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 5dz2 in character
and supportive of 4f105d1 and 4f115d1 ground-state configurations, respectively.