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Pentafluorophenyl triazolium carbenes, widely used in NHC-catalysis, can decompose by several mechanisms. Under high concentration conditions, the azolium may undergo a pentafluorophenyl exchange by a proposed S N Ar mechanism to give an inactive salt. In the presence of appropriate substrates, cyclization on the ortho -position of the arene can occur, also by S N Ar. These adducts provide a potential pathway for catalyst decomposition and serve as a caveat to the development of new reactions and catalysts.