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 EC4R42-, 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-Me22-, 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) 22- (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).