In the isoelectronic series methylpnenylborenide, methylphenylcarbene, and methylphenylnitrenium, fundamental differences are-predicted for singlet state geometries, singlet-triplet state splittings, barriers to singlet 1,2-hydrogen migration, and sensitivity of 1,2-hydrogen migration to solvent effects in n-heptane and acetonitrile. We conclude that isoelectronic analogies are dangerous for systems having different formal charges, and that the interaction of the divalent center with a conjugating substituent is very sensitive to the electron donating or withdrawing nature (and power) of the hypovalent atom. Solvent effects on the singlet-triplet splitting result from static polarity differences whereas the solvent effects on 1,2-hydrogen migration result primarily from polarizability differences. For the experimentally characterized carbene case, extensive comparison of calculated and measured results is provided.