The insertion of CO into metal–carbon bonds is an important elementary reaction in organometallic chemistry. Reactions of CO with mononuclear transition-metal alkylidene (or carbene) complexes have been well-studied. In contrast, there had been no example of a reaction of mononuclear rare‐earth metal alkylidene complexes with CO so far.

Yaofeng Chen and Li Xiang, Shanghai Institute of Organic Chemistry, University of the Chinese Academy of Sciences, Qian Peng, Nankai University, Tianjin, China, and colleagues have performed a reaction of CO with a mononuclear rare-earth metal alkylidene complex. The product is the first rare-earth phosphonioketene: a scandium phosphonioketene (pictured). The team started from a scandium phosphoniomethylidene [LSc(CHPPh₃)I] (L = [MeC(NDIPP)CHC(NDIPP)Me]⁻, DIPP = 2,6‐(ⁱPr)₂C₆H₃). This complex reacts with CO in toluene at room temperature to give the desired product in 94 % yield.

The scandium phosphonioketene shows high reactivity. A series of unsaturated substrates, including carbon dioxide, ketones, imines, nitriles, and isocyanides, can easily insert into the highly polar Sc–C bond in this complex under mild conditions.

• Scandium Phosphonioketene: Synthesis, Bonding and Reactivity,
  Weiqing Mao, Yaya Wang, Li Xiang, Qian Peng, Xuebing Leng, Yaofeng Chen,
  Chem. Eur. J. 2019.
  https://doi.org/10.1002/chem.201902008