Metal Halide Ammines. 1. Thermal Analyses, Calorimetry, and Infrared Spectra of Cadmium Halide (CI, Br, I) Derivatives

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1 Metal Halide Ammines. 1. Thermal Analyses, Calorimetry, and Infrared Spectra of Cadmium Halide (CI, Br, I) Derivatives K. C. PATIL AND E. A. SECCO Chemistry Departpent, St. Francis Xavier University, Antigonish, Nova Scotia Received May 26, 1971 The thermal analyses of cadmium halide hexammines reveal that decomposition occurs by loss of ammonia via the diammine and the monoammine. Calorimetric measurements along with related enthalpic values for the intermediate stable stages of the decomposition reaction are given. The infrared spectral data of the diammines and monoammines are presented with their assignments. Les analyses thermiques d'hexarnmines d'halogtnure de cadmium indiquent que la dtcomposition se fait par perte d'ammoniaque via la diammine ef la monoammine. Les mesures calorimttriques ainsi que les valeurs d'enthalpie correspondantes sont donntes pour les Btapes intermediaires stables. Les donntes relatives aux spectres infrarouges des diamines et monoamines sont prtsenttes avec leurs attributions. Canadian Journal of Chemistry, 49, 3831 (1971) Introduction The reaction between cadmium halides (Cl, Br, I) and ammonia in aqueous solution is known to yield ammoniates or ammines of varying compositions depending on the reaction conditions, such as solution concentration and temperature (1-3). Although the kinetics of formation of cadmium halide diammines have been reported (4-6), no study of the mode of decomposition of these ammines has appeared in the literature. The purpose of this paper is to report on the mode of decomposition, heat of decomposition, relative thermal stability, and infrared spectra of cadmium halide (Cl, Br, I), monoammines, diammines, and hexammines. Experimental All the halide diammines were prepared by the addition of liquor ammonia to saturated aqueous solutions (10.25 m) of the respective cadmium halide. The resulting precipitate was washed free from halide with distilled water, followed by washing with alcohol, and dried at room temperature in air. Dilute aqueous solutions (<0.25 m) of cadmium halide are reported to yield basic halides with ammonia (7). Cadmium halide hexammines were prepared by refluxing the anhydrous cadmium halide with anhydrous liquid ammonia for 34 h. The ammonia was Matheson compressed gas with a stated purity 99.99% grade. The apparatus consisted of a conical Pyrex flask containing a magnetic stirrer fitted to a condensor charged with an acetone - Dry Ice mixture (ca "C). The heating - curves recorded for differential thermal analysis, thermogravimetry, and calorimetry were obtained using the instrumentation described previously (8, 9). The infrared spectra of the compounds were re- corded on a Perkin-Elmer Model spectrophotometer using the standard pellet technique with KBr as the matrix material. Results and Discussion Thermogravimetry The results of thermogravimetry are given in Table 1. The proposed reactions for each compound are those which best fit the observed weight loss; the intermediate and final products of decomposition were further confirmed by infrared and correlated with DTA curves. The decomposition course of all the hexammines occurred via the following sequence [4] CdX2(,) -+ CdX,,,,, where (X = C1, Br, I). The hexammines are relatively unstable, decomposing to the diammines after a few days in storage at room temperature. The diammines prepared by aqueous precipitation show the corresponding steps 2,3, and 4 in their decomposition pattern. Dlferential Thermal Analysis The results of DTA measurements are presented in Table 2 and the discussion is given to accompany the TG results. The three endotherms corresponding to reaction eqs. 1 to 3 appeared for the bromide and iodide ammines; an additional endotherm due to

2 3832 CANADIAN JOURNAL OF CHEMISTRY. VOL. 49, 1971 TABLE 1. Thermogravimetric analyses % of total weight lost Step Temperature Compound number range* ("C) Observed Calculated Reaction *Onset and terminal temperatures. Compound TABLE 2. Differential thermal analyses Peak number* *All endotherms except those marked with a. the melting of the monoammine is observed in all compounds. The melting temperatures of the monoammines decrease in the order of C1 + Br -t I which is interpreted in terms of stronger covalency of the Cd-N bond in going from chloride to iodide ammines. The DTA curve of the chloride hexammine, Fig. 1, displays two endotherms attributed to steps 1 and 2 leading to the monoammine. The monoammine subsequently decomposed to the anhydrous chloride via two steps Peak temperature (OC) Reaction [1 [2 I Fusion CdBr2.NH3 [3 I Fusion CdBr, Fusion &I,.NH~ [3 I Fusion CdI, gravimetrically to correspond to the composition CdCl,.+NH,. The small exotherm observed at 370 "C is attributed to crystallization of CdCI,, confirmed by X-ray diffraction patterns of the residues immediately prior to the exothermic peak and immediately after this peak. The absence of a crystallization exotherm for CdBr, and CdI, is most likely due to the fact that these anhydrous halides result from a liquid phase decomposition reaction whereas anhydrous CdCI, results from a solid phase reaction. [3bl CdCl,.+NH,<,) + CdCI,(s) + +NH3(,) which accounts for the two endotherms at 330 and 360 "C, respectively. The residue after the 330 "C endotherm was determined thermo- Calorimetric Measurements The calorimetric data, along with the related enthalpy values for decomposition of the halide ammines including two halide hydrates for comparison purposes, are presented in Table 3

