Chemistry

N. V. TRONEVA

X-ray \(L_{\mathrm{III}}\)-Absorption Spectra and the Nature of Bonding in Complex Compounds of \(\mathrm{Re}^{\mathrm{II}}\), \(\mathrm{Re}^{\mathrm{IV}}\), \(\mathrm{Re}^{\mathrm{VII}}\)

(Presented by Academician I. I. Chernyaev, April 13, 1962)

The study of the fine structure of X-ray spectra makes it possible to judge the nature of the chemical bond and the charge of the central atom in coordination compounds \((^{1-5})\). Thus, for example, in \((^2)\) an approximate method is given for calculating the charge of osmium atoms in complexes from the decomposition of the contour of the absorption curve into a series of lines and the true absorption edge. The development of the theory of short-range order in X-ray spectroscopy \((^6,\,^7)\) will probably make it possible to determine the effective charge of the central atom, the radius of the first coordination sphere, and the mean potential of an electron in the crystal lattice of solids.

In the present communication are given the results of an experimental study of the rhenium \(L_{\mathrm{III}}\)-absorption spectra in different valence states: \(\mathrm{Re}^{\mathrm{II}}\), \(\mathrm{Re}^{\mathrm{IV}}\), \(\mathrm{Re}^{\mathrm{VII}}\), and also of metallic rhenium. On the basis of an analysis of the absorption curves of rhenium in the metal, \(\mathrm{NH_4HReCl_4}\), \(\mathrm{RePy_2Cl_4}\), \((\mathrm{NH_4})_2\mathrm{ReCl_6}\), and \(\mathrm{NH_4ReO_4}\), an attempt has been made to estimate the effective charge of rhenium in the vicinity of the \(5d\)-shell, as well as the mean potential of the electron in the crystal lattice and the radius of the first coordination sphere on the basis of the theory of short-range order \((^7)\). The rhenium \(L_{\mathrm{III}}\)-absorption spectra were obtained on a short-wavelength X-ray spectrograph with a bent quartz crystal \((10\overline{1}0)\) in the second order of reflection. The dispersion was \(8.25\ \text{XU}/\mathrm{mm}\), resolving power 10000. The absorbers were prepared by mixing finely ground powder with Canada balsam; the mixture was poured onto tissue paper. The surface density was \(10\text{--}20\ \mathrm{mg/cm^2}\). The X-ray source was a BSV tube with a molybdenum anode; tube voltage \(15\text{--}20\ \mathrm{kV}\), current \(20\ \mathrm{mA}\); registration was photographic, exposure \(1\text{--}3\) hours.

Fig. 1. Rhenium X-ray \(L_{\mathrm{III}}\)-absorption spectra

The blackening curve was constructed for each photograph by the method of \((^8)\), using the \(K_{\alpha,\alpha_2}\)- and \(K_{\beta_1,\beta_3}\)-lines of molybdenum. The blackening-distribution curves \(S = S(l)\) were recalculated into distribution curves of the relative absorption coefficients \(\mu |E| \sim -\lg I(l)\); the results were averaged over twelve

absorption curves obtained from three or four photographs over a length of 6 mm (300–400 eV) in the vicinity of the \(L_{\mathrm{III}}\) absorption edge. In calculating the wavelengths, the nearest molybdenum lines were used as comparison lines.

The results of the measurements are given in Fig. 1 and in Table 1, where: \(\Delta E\) is the shift of the white line of the compound toward higher energies relative to metallic rhenium; \(\Delta E_i\) are the distances of the maxima (\(i\) even) and minima (\(i\) odd) of the fine structure of the \(L_{\mathrm{III}}\)-absorption spectra from the inflection point on the long-wavelength side (\(i=1\)).

Table 1

As can be seen from Fig. 1 and the data of Table 1, there is a shift of the white line (\(i=2\)) of the rhenium \(L_{\mathrm{III}}\)-absorption spectrum with a change in formal valence. In works \(^{(9,10)}\) it was established for the rhenium \(L_{\mathrm{III}}\)-absorption spectra in the metal, \(\mathrm{ReO_2\cdot 2H_2O}\), \(\mathrm{K_2ReCl_6}\), and \(\mathrm{KReO_4}\) that the shift of the white line increases in proportion to the formal valence. In the case of the compounds we studied, \((\mathrm{NH_4})_2\mathrm{ReCl_6}\) and \(\mathrm{RePy_2Cl_4}\), shifts of the white line by \(+3\) eV and \(+1\) eV, respectively, are observed, despite the fact that in both compounds rhenium is formally tetravalent. The \(\mathrm{Re}^{\mathrm{IV}}\) compounds considered differ in the character of the immediate environment of the central rhenium atom and, consequently, in the effective charge of rhenium.

If one takes into account that the physical meaning of charge consists in the degree to which electrons of the central atom are withdrawn by the atoms of the immediate environment, then the charge in the vicinity of the \(5d\) shell can be estimated by interpolation from the shift of the white line, as was done in work \(^{(5)}\) for calcium compounds. The charge values in the vicinity of the \(5d\) shell obtained in this way are given in Table 1.

