Solution [Cu(amm)]2+ is a Strongly Solvated Square Pyramid: A Full Account of the Copper K-edge XAS Spectrum Within Single-Electron Theory

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Abstract
The solution structure of Cu(II) in 4 M aqueous ammonia, [Cu(amm)]2+, was assessed using copper K-edge extended X-ray absorption fine structure (EXAFS) and Minuit XANes (MXAN) analyses. Tested structures included trigonal planar, planar and D2d-tetragonal, regular and distorted square pyramids, trigonal bipyramids, and Jahn−Teller distorted octahedra. Each approach converged to the same axially elongated square pyramid, 4 × Cu−Neq = 2.00 ± 0.02 Å and 1 × Cu−Nax=2.16 ± 0.02 Å (EXAFS) or 2.20 ± 0.07 Å (MXAN), with strongly localized solvation shells. In the MXAN model, four equatorial ammonias averaged 13° below the Cu(II) xy-plane, which was 0.45 ± 0.1 Å above the mean N4 plane. When the axial ligand equilibrium partial occupancies of about 0.65 ammonia and 0.35 water were included, EXAFS modeling found Cu−Lax distances of 2.16 and 2.31 Å, respectively, reproducing the distances found in the crystal structures of [Cu(NH3)5]2+ and [Cu(NH3)4(H2O)]2+. A transverse axially localized solvent molecule was found at 2.8 Å (EXAFS) or 3.1 Å (MXAN). Six second-shell solvent molecules were also found at about 3.4 ± 0.01 (EXAFS) or 3.8 ± 0.2 Å (MXAN). The structure of Cu(II) in 4 M pH 10 aqueous NH3 may be notationally described as {[Cu(NH3)4.62(H2O)0.38](solv)}2+·6solv, solv = H2O, NH3. The prominent shoulder and duplexed maximum of the rising K-edge XAS of [Cu(amm)]2+ primarily reflect the durable and well-organized solvation shells, not found around [Cu(H2O)5]2+, rather than two-electron shakedown transitions. Not accounting for solvent scattering thus may confound XAS-based estimates of metal−ligand covalency. [Cu(amm)]2+ continues the dissymmetry previously found for the solution structure of [Cu(H2O)5]2+, again contradicting the rack-bonding theory of blue copper proteins.