Reduction of Rare‐Earth Stannole Sandwich Complexes to Tin‐Based Radical Ligands and Tin–Tin Bonds

f-Element organometallic chemistry is dominated by cyclopentadienyl ligands. In contrast, isoelectronic metallole ligands with the general formula [EC4R4]2-, where E is a heavier group 14 element, are rare in the f-block, particularly stannole ligands. Here, we describe the synthesis of the dimetallic stannole complexes [(η5-CpSn)M(η5-Cpttt)]2 (1M ; M = Y, Gd, Dy; CpSn = [SnC4-2,5-(SiMe3)2-3,4-Me2]2-, Cpttt = [1,2,4-C5 tBu3H2]-), which form by virtue of Sn→M dative bonds. One-electron reduction of 1M with KC8/2.2.2-cryptand produces the mono-anionic complexes [{(η5-CpSn)M(η5-Cpttt)}2]- (2M), and two-electron reduction gives di-anionic [{(η5-CpSn)M(η5-Cpttt)}2]2- (3M) as [K(2.2.2-crypt)]+ salts. Studies of the stannole complexes using crystallography, UV/vis and EPR spectroscopy, magnetometry and computational methods reveal that the reduction steps generate tin-tin bonds through population of a delocalized molecular orbital that spans the {M2Sn2} rings, with attendant dearomatization of the stannole rings. Complexes 2M are the first tin-radical ligands bound to rare earth elements. Spin density calculations of 2Y and 2Gd reveal significant build-up of unpaired spin on the tin atoms, with magnetic measurements on 2Gd yielding an unprecedentedly large tin-gadolinium exchange coupling constant of -112 cm-1 (-2J formalism).