[Cu(aq)]2+ is structurally plastic and the axially elongated octahedron goes missing

High resolution (k = 18 A−1 or k = 17 A−1) copper K-edge EXAFS and MXAN (Minuit X-ray Absorption Near Edge) analyses have been used to investigate the structure of dissolved [Cu(aq)]2+ in 1,3-propanediol (1,3-P) or 1,5-pentanediol (1,5-P) aqueous frozen glasses. EXAFS analysis invariably found a single axially asymmetric 6-coordinate (CN6) site, with 4×Oeq = 1.97 A, Oax1 = 2.22 A, and Oax2 = 2.34 A, plus a second-shell of 4×Owater = 3.6 A. However, MXAN analysis revealed that [Cu(aq)]2+ occupies both square pyramidal (CN5) and axially asymmetric CN6 structures. The square pyramid included 4×H2O = 1.95 A and 1×H2O = 2.23 A. The CN6 sites included either a capped, near perfect, square pyramid with 5×H2O = 1.94 ± 0.04 A and H2Oax = 2.22 A (in 1,3-P) or a split axial configuration with 4×H2O = 1.94, H2Oax1 = 2.14 A, and H2Oax2 = 2.28 A (in 1,5-P). The CN6 sites also included an 8-H2O second-shell near 3.7 A, which was undetectable about the strictly pyramidal sites. Equatorial angles averaging 94° ± 5° indica...