiH₂(Bu^tNCHCHNBu^t), providing evidence for an energetically significant contribution from the silicon 3pπ orbital to the ring π system of Si(Bu^tNCHCHNBu^t). Theoretical calculations on the model compounds Si(HNCHCHNH), Si(HNCH₂CH₂NH) and SiH₂(HNCHCHNH) have been carried out and there is good agreement between the calculated and experimental geometries. Trends in the orbital energies corresponded closely with the trends found for the ionization energies of the photoelectron bands. Calculated singlet–triplet splittings predict singlet ground states for the model silyienes. Calculated heats of hydrogenation at the ring carbons of Si(HNCHCHNH) and SiH₂(HNCHCHNH) show the latter to be more exothermic by 13 kcal mol^–1 providing evidence for aromatic cyclic (4n+ 2)-π delocalization in Si(Bu^tNCHCHNBu^t). Estimates of Si–N double-bond character are obtained by ab initio calculations on rotamers of the model compound Si(NH₂)₂ and compared with analogous values for C(NH₂)₂.