The smaller charge of the central rhenium atom in the vicinity of the \(5d\) shell in \(\mathrm{NH_4ReO_4}\), compared with \((\mathrm{NH_4})_2\mathrm{ReCl_6}\), suggests \(d^3s\)-hybridization in \(\mathrm{NH_4ReO_4}\), rather than \(sp^3\)-hybridization.

As is evident from the data of Table 1 and Fig. 1, the dependence of the short-wave structure of the rhenium \(L_{\mathrm{III}}\)-absorption edge on the coordination number is quite clearly expressed, and consequently \({}^{(14)}\) also on the interatomic distance for metallic rhenium, \((\mathrm{NH}_4)_2\mathrm{ReCl}_6\), and \(\mathrm{NH}_4\mathrm{ReO}_4\), whose structures are known \({}^{(11,12,14)}\). This confirms the theory of short-range order \({}^{(6)}\). On the basis of the similarity of the short-wave fine structure of \((\mathrm{NH}_4)_2\mathrm{ReCl}_6\) and \(\mathrm{RePy}_2\mathrm{Cl}_4\) (Fig. 1), one may suppose that in \(\mathrm{RePy}_2\mathrm{Cl}_4\) the coordination is also octahedral or close to octahedral.

On the basis of the experimental data obtained, an attempt was also made to determine the limits of applicability of the approximate formula \({}^{(7)}\) for the compounds considered with unknown structure. The conditions for maxima of the relative absorption coefficient under scattering of \(2p\)-electrons ejected from the absorbing atom by atoms of the first coordination sphere are written as:

\[ 2k_m r_1 + 2\eta_2(k_m)=\frac{\pi}{2}+2m\pi . \tag{1} \]

Here: \(r_1\) is the radius of the first coordination sphere,

\[ k_m=\frac{\sqrt{2m}}{\hbar}\cdot \sqrt{\Delta E_m+A}, \]

\(\Delta E_m\) are the distances of the maxima of the fine structure from the inflection point on the long-wave side \((m\ne i)\), \(A=U_0-\varphi\), \(U_0\) is the mean potential of the electron in the lattice, \(\varphi\) is the electron work function, and \(\eta_2\) is the atomic scattering phase of the \(2p\)-electron.

If it is assumed that, in some interval of values of \(k_m\), the scattering phase \(\eta_2\) is constant, then for three neighboring maxima of the fine structure with energies \(\Delta E_m\), \(\Delta E_{m+1}\), and \(\Delta E_{m+2}\), the three conditions (1) are satisfied simultaneously if there exists a real solution of the equation:

\[ f(A)=\sqrt{\Delta E_m+A}-2\sqrt{\Delta E_{m+1}+A}+\sqrt{\Delta E_{m+2}+A}=0 . \tag{2} \]

By substituting successively different values of \(A\) from 5 to 40 eV for various sets of three neighboring experimental values \(\Delta E_i\), it was found that for metallic rhenium at \(A=17.5\) eV equation (2) is valid for all sets beginning with the set 34, 74, 123 eV (see Table 1); consequently, these maxima of the fine structure of the \(L_{\mathrm{III}}\)-absorption edge are due to scattering of the \(2p\)-electron ejected upon absorption by atoms of the first coordination sphere. The radius of the first coordination sphere can be estimated from the formula:

\[ r_1=\frac{6.15\cdot 10^{-8}}{\sqrt{\Delta E_{m+1}+A}-\sqrt{\Delta E_m+A}}, \tag{3} \]

whence for metallic rhenium one obtains \(r_1=2.7\ \text{Å}\). This agrees with data on interatomic distances in metallic rhenium \({}^{(14)}\). In the case of \(\mathrm{NH}_4\mathrm{HReCl}_4\) and \(\mathrm{RePy}_2\mathrm{Cl}_4\), only for one set, 112, 189, 281 eV, was such a value of \(A\) found \((A=27.5\ \text{eV})\) for which condition (2) is satisfied; in this case formula (3) gave \(r_1=2.2\ \text{Å}\).

For the compounds \((\mathrm{NH}_4)_2\mathrm{ReCl}_6\) and \(\mathrm{NH}_4\mathrm{ReO}_4\), no set of \(\Delta E_i\) was found for which condition (2) would be satisfied in the indicated interval of values of \(A\). This is apparently due to the presence of a periodic field of the ionic environment in these compounds.

Thus, the approximate theory of the fine structure of x-ray absorption spectra \({}^{(7)}\) proved applicable for calculating certain parameters not only of metallic rhenium, but also of the coordination compounds \(\mathrm{NH}_4\mathrm{HReCl}_4\) and \(\mathrm{RePy}_2\mathrm{Cl}_4\). This is apparently associated with the covalent character of the rhenium bonds in these compounds, as well as with the presence of \(\mathrm{Re—Re}\) bonds in \(\mathrm{NH}_4\mathrm{HReCl}_4\), whose structure is probably analogous to the structure of \((\mathrm{PyH})\mathrm{HReCl}_4\) \({}^{(13)}\).

In conclusion I express my gratitude to I. B. Borovsky for valuable advice, and also to V. G. Tronev, G. K. Babashkina, and A. S. Kotelnikova for providing preparations of rhenium compounds.

Received
10 IV 1962

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