The alpha -1 and alpha -2 isomers of the monovacant Wells-Dawson heteropolyoxoanion [P2W17O61](10-) are complexants of trivalent rare-earth (RE) ions and serve to stabilize otherwise reactive tetravalent lanthanide (Ln) and actinide (An) ions in aqueous solution. Aspects of the bonding of Ln ions with alpha -1-[P2W17O61](10-) and alpha -2-[P2W17O61](10-) were investigated to address issues of complex formation and stability. We present structural insights about the Ln(III) coordination environment and hydration in two types of stoichiometric complexes, [Ln(alpha -1-P2W17O61)](7-) and [Ln(alpha -2-X2W17O61)(2)](17-) (for Ln drop Sm, Eu, Lu; X drop P, As). The crystal and molecular structures of C(H2O)(4)-Lu(alpha -1-P2W17O61)(2)](17-) (1) and [Lu(alpha -2-P2W17O61)(2)](17-) (2) were solved and refined through use of single-crystal X-ray diffraction. The crystallographic results are supported with corresponding insights from XAFS (X-ray absorption fine structure) for a series of nine solid-state complexes as well as from optical luminescence spectroscopy of the Eu(III) analogues in aqueous solution. All the Ln ions are eight-coordinate with oxygen atoms in a square antiprism arrangement. For the 1:1 stoichiometric Ln/alpha -1-[P2W17O61](10-) complexes, the Ln ions are bound to four O atoms of the lacunary polyoxometalate framework in addition to four O atoms from solvent (water) molecules as I(H2O)(4)Ln(alpha -1-P2W17O61)](7-) This structure (1) is the first of its kind for any metal complex of alpha -1-[P2W17O61](10-), and the data indicate that the general stoichiometry [(H2O)(4)Ln(alpha -1-P2W17O61)](7-) is maintained throughout the lanthanide series. For the 1:2 stoichiometric Ln/alpha -2-[X2W17O61](10-) complexes, no water molecules are in the Ln-Os coordination sphere. The Ln ions are bound to eight O atoms-four from each of two heteropolyanions-as [Ln(alpha -2-X2W17O61)21 (17-). The average Ln-O interatomic distances decrease across the lanthanide series, consistent with the decreasing Ln ionic radius.