Deprotonation of the yttrium–arsine complex [Cp' 3 Y{As(H) 2 Mes}] (1) (Cp' = h 5 -C 5 H 4 Me, Mes = mesityl) by nBuLi produces the m-arsenide complex [{Cp' 2 Y[m-As(H)Mes]} 3 ] (2). Deprotonation of the As À H bonds in 2 by nBuLi produces [Li(thf) 4 ] 2 [{Cp' 2 Y(m 3 -AsMes)} 3 Li], [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. Rare-earth metal compounds containing soft heteroatom donor ligands have attracted considerable interest in recent years. [1–6] The combination of Lewis acidic M 3+ cations with heavy p-block donor atoms results in a hard–soft mismatch that can lead to unusual bonding properties and to distinct reactivity. Within this context, rare-earth metal complexes of anionic phosphorus donor ligands such as phosphide (R 2 P À) have been extensively studied. [7] A key development occurred in 2008, when a lutetium phosphinidene (RP 2À) complex was structurally characterized and its phosphinidene transfer reactivity towards aldehydes and ketones demonstrated. [8]