The most salient difference between carbenes and silylenes is their singlet-triplet gap, which is much larger for silylenes. The difference reflects the much larger valence s-p energy difference for third period elements than for second-period elements. Unlike simple carbenes such as methylene (CH2), which exhibit triplet ground states, virtually all silylenes are ground-state singlets (except for rare reports of triplet silylenes ). The vertical and adiabatic IPs of other simple silylenes such as SiF2, SiCl2, and Me2ImSi, are very similar to those calculated for the analogous, nucleophilic carbenes. As in the case of carbenes, substituents make an enormous difference in determining the first IP of silylenes, which increases by almost 4 eV on going from 2-silaimidazol-2-ylidene Me2ImSi to SiF2. In general, the stabilized silylenes exhibit lower IPs than the simpler ones (Table ), indicating weaker nucleophilicity of the Si lone pair in the former compounds. On the other hand, the simpler silylenes exhibit positive electron affinities, indicating susceptibility to nucleophilic attack. The present calculations underscore two unusual features of the divalent Si pcomplexes. First, they exhibit unusually high singlet-triplet gaps relative to simple silylenes.