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

High resolution (k = 18 Å-1 or k = 17 Å-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 Å, Oax1 = 2.22 Å, and Oax2 = 2.34 Å, plus a second-shell of 4×Owater = 3.6 Å. 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 Å and 1×H2O = 2.23 Å. The CN6 sites included either a capped, near perfect, square pyramid with 5×H2O = 1.94 ± 0.04 Å and H2Oax = 2.22 Å (in 1,3-P) or a split axial configuration with 4×H2O = 1.94, H2Oax1 = 2.14 Å, and H2Oax2 = 2.28 Å (in 1,5-P). The CN6 sites also included an 8-H2O second-shell near 3.7 Å, which was undetectable about the strictly pyramidal sites. Equatorial angles averaging 94° ± 5° indicated significant departures from tetragonal planarity. MXAN assessment of the solution structure of [Cu(aq)]2+ in 1,5-P prior to freezing revealed the same structures as previously found in aqueous 1M HClO4, which have become axially compressed in the frozen glasses. [Cu(aq)]2+ in liquid and frozen solutions is dominated by a 5-coordinate square pyramid, but with split axial CN6 appearing in the frozen glasses. Among these phases, the Cu-O axial distances vary across 1 Å, and the equatorial angles depart significantly from the square plane. Although all these structures remove the dx2-y2 , dz2 degeneracy, no structure can be described as a Jahn-Teller (JT) axially elongated octahedron. The JT-octahedral description for dissolved [Cu(aq)]2+ should thus be abandoned in favor of square pyramidal [Cu(H2O)5]2+. The revised ligand environments have bearing on questions of the Cu(i)/Cu(ii) self-exchange rate and on the mechanism for ligand exchange with bulk water. The plasticity of dissolved Cu(ii) complex ions falsifies the foundational assumption of the rack-induced bonding theory of blue copper proteins and obviates any need for a thermodynamically implausible protein constraint.