Yttrium Complexes of Arsine, Arsenide, and Arsinidene Ligands**

Deprotonation of the yttrium-arsine complex [Cp'3Y{As(H)2Mes}] (1) (Cp'=η(5)-C5H4Me, Mes=mesityl) by nBuLi produces the μ-arsenide complex [{Cp'2Y[μ-As(H)Mes]}3] (2). Deprotonation of the As-H bonds in 2 by nBuLi produces [Li(thf)4]2[{Cp'2Y(μ3-AsMes)}3Li], [Li(thf)4]2[3], in which the dianion 3 contains the first example of an arsinidene ligand in rare-earth metal chemistry. The molecular structures of the arsine, arsenide, and arsinidene complexes are described, and the yttrium-arsenic bonding is analyzed by density functional theory.