THE ABSORPTION SPECTRA OF SOLID HYDRATED NICKEL SULPHATE BY HERMANN HARTMANN AND HEINZ MULLER Institut fur phys. Chemie Universitat, Frankfurt am Main Robert Mayer Strasse 11 Received 9th June, 1958 In order to test Hartmann and Furlani's quantum-mechanical calculations of the energy level diagram of the complex ion Ni(H20)2,+ for octahedral and tetragonal symmetry, the spectra of the crystalline hexa- and heptahydrates of nickel sulphate have been determined at room temperature and at - 205°C. The number and sequence of main and intercombination bands satisfy the requirements of the theory. Distortion of the octahedral symmetry of the complex ion is shown by the splitting of the main bands. The extent of distortion can be deduced from the relative displacement of the absorption curve, The spectrum suggests the occurrence of multiplet splitting of the triplet terms.Hartmann and Use's theory of the light absorption of complex ions1 has given good agreement with experimental results in a large number of cases 2 and was therefore applied to the Ni(HzO)z+ ion. The shape of the absorption curve for this ion is more complicated than those of the ions investigated before. In order to give a theoretical account of the fine structure of this spectrum, distortion of the octahedral symmetry of the ligand arrangement and intercombinations were taken into consideration. The possibility of multiplet splitting of levels was also discussed. The results of these quantum-mechanical calculations 3 have now been tested by spectroscopic measurements.4 The crystalline hexa- and heptahydrates of nickel sulphate were used.These are obtained by crystallization from hot solutions and from solutions at room temperature, respectively. In both hydrates the structural units are octahedral Ni(H20)e and tetrahedral SOa- ions but the heptahydrate contains in addition an isolated seventh water molecule which is responsible for more extensive dis- tortion of the octahedra and tetrahedra than is met in the hexahydrate. Large crystals (several centimetres long) of both salts were grown. Parallel-sided plates were obtained by careful grinding of the crystals, carbon tetrachloride being used as cooling liquid. The spectroscopic investigations covered the region 2400-9100 A. A Carl- Zeiss 4 2 4 quartz spectrograph was used for the ultra-violet region, and at longer wavelengths a Steinheil Universal-Spektrograph GH with three glass prisms and a focal length of 1.6 m was used.Infra-red-sensitized plates were employed above 6000A. In order to bring out the fine structure, the spectra were taken not only at room temperature but also at the lowest possible temperatures. By applying Simon's desorption technique it was possible to attain temperatures down to - 205" to - 207.5" and to maintain them for the duration of the exposures. All four spectra have the same general form. In particular, the main absorp- tion peaks occur at analogous positions in all four curves. As an example, the spectrum of the hexahydrate at room temperature is shown in fig. 1. There are two main absorption regions of the Ni(H20)3+ ion in the wavelength range in- vestigated.They are indicated by the numerals II and III. Another absorption region occurs at still longer wavelengths for which the experimental technique employed was unsuitable. As this infra-red band is also important to the dis- cussion of the theoretical results we have reserved for it the symbol I. In addition 4950 SPECTRA OF NICKEL SULPHATE several weaker absorption bands, designated by small letters, can be recognized in fig. 1. Lastly, the main bands exhibit fine structure, an enlargement of which is reproduced in fig. 2. The main components of the strong absorption which F~G. 1.-Absorption curve of [Ni(H20)6]S04 at 20°C. FIG. 2.-Fine structure of the main bands 2 5 0 0 0 26OOOcm" 350 300mm of the hexahydrate at 20°C.recur in all the spectra are indicated by capital letters. The main difference between the individual spectra is a largely regular displacement of the entire ab- sorption curve along the abscissa. The average shift of the main maxima is schematically shown in fig. 3. The wavelength difference of the two low-tem- perature spectra is remarkably slight (39 cm-I), that of the hexahydrate occurring at shorter wavelengths. At room temperature both spectra are displaced towards longer wavelengths, the spectrum of the heptahydrate being shifted more than twice as far (367 cm-1) as that of the hexahydrate (142 cm-1). The broad, strong maximum of the heptahydrate which occurs at room temperature in place of the partial bands C and D is also noteworthy.This band is also a characteristic feature of the spectrum of the ion in aqueous solution. The shape and position of this spectnun agree almost exactly with that of the heptahydrate at room temperature. The absorption curves described are of special interest when compared with Hartmann and Furlani's quantum-mechanical calculations of the splitting of energy levels in an electrostatic field of six ligands. The calculated energy levels are shown in fig. 4.H . HARTMANN AND H. MULLER 51 In an electrostatic field of symmetry Oh the 3Fground state of the Ni2+ ion is split into three components. For the absorption spectra the next higher 3P level must also be taken into account. There are thus three transitions from the new ground state whose energies are of the order of light quanta.Since transitions from the triplet ground state to other triplet levels are possible only in combination with suitable vibration transitions, the intensities of the light absorptions must be lower than that caused by pure electronic transitions. The spectrum of the Ni(H20)4+ ion does indeed contain three main bands whose intensities are of the order of magnitude found in analogous spectral transitions of structurally NiSO+* 6H20 405' 367cm-I FIG. 4.-Energy level diagram of the Ni(H20);+ ion. NiSO+: 7H10 -205' In addition to the transitions between the levels produced by splitting of the triplet terms one may also expect transitions from the triplet ground state to the levels produced by splitting of the singlet terms. Although such " intercombinations " between terms of different multiplicity are forbidden, they occur in larger atoms with a low transition probability. The intensities of intercombination bands should therefore be low.It seemed plausible to attribute the bulges at the shorter wavelength side of the main band II and on both sides of the main band III to intercombinations of this kind. The cal- culations showed in addition that most intercombination bands must lie so close to the broad main bands as to be superimposed on them. In order to confirm them experimentally it seemed admissible to take the spectra under high dis- persion at low temperatures. Unfortunately the structure of the band regions is so complicated that it cannot be resolved into Gaussian error curves. However, they are sdciently intense to be recognizable even in the immediate vicinity of the most intense maxima under these rather favourable experimental conditions.Fig. 5 is a schematic diagram of the positions of the observed maxima and of the calculated transitions. The weaker bands are distinguished from the stronger ones by being drawn as lines of half length. The labelling of the bands is the same as in the earlier diagrams. The number and sequence of the observed weaker bands agrees well with the intercombinations required by theory. In the spectrum of the hexahydrate all main bands are split. The octahedral symmetry of the ligand field is therefore disturbed. The occurrence of two components indicate a resultant D4h symmetry, in agreement with crystal struc- ture analysis.It is peculiar that the spectrum of the heptahydrate has the same form as that of the hexahydrate. However, according to the crystallographic measurements, the Ni(H20);+ octahedron is more strongly distorted. The spectra52 SPECTRA OF NICKEL SULPHATE at room temperature do not permit any definite conclusions about the extent of band splitting, and hence about the degree of distortion. According to theory the displacement of the bands to shorter wavelengths is, in addition to the above splitting, a measure of the distortion. The remarkably large displacement of the absorption curve for the heptahydrate, as compared with the hexahydrate, clearly indicate therefore that the distortion is greater in the heptahydrate. All the details of the absorption curves discussed so far can be well reconciled with the requirements of the theoretical treatment of the Ni(&,O)~+ ion.Only the occurrence of the maximum in the main band I1 is not indicated by the theory and cannot be accounted for on the basis of the energy level diagram derived from the ligand-field splitting. To explain it one could invoke multiplet-splitting due to the interaction of spin and orbital angular momenta. NiSO;6H10 +lao 11 u I , I Ill I I I NiS0;6H10 -205' 1 1, I I l l I I Ill I NiSO~7H10t181 I II I1 AB CDEa b FGC d C f CD Ea b FGc d 1 1 Cf b FGc d AB CDEa UiS0,+.7HLO -205O )I 11 CDEa b FGc v 10000 2 0 0 0 0 3 0 0 0 0 4 0 0 0 0 crni FIG. 5.-Comparison of calculated transitions with observed absorption maxima. The difference in the symmetry of the complex ion in the two hydrates, which has been deduced from X-ray measurements at room temperature, is not brought out in the shape of the spectra. Even the low-temperature spectra give no in- dication that a symmetry lower than D4h occurs in the heptahydrate.On the contrary, their remarkable agreement suggests that at this temperature, there can be hardly any difference in the type of distortion of the complex ion in the two hydrates. Conclusions about the extent of distortion may, however, be drawn from the relative displacement of the spectra. This is slight at - 205" (39 cm-1). At this temperature the deformation in the two hydrates is therefore little different and must be much smaller than in the hexahydrate at 18", i.e. considerably less than 1 %. The displacement of the absorption curves on warming the hydrates to room temperature is so great that it cannot be attributed to the usual thermal dilatation but must be due to distortion. It is entirely plausible that the influence of temperature should be greater for the more loosely constructed heptahydrate (density = 2-07) than for the hexahydrate (density = 2.24). The magnitudes of the displacements (hexahydrate : 142 cm-1, heptahydrate : 367 cm-1) agree well with the changes in ligand distances found by X-ray crystallography. These amount to 1 % for the hexahydrate and nearly 10 % for the heptahydrate. The spectroscopic investigations have confirmed all the requirements of the theory. As theoretically calculated energy-level differences can be unreliable owing to the approximations made, we would point out that in the present case, the discrepancies are, on the average, only about 5 % and always less than 10 %. 1 Ilse and Hartmann, 2. physik. Chem., 1951, 197, 239. 2 Hartmann and Schlafer, Angew. Chem., 1958,70, 155. 3 Furlani, 2. physik. Chem., 1957, 10, 291. 4 Heinz Miiller, DipZomarbeit (Frankfurt am Main, 1957).