This study identifies the principles that govern the formation and stability of Ln complexes of the (α(1)-P2W17O61)(10-) isomer. The conditional stability constants for the stepwise formation equilibria, K-1cond and K-2cond, determined by P-31 NMR spectroscopy, show that the high log K-1cond/log K-2cond ratio predicts the stabilization of the 1:1 Ln/(α(1)-P2W17O61)(10-) species. The value of log K-1cond increases as the Ln series is traversed, consistent with the high charge/size requirement of the basic α(1) defect site. The conditional stability constants, K-2, are very low and are highly dependent on the countercations in the buffer. The source of the instability is understood from the crystal structures of the early-mid lanthanide analogues, where the close contact of the (α(1)-P2W17O61)(10-) units result in severe steric encumbrance. The electronic properties of the a, defect along with the lanthanide ionic radii and countercation composition are important parameters that need to be considered for a rational synthesis of lanthanide polyoxometalates.