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)₄^2- 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 β_s,p bonding parameter of −5 found appropriate for Ni coordination in our previous studies of single Ni(CN)₄^2- planes and a helically stacked Cs₂[Ni(CN)₄]·H₂O crystal was changed to −3 for the more parallel-stacked Ni(CN)₄^2- planes in this case, while β_d was retained at −41. Crystal data are as follows:  Na₂[Ni(CN)₄]·3H₂O, triclinic space group P1̄, a = 7.2980(10) Å, b = 8.8620(10) Å, c = 15.132(2) Å, α = 89.311(5)°, β = 87.326(5)°, γ = 83.772(6)°, V = 971.8(2) ų, T = 100 K, Z = 4, R = 0.024, R_w = 0.064; Sr[Ni(CN)₄]·5H₂O, monoclinic space group C2/m, a = 10.356(2) Å, b = 15.272(3) Å, c = 7.1331(10) Å, β = 98.548(12)°, V = 1115.6(3) ų, T = 100 K, Z = 4, R = 0.024, R_w = 0.059; Rb₂[Ni(CN)₄]·1.05H₂O, triclinic space group P1̄, a = 8.6020(10) Å, b = 9.6930(10) Å, c = 12.006(2) Å, α = 92.621(6)°, β = 94.263(6)°, γ = 111.795(10)°, V = 924.0(2) ų, T = 100 K, Z = 4, R = 0.034, R_w = 0.067.