The electronic spectra of the hexafluoronickelate(III), hexafluorocuprate(III), and hexafluoroargentate(III) anions have been studied by diffuse reflectance between 4 and 45 kK for the compounds K3NiF6, Na3NiF6, Cs2KCuF6, and Cs2KAgF6 The CuF63- ion shows prominent ligand field bands at 14.1 and 20.4 kK, assigned to 3A2g → 3T2g and 3A2g → 3T1g transitions, respectively, with a weaker peak at 9.6 kK and a faint shoulder at 16.4 kK. The latter are ascribed to spin-forbidden transitions to 1Eg and 1A1g states and the intense bands found at 29.5 and 37.2 kK to π → eg charge-transfer excitations. The AgFe3-ion similarly shows the 3T2g and 3T1g bands at 18.4 and 23.4 kK, but the well-marked shoulder at 12.9 kK is now assigned to the 1A1g excitation and the very weak peak at 6.3 kK to the 1Eg transition, while the charge-transfer bands appear at 27.5 and 37.5 kK. In both compounds spin-orbit coupling is important in determining the relative intensities of the spin-forbidden transitions and the widths of the 3T2g bands. Fitting of the d-d bands using the Tanabe-Sugano strongfield matrices yields, for the copper complex, Dq = 1410 cm-1, B35 = 641 cm-1, B33 = 536 cm-1, β35 = 0.55, and β33 = 0.46, and for the silver complex, Dq = 1840 cm-1, B35 = 472 cm-1, B33 = 375 cm-1, β35 = 0.61, and β33 = 0.48. The spectrum of the NiF63- anion is analyzed in terms of a 2Eg (t2g6eg) low-spin ground state, in which appreciable tetragonal Jahn-Teller distortion occurs. The ligand field bands are found at 6.8, 12.3, 15.7, and 19.2 kK in K3NiF6 and the spectrum of Na3NiF6is identical except for the shift of the first two bands to 6.1 and 12.8 kK. The lowest energy band is assigned to a 1A1 → 1B1 transition arising from the tetragonal ground state splitting, and the three other bands are similarly interpreted in terms of the D4 splitting of the Oh 2T1g, 2T2g, and 2T1g (t2g5eg2) levels. The intense bands at 32.0 and 37.0 kK are again assigned to π →eg charge-transfer transitions. The d-d bands were fitted using the d7 strongfield matrices for D4 symmetry, assuming Ds/Dt = 3.0. This gave Dq = 1620 cm-1, B = 703 cm-1, β = 0.63, and Dq = 423 cm-1 for K3NiF6 and Dt = 388 cm-1 for the sodium compound. In all cases the degree of covalency indicated is high for fluoride complexes, but for Ni3+ and Cu3+ β reflects closely the decreasing stability of the higher oxidation states toward the end of the first transition series. © 1969, American Chemical Society. All rights reserved.
