We have discovered a low pressure and low temperature process for the single step conversion of ethylene and carbon monoxide to either propionic acid or anhydride utilizing surprisingly active, inexpensive Cr group based catalysts which operate at very high rates under low-to-moderate pressures (30-70 atm) and temperatures (150-200 degrees C). Mechanistic investigations of the Mo based process, which is clearly the most active metal of the group, imply that catalysis is initiated by a rate limiting CO dissociation from Mo(CO)(6). This dissociation appears to be followed by a process which ultimately transfers an I atom from EtI to the coordinatively unsaturated Mo(CO)(5), probably via an inner sphere, electron transfer process. Subsequent reaction of the resultant ethyl radical with Mo(CO)(6) probably generates Very reactive odd electron Mo species which are capable of rapid catalysis via classical olefin carbonylation mechanisms. This discovery represents the first case of an efficient carbonylation process based on the Cr group metals and a unique method for initiating carbonylation catalysis. A general description of this process and the mechanistic proposal, which is based on detailed kinetics, spectroscopy and model reactions, will be presented.