Expanding Rare-Earth Oxidation State Chemistry to Molecular Complexes of Holmium(II) and Erbium(II)

The first molecular complexes of holmium and erbium in the +2 oxidation state have been generated by reducing Cp′3Ln [Cp′ = C5H4SiMe3; Ln = Ho (1), Er (2)] with KC8 in the presence of 18-crown-6 in Et2O at −35 °C under argon. Purification and crystallization below −35 °C gave isomorphous [(18-crown-6)K][Cp′3Ln] [Ln = Ho (3), Er (4)]. The three Cp′ ring centroids define a trigonal-planar geometry around each metal ion that is not perturbed by the location of the potassium crown cation near one ring with K–C(Cp′) distances of 3.053(8)–3.078(2) Å. The metrical parameters of the three rings are indistinguishable within the error limits. In contrast to Ln2+ complexes of Eu, Yb, Sm, Tm, Dy, and Nd, 3 and 4 have average Ln–(Cp′ ring centroid) distances only 0.029 and 0.021 Å longer than those of the Ln3+ analogues 1 and 2, a result similar to that previously reported for the 4d1 Y2+ complex [(18-crown-6)K][Cp′3Y] (5) and the 5d1 La2+ complex [K(18-crown-6)(Et2O)][Cp″3La] [Cp″ = 1,3-(Me3Si)2C5H3]. Surprisingly, the UV–vis spectra of 3 and 4 are also very similar to that of 5 with two broad absorptions in the visible region, suggesting that 3–5 have similar electron configurations. Density functional theory calculations on the Ho2+ and Er2+ species yielded HOMOs that are largely 5d z 2 in character and supportive of 4f105d1 and 4f115d1 ground-state configurations, respectively.