Solution [Cu(amm)]2+ is a strongly solvated square pyramid:: A full account of the copper K-edge XAS spectrum within single-electron theory

The solution structure of Cu(II) in 4M 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 D-2d-tetragonal, regular and distorted square pyramids, trigonal bipyramids, and Jahn-Teller distorted octahedra. Each approach converged to the same axially elongated square pyramid, 4 x Cu-N-eq = 2.00 +/- 0.02 angstrom and 1 x Cu-N-ax=2.16 +/- 0.02 angstrom (EXAFS) or 2.20 +/- 0.07 angstrom (MXAN), with strongly localized solvation shells. In the MXAN model, four equatorial ammonias averaged 13 degrees below the Cu(II) xy-plane, which was 0.45 +/- 0.1 angstrom above the mean N-4 plane. When the axial ligand equilibrium partial occupancies of about 0.65 ammonia and 0.35 water were included, EXAFS modeling found Cu-L-ax distances of 2.16 and 2.31 angstrom, respectively, reproducing the distances found in the crystal structures of [Cu(NH3)(5)](2+) and [Cu(NH3)4(H2O)](2+). angstrom transverse axially localized solvent molecule was found at 2.8 angstrom (EXAFS) or 3.1 angstrom (MXAN). Six second-shell solvent molecules were also found at about 3.4 +/- 0.01 (EXAFS) or 3.8 +/- 0.2 angstrom (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(am M)]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.