The goals of this project are to understand the factors that control the reactions of novel electron-deficient and multiply bonded species that are heavier analogs of the reactive intermediates of organic chemistry containing second- , third- and fourth-row elements in groups 13, 14, and 15. It is hoped that by generating new novel molecules, exploring their reactions, and determining their reaction mechanisms, it will be possible to extend the powerful mechanistic ideas of organic chemistry to the prediction and rationalization of bond-making and bond-braking processes throughout the periodic table. Considerable progress has already been made. Among the highlights of the past five years has been the development of general routes for the formation of phosphinidenes, molecules containing mono-valent phosphorus atoms. Their chemistry has been explored. A triplet ground electronic state was established for mesitylphosphinidene Mes-P, the first triplet ground state of a carbene analog to have been observed.