Homoleptic trivalent selenolates of La and Y have been isolated from the reaction of Ln[N(SiMe(3))(2)](3) (Ln = La, Ce) with 3HSeSi(SiMe(3))(3) in hexanes. Variable-temperature NMR studies suggest that La[SeSi(SiMe(3))(3)](3) behaves as a three-coordinate monomer in toluene, whereas data for the Y analogue are consistent with the dimeric structure {Y[SeSi(SiMe(3))(3)](2)[mu-SeSi(SiMe(3))3]}(2). EXSY experiments have been used to investigate ligand exchange in the Y compound. THF adducts Ln[SeSi(SiMe(3))(3)](3)(THF)(2) (Ln = La, Sm, Yb) have been isolated and fully characterized. Tellurolate species Ln[TeSi(SiMe(3))(3)](3) (Ln = La, Ce), prepared by the analogous reaction with 3HTeSi(SiMe(3))(3) in hydrocarbons, were characterized by NMR spectroscopy and by the isolation of derivatives. Addition of DMPE (DMPE = 1,2-bis(dimethylphosphino)ethane) to these reactions gave isolable adducts Ln[TeSi(SiMe(3))(3)](3)(DMPE)(2) (Ln = La, Ce, Y) which have been characterized by multinuclear and variable-temperature NMR spectroscopy. in addition, the X-ray crystal structure of La[TeSi(SiMe(3))(3)](3)(DMPE)(2) has been determined : it crystallizes at -40 degrees C from hexanes in the space group C2/c with a = 51.207(11) Angstrom, b = 15.725(3) Angstrom, c = 18.903(3) Angstrom, beta = 92.698(15)degrees, V = 15204(9) Angstrom(3), d(calcd) = 1.37 g cm(-3), and Z = 8. Exchange spectroscopy has also been used to investigate the process by which these molecules rearrange in solution, The homoleptic tellurolates are unstable in solution as monitored by NMR spectroscopy; preparative-scale reactions led to the isolation of a remarkable tellurolate/telluride cluster Ce5Te3[TeSi(SiMe(3))(3)](9), which has been structurally characterized by X-ray crystallography. Ce5Te3 [TeSi(SiMe(3))(3)](9) crystallizes at -40 degrees C from hexanes in the space group P6(3)/m with a = 20.456(5) Angstrom, b = 20.456(5) Angstrom, c = 28.063(9) Angstrom, gamma = 120 degrees, V = 10170(8) Angstrom(3), d(calcd) 1.46 g cm(-3), and Z = 2.