Tautomerizations, protonations, and electrophilic additions of .eta.2-coordinated pyrroles

A series of complexes is synthesized of the form [Os(NH 3 ) 5 (2,3-η 2 -pyrrole)] 2+ , with pyrrole and various alkylated pyrroles. In contrast to the free ligands, these complexes can be protonated chemo- and stereoselectively at the β carbon, away from the metal, to produce pyrrolium species whose acidities range in pK a from 4.2 to 7.5. In the presence of a weak base, two of these 3H-pyrrolium species can be converted to the corresponding 2H-pyrrolium tautomer, [Os(NH 3 ) 5 (3,4-η 2 -2H-pyrrolium)] 3+