The first molecular complexes of holmium and erbium in the +2 oxidation state have been generated by reducing Cp-3'Ln [Cp' = C5H4SiMe3; Ln = Ho (1), Er (2)] with KC8 in the presence of 18-crown-6 in Et2O at -35 degrees C under argon. Purification and crystallization below -35 degrees C gave isomorphous [(18-crown-6)K][Cp-3'Ln] [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) angstrom. The metrical parameters of the three rings are indistinguishable within the error limits. In contrast to Ln(2+) 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 angstrom longer than those of the Ln(3+) analogues 1 and 2, a result similar to that previously reported for the 4d(1) Y2+ complex [(18-crown-6)K][Cp-3'Y] (5) and the 5d(1) La2+ complex [K(18-crown-6)(Et2O)][Cp-3 '' La] [Cp '' = 1,3-(Me3Si)(2)C5H3]. 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 4f(10)5d(1) and 4f(11)5d(1) ground-state configurations, respectively.