A series of lanthanide complexes containing a chalcogenolate ligand supported by two Tp(Me,Me) (tris-3,5-dimethylpyrazolylborate) groups has been prepared and crystallized and provides direct comparisons of bonding to hard and soft ligands at lanthanide centers. Reaction of [Sm(Tp(Me,Me))(2)Cl] with NaOR (R = Ph, Ph-Bu-t) gives [Sm(Tp(Me,Me))(2)OR] (1a and 1b, respectively) in good yields. Reductive cleavage of dichalcogenides by samarium(II) was used to prepare the heavier congeners. Complexes of the type [Sm(Tp(Me,Me))(2)ER] for E = S, R = Ph (2a), E = S, R = Ph-4-Me (2b), E = S, R = CH2Ph (2c), E = Se, R = Ph (3a), E = Se, R = Ph-4-Bu-t (3b), E = Se, R = CH2Ph (3c), and E = Te, R = Ph (4) have been prepared together with the corresponding complexes with Tp(Me,Me,4-Et) as ancillary. The X-ray crystal structures of 1b, 2b, 3a, 3b, and 4 have been determined. The crystal of 1b (C40H57B2N12OSm . C7H8) was monoclinic, P2(1)/c, a = 10.6845(6) Angstrom, b = 18.5573(11) Angstrom, c = 24.4075(14) Angstrom, beta = 91.616(2)degrees Z = 4. The crystal of 2b (C37H51B2Ni2SSm) was monoclinic, P2(1)/n, a = 15.0154(9) Angstrom, b = 13.1853(8) Angstrom, c = 21.1254(13) Angstrom, beta = 108.628(2)degrees, Z = 4. The crystal of 3a (C36H49B2N12SeSm . C7H8) was triclinic, P (1) over bar, a = 10.7819(6) Angstrom, b = 19.3011(10) Angstrom, c = 23.0235(12) Angstrom, alpha = 79.443(2)degrees, beta = 77.428(2)4 gamma = 89.827(2)degrees, Z = 4. The crystal of 3b (C40H57B2N12SeSm) was triclinic, P (1) over bar, a = 10.1801(6) Angstrom, b = 10.2622(6) Angstrom, c = 23.4367(14) Angstrom, alpha = 88.313(2)degrees, beta = 86.268(2)degrees, gamma = 62.503(2)degrees Z = 2. The crystal of 4 (C36H49B2N12- TeSm . C7H8) was monoclinic, P2(1)/c, a = 18.7440(10) Angstrom, b = 10.3892(6) Angstrom, c = 23.8351(13) Angstrom, beta = 94.854(2)degrees Z = 4. The compounds form an isoleptic series of seven-coordinate complexes with terminal chalcogenolate ligands. Examination of Ib and other crystallographically characterized lanthanide all;oxides suggests that there is little correlation between bond angle and bond length. The structures of 3a and 3b, however, contain molecules in which one of the pyrazolylborate ligands undergoes a major distortion arising from twisting around a B-N bond so as to give an effectively eight-coordinate complex with pi-stacking of the phenyl group with one pyrazolyl ring. These distortions shed light on the fluxionality of these systems.