A Trinuclear Gadolinium Cluster with a Three-Center One-Electron Bond and an S = 11 Ground State

[Image: see text] The recent discovery of metal–metal bonding and valence delocalization in the dilanthanide complexes (Cp(iPr5))(2)Ln(2)I(3) (Cp(iPr5) = pentaisopropylcyclopentadienyl; Ln = Y, Gd, Tb, Dy) opened up the prospect of harnessing the 4f(n)5d(z(2))(1) electron configurations of non-traditional divalent lanthanide ions to access molecules with novel bonding motifs and magnetism. Here, we report the trinuclear mixed-valence clusters (Cp(iPr5))(3)Ln(3)H(3)I(2) (1-Ln, Ln = Y, Gd), which were synthesized via potassium graphite reduction of the trivalent clusters (Cp(iPr5))(3)Ln(3)H(3)I(3). Structural, computational, and spectroscopic analyses support valence delocalization in 1-Ln resulting from a three-center, one-electron σ bond formed from the 4d(z(2)) and 5d(z(2)) orbitals on Y and Gd, respectively. Dc magnetic susceptibility data obtained for 1-Gd reveal that valence delocalization engenders strong parallel alignment of the σ-bonding electron and the 4f electrons of each gadolinium center to afford a high-spin ground state of S = 11. Notably, this represents the first clear instance of metal–metal bonding in a molecular trilanthanide complex, and the large spin–spin exchange constant of J = 168(1) cm(–1) determined for 1-Gd is only the second largest coupling constant characterized to date for a molecular lanthanide compound.