A qualitative model which is based on the theory of Press and Shaik is; presented in this work. It was found that a knowledge of the ground spin state and the singlet-triplet splitting (Delta E(st)) is of great importance in understanding singlet chemical addition reactions, since a small value of Delta E(st) (Delta E(st) = E(triplet) - E(singlet)) was found to lead to a low barrier height and a larger exothermicity. Using the "isolobe analogy", the same predictions were also applied to organometallics and other related systems.