“…As reviewed in 1987, radical monoanions , routinely obtained by the one-electron reduction of neutral precursors or by the one-electron oxidation of dianions, are among the most common radical ligands for main-group and transition-metal centers, either as small ions such as O 2 •– (superoxide) or as larger π systems such as pyridine-type heterocycles. The presence of additional negative functions such as RO – or R 2 N – can give rise to radical-dianion ligands such as 1 or 2 or to radical trianions such as those in reduced metalloporphyrins; however, even small such species like N 2 •3– are known to act as ligands in coordination compounds. , From such recognition followed the development and identification of related new species like NO •2– . , Neutral radical ligands are generally less basic but do exist in the form of NO • , R 2 NO • , verdazyls, or 1-methylpyrazinyl (mpz • , 3 ); even radical cations can serve as ligands, as has been shown for the 2,2′-bipyridine/1,1′-dialkyl-4,4′-bipyridinium hybrid 4 …”