3 PATIL AND SECCO: METAL HALIDE AMMINES. I 3833 TABLE 3. Calorimetric data Heat of decompositionf AHfO (kcal/mol) AH of compound AH per mol of Literature decomposed NH, or H20 formed This Compound Reaction (kcal/mol) (kcal/mol) workf Value Reference *Includes AH(fusion) of CdCI2.NH3 which overlaps the decomposition reaction.?all determinations were done in duplicate; AHvalues accompanied by average deviations. $AHr" yalues ;f compounds used in our calculations are from ref. 3 below. The above AHr' values were calculated using Kopp's Rule approximat~on, 1.e. AC, e 0. FIG. 1. Differential thermal analysis heating curve A for CdC12.6NH3 and the cooling curve A' from 300 "C illustrating the reversibility of the process associated with the third endotherm. The heating curve shows the additional endotherms identified with reactions 3a and 36 along with the exotherm due to crystallization. according to the following equations showing the onset and terminal temperatures, "C Our AH," values for CdC12.NH3, CdC12.6NH3, and CdBr2-6NH3 are in excellent agreement with the literature whereas a noticeable disparity is evident in the AH," values for the other compounds. The column of values for AH per mole of NH, or H,O formed indicates an increasing trend, CdCl, + CdBr, + CdI,. This trend is consistent with a prediction by Chatt and co-workers (10) which suggested that the relative affinities of the halide ions for Cd2+ are in the direction C1 < Br < I. The order of Cd-halogen bond strength is C1 +- Br -t I where the difference is reported as 8-10 kcal/mol (1 I), thus a parallel increasing trend for the ammonia ligands is anticipated. However, in addition to the factors of ionic size and polarizability of the iigand in contributing to the stability of the metal halogen complexes (12), the role of lattice energy must also be recognized in crystalline compounds. [7] CdCI2.6NH3(,, ---+ Infrared Spectral Data CdCI2.2NH3(,) + 4NH3(,) The infrared spectra of cadmium halide hexammines (13, 14) and diammines (15, 16) "C [81 CdBr2.6NH3(,) already appear in the literature. In Table 4 the CdBr2~2NH3(,, + 4NH3,,, infrared absorption frequencies, with their

4 3834 CANADIAN JOURNAL OF CHEMISTRY. VOL. 49, 1971 Compound V(NH)~S V(NH)S TABLE 4. Infrared absorption band frequencies* Frequency (cm-i) assignment CdC12'2NH (s) 3 160(w) 3260(s) *vs = very strong; s = strong; m = medium; w = weak; b = broad; sp = sharp; sh = shoulder. assignments, are presented for the diammines, monoammines, and the deuterated analogues of the diammines over the cm-i region. The v,/vd ratios fall in the range for the NH, vibrations. The vibrational assignments are made on the basis of a simple 1 :1 model (i.e. Cd-NH,) where the normal modes of vibration are represented by those of a tetrahedral ZXY, molecule of C,, symmetry. Our data show a high degree of internal consistency for the various assignments in all ammines; the data are also in agreement with the existing literature values for the diammines. The symmetric stretching v(,,,, for the deuterated analogue is not discernible since its expected weak band at ca cm-' is masked by v, of GO, present in the atmosphere. Slight shifts of cm-' are noticeable for v(,,,, and p(,,,,= in going from CdCI,.NH, -t CdBr,.NH, -t CdI,.NH,. The observation of the 370 cm-' band in the monoammines confirms the v(,-,, assignment and is a step towards resolving the existing con- troversy (16, 17) regarding the assignment of v(,-,, and 6(,-,-,, in the hexammines. The bending mode 6(,-,-,, predicted to be <200 cm-' was not observed in the diammines since this frequency is beyond the limits of our instrument. The slight isotopic shift evident in v(,-,,, vh/v, = 1.06, suggests that this mode involves the entire NH, (or ND,) mass unit; such an assumption leads to a calculated v,/vd of G. G. URAZOV and A. K. KIRAKOSYAN. Zh. Neorg. Khim. 2, 1332 (1957). 2. A. K. KIRAKOSYAN. RUSS. J. Inorg. Chem. 5, 101 (1960). 3. A. K. KIRAKOSYAN. RUSS. J. Inorg. Chem. 5, 876 (1960). 4. M. M. PAVLYUCHENKO and T. I. TORGONSKAYA. C.A. 57: (Geterongeunye Khim. Reaktsii 184 (1961)). 5. M. M. PAVLYUCHENKO and G. A. LAZERKO. Zh. Fiz. Khim. 29, 1064 (1955). 6. G. A. LAZERKO and M. M. PAVLYUCHENKO. C.A. 51: 16177~. 7. A. K. KIRAKOSYAN. RUSS. J. Inorg. Chem. 7, 1327 (1962) K. SGVASTAVA and E. A. S~cco. Can. J. Chem. 45, 579 (1967).

5 PATIL AND SECCO: METAL HALIDE AMMINES. I P. RAMAMURTHY and E. A. S~cco. Can. J. Chem. 46, 3605 (1968). 10. S. AHRLAND, J. CHATT, and N. R. DAVIES. Q. Rev. 12, 265 (1958). 11. A. J. POE and M. S. VAIDYA. J. Chem. Soc (1961). 12. A. B. BLAKE and F. A. COTTON. Inorg. Chem. 3, 5 (1963). 13. J. M. TERRASSE, H. POULET, and J. P. MATHIEU. Spectrochim. Acta, 20, 305 (1964). 14. L. SACCONI, A. SABATINI, and P. GANS. Inorg. Chem. 3, 1772 (1964). 15. G. M. BARROW, R. H. KRUGER, and F. BASOLO. J. Inorg. Nucl. