The Synthesis, Structures and Polymorphism of the Dimeric Trivalent Rare‐Earth 3,5‐Dimethylpyrazolate Complexes [Ln(Me2pz)3(thf)]2

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A variety of rare‐earth 3,5‐dimethylpyrazolate (Me2pz) complexes have been synthesised by (i) the direct reaction of Hg‐activated metal with Me2pzH as a pro‐ligand at elevated temperatures, (ii) by redox transmetalation/protolysis with the lanthanoid element, Hg(C6F5)2, and Me2pzH, and (iii) by protolysis of tris[bis(trimethylsilyl)amido]cerium(III) with Me2pzH. Each product, upon crystallisation from tetrahydrofuran (thf), formed a dimeric complex, [Ln(Me2pz)3(thf)]2 (Ln = La, Ce, Pr, Nd, Ho, Yb, or Lu). Despite the common formulation, two completely different structures were observed in two distinct crystallographic “domains of existence”, together presumptively spanning the gamut of Ln and Y. For the larger rare‐earth ions (La–Pr), there are two terminal η2‐Me2pz ligands and one thf donor on each Ln atom, with the metal atoms being linked by a pair of bridging pyrazolate ligands of an uncommon type (η2:η5), resulting in formal ten‐coordination. A Me2pzH complex [Ce(Me2pz)3(Me2pzH)], although not isomorphous, has a similar structure. For the smaller rare‐earth elements (Nd–Lu), the bridging is entirely different, with two μ‐κ1(N):κ1(N) pyrazolate and two unusual bridging thf ligands. Each Ln atom also has two chelating Me2pz ligands, resulting in formal eight‐coordination. Crystallisation of [Nd(Me2pz)3(thf)]2 from pyridine yields monomeric, nine‐coordinate [Nd(Me2pz)3(pyridine)3] with only chelating Me2pz ligands.