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 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 x Cu-Neq=2.00+/-0.02 A and 1 x Cu-Nax=2.16+/-0.02 A (EXAFS) or 2.20+/-0.07 A (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 A 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 A, 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 A (EXAFS) or 3.1 A (MXAN). Six second-shell solvent molecules were also found at about 3.4+/-0.01 (EXAFS) or 3.8+/-0.2 A (MXAN). The structure of Cu(II) in 4 M pH 10 aqueous NH 3 may be notationally described as {Cu(NH 3)4.62(H2O)0.38](solv)}(2+).6solv, solv=H2O, NH 3. 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.