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To advance our ability to control the electronic properties of divalent lanthanides, the interplay between deformation densities, 4f interelectronic repulsion, and ligand field effects is discussed to predict the nature of their ground states. 

The importance of the specific trialkylsilyl substituent in the cyclopentadienyl chemistry of C5H4SiR3 ligands has been demonstrated by the synthesis of low oxidation-state thorium complexes. Although the structure of the disilyl-substituted cyclopentadienyl Th(III) complex, [C5H3(SiMe3)2]3ThIII (Cp″3ThIII), was reported in 1986, no monosilyl-substituted analogues, (C5H4SiR3)3ThIII (R = alkyl, aryl), have been isolated to date, even though analogues are well known in U(III) chemistry. We now report that crystalline tris(monosilyl-substituted cyclopentadienyl) Th(III) and Th(II) complexes can be isolated when R = isopropyl, i.e., using the (triisopropylsilyl)cyclopentadienyl ligand, C5H4SiiPr3 (CpTIPS). The salt metathesis reaction between three equiv of KCpTIPS and ThIVBr4(DME)2 (DME = 1,2-dimethoxyethane) afforded the colorless Th(IV) complex, CpTIPS3ThIVBr, 1, which was identified spectroscopically and crystallographically. KC8 reduction of 1 in THF produced dark blue CpTIPS3ThIII, 2, in crystalline form. The complex was identified by X-ray crystallography, EPR, and UV–visible spectroscopy in contrast to ″(C5H4SiMe3)3ThIII,″ which has never been isolated due to its instability. This Th(III) complex can be reduced further with KC8 in the presence of 2.2.2-cryptand (crypt) to make [K(crypt)][CpTIPS3ThII], 3, which is only the second crystallographically characterized Th(II) complex isolated since (Cp″3ThII)1– was discovered in 2014. Spectroscopic, crystallographic, and density functional theory (DFT) analyses are consistent with 6d1 and 6d2 electron configurations for the Th(III) and Th(II) complexes, respectively. The importance of the triisopropylsilyl substituent and the role that steric factors play in the successful isolation of Th(III) and Th(II) complexes were evaluated by Guzei solid angle calculations and electrochemical studies. The results suggest that both electronic and steric effects should be considered in the isolation of Th(III) and Th(II) complexes.