Trace metal impurities in catalysis

Metal-catalysed transformations are a powerful tool in organic chemistry and the enormous progress, which has been made in the last few decades, was one more time honoured by the Nobel Prize in Chemistry in 2010. Many metal-containing compounds have been applied in carbon-carbon and carbon-heteroatom bond formations. However, not every component originally claimed as catalyst turned out to be the active ingredient in the end. Sometimes trace metal impurities were the actual catalytic species. In this tutorial review, we will highlight recent findings in transition metal-catalysed cross-coupling reactions and detail several reports from the past, which illustrate that "trace metal catalysis" is not a newly discovered phenomenon.