Figure compares the conventional 1 H NMR (Figure ) and the CEST spectrum (Figure ) of 1 dissolved in methanol-d4 and pyridine (for experimental details see ESI). While the conventional 1 H NMR spectrum exhibits only a single signal at -22.72 ppm, the CEST spectrum consists of four contributions, a strong signal at -22.72 ppm with a signal reduction of 30%, two weaker signals of practically equal intensity at -23.69 and -25.71 ppm with a signal reduction of ca. 15% and a broad signal at -26.71 ppm with a signal reduction of ca. 10%. By virtue of the chemical shift we attribute the strong signal at -22.72 ppm, present in both spectra to the hydrides of species 1.