The X-ray structure of 1 (Figure ) confirmed the coordination of two  5 -Cp ttt and one  2bound borohydride to the Lu III center. In comparison with the isomorphous Tm III complex, 21c the Lu-Cp(ctr) (ctr = ring centroid) separation is slightly shorter (2.322 Å for Lu vs. 2.356 Å for Tm), in agreement with the smaller ionic radius of Lu. The 1 H NMR spectrum of 1 displays three resonances for the coordinated Cp ttt ligands, consistent with an overall C2v symmetric species in solution. One notable feature in the IR spectrum of 1 is the presence of three moderately intense bands in the region 2000-2500 cm -1 , more precisely one doublet band at 2459 and 2413 cm -1 associated with one additional band at 2112 cm -1 , arising from the terminal  2 -bound BH4. These bands are blue-shifted by ca. 50 cm -1 with respect to those in [LuCp2(BH4)(THF)] initially reported by Schumann and coworkers. To access [Lu(Cp ttt )2] (2), reduction of 1 was performed by treatment with excess KC8 in pentane at room temperature under an argon atmosphere for 5 days (Figure ). As 2 is extremely sensitive towards N2, special precautions must be taken to avoid adventitious reaction with traces of N2, even when working in an argon-filled glovebox (see Supplementary Information for details). It should be noted that previous reports on the attempted isolation of non-classical divalent lanthanide [Ln(Cp ttt )2] complexes highlighted the difficulty in isolating these species owing to their high reactivity and solubility. 36 Building on our recent success in crystallizing the related Tm II complex [Tm(Cp ttt )2], orangered crystals of 2 suitable for X-ray diffraction (XRD) studies were isolated in 18% yield after crystallization from pentane at low temperature (-40 °C). Although the isolated crystalline yield is relatively low, which can be traced back to the very high solubility of 2 in hydrocarbon solvents, the mother liquor was found to be composed of 2 along with variable amounts of clean thermolysis products (see below), as evidenced by 1 H NMR analysis.