A Solid-State Spectral Effect in Eclipsed Tetracyanonickelates: X-ray Crystal Structure, Polarized Specular Reflectance Spectroscopy, and ZINDO Modeling of Sr[Ni(CN)4]·5H2O, Rb2[Ni(CN)4]·H2O, and Na2[Ni(CN)4].3H2O

Crystal structures of three Ni(CN)42- salts all with eclipsed ligands and varying axial stacking arrangements are presented. The absorption spectra of all three salts show a slight red shift in the x,y-polarizations and a large red shift in their z-polarizations upon crystallization from solution. Semiempirical ZINDO calculations provide a good model of the solid state, even with only a three-molecule segment, allowing reproduction of the red-shifting and intensity increase upon crystallization found experimentally. The modified nickel (beta)s,p bonding parameter of -5 found appropriate for Ni coordination in our previous studies of single Ni(CN)42- planes and a helically stacked Cs2[Ni(CN)4].H2O crystal was changed to -3 for the more parallel-stacked Ni(CN)42- planes in this case, while (beta)d was retained at -41. Crystal data are as follows: Na2[Ni(CN)4].3H2O, triclinic space group P1, a = 7.2980(10) (angstrom), b = 8.8620 (10) (angstrom), c = 15.132(2) (angstrom), (alpha)= 89.311(5)°, (beta)= 87.326(5)°, (gamma)= 83.772(6)°, V = 971.8(2) (angstrom)3, T = 100 K, Z = 4, R = 0.024, Rw = 0.064; Sr[Ni(CN)4]·5H2O, monoclinic space group C2/m, a = 10.356(2) (angstrom), b = 15.272(3) (angstrom), c = 7.1331(10) (angstrom), (beta)= 98.548(12)°, V = 1115.6(3) (angstrom)3, T = 100 K, Z = 4, R = 0.024, Rw = 0.059; Rb2 [Ni(CN)4].1.05H2O, triclinic space group P , a = 8.6020(10) (angstrom), b = 9.6930(10) (angstrom), c = 12.006(2) (angstrom), (alpha)= 92.621(6) °, (beta)= 94.263(6) °, (gamma)= 111.795(10) °, V = 924.0(2) (angstrom)^3, T = 100 K, Z = 4, R = 0.034, Rw = 0.067.