The alpha -1 and alpha -2 isomers of the monovacant Wells-Dawson heteropoly
oxoanion [P2W17O61](10-) are complexants of trivalent rare-earth (RE) ions
and serve to stabilize otherwise reactive tetravalent lanthanide (Ln) and a
ctinide (An) ions in aqueous solution. Aspects of the bonding of Ln ions wi
th 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 t
ypes 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) an
d [Lu(alpha -2-P2W17O61)(2)](17-) (2) were solved and refined through use o
f single-crystal X-ray diffraction. The crystallographic results are suppor
ted with corresponding insights from XAFS (X-ray absorption fine structure)
for a series of nine solid-state complexes as well as from optical lumines
cence spectroscopy of the Eu(III) analogues in aqueous solution. All the Ln
ions are eight-coordinate with oxygen atoms in a square antiprism arrangem
ent. 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 thr
oughout the lanthanide series. For the 1:2 stoichiometric Ln/alpha -2-[X2W1
7O61](10-) complexes, no water molecules are in the Ln-Os coordination sphe
re. The Ln ions are bound to eight O atoms-four from each of two heteropoly
anions-as [Ln(alpha -2-X2W17O61)21 (17-). The average Ln-O interatomic dist
ances decrease across the lanthanide series, consistent with the decreasing
Ln ionic radius.