Transition-metal compounds that contain carbonyl (CO) and thiocarbonyl (CS) ligands have been extensively studied, and their chemical properties have been disclosed. In contrast, the heavier chalcogen isologues, namely selenocarbonyl (CSe) and tellurocarbonyl (CTe) complexes have received less attention. Similarly, although nitrosyl (NO) complexes of transition-metals are well studied, their sulfur and selenium congeners, i.e., thionitrosyl (NS) and selenonitrosyl (NSe) complexes are less explored. These limitations should be related to the lack of suitable reagents and efficient synthetic methods available for synthesizing them.

Recent notable achievements in this field include among others the discovery of CSe and CTe ligands that can adopt various coordination modes to bridge multiple metals, the synthesis of hitherto unknown CTe complexes with a half-sandwich structure that enables the systematic investigation regarding a series of half-sandwiched CE complexes, and the increasing number of selenonitrosyl (NSe) complexes, only one of which existed ten years ago. This chapter mainly describes the recent-ten-year developments in the chemistry of the transition-metal complexes with heavier chalcogenocarbonyl, CE (E = Se, Te) and heavier chalcogenonitrosyl, NE (E = S, Se) ligands. Some review articles on the chemistry of CE and NE complexes have been reported and will be mentioned in each section.