The partitioning behavior of pentacyanonitrosilmetallate complexes-[Fe(CN)(5)NO](2-), [Mn(CN)(5)NO](3-), and [Cr(CN)(5)NO](3-)-has been studied in aqueous two-phase systems (ATPS) formed by adding poly(ethylene oxide) (PEO; 4000 g mol(-1)) to an aqueous salt solution (Li2SO4, Na2SO4, CUSO4, or ZnSO4). The complexes partition coefficients (K-complex) in each of these ATPS have been determined as a function of increasing tie-line length (TLL) and temperature. Unlike the partition behavior of most ions, [Fe(CN)(5)NO](2-) and [Mn(CN)(5)NO](3-) anions are concentrated in the polymer-rich phase with K values depending on the nature of the central atom as follows: K[Fe(CN)5NO]2- >> K[Mn(CN)5NO]3- > K[Cr(CN)5NO]3-. The effect of ATPS salts in the complex partitioning behavior has also been verified following the order Li2SO4 > Na2SO4 > ZnSO4. Thermodynamic analysis revealed that the presence of anions in the polymer-rich phase is caused by an EO-[M(CN)(5)NO](x-)(M = Fe, Mn, or Cr) enthalpic interaction. However, when this enthalpic interaction is weak, as in the case of the [Cr(CN)(5)NO](3-) anion (K[Cr(CN)5NO]3- < 1), entropic driving forces dominate the transfer process, then causing the anions to concentrate in the salt-rich phase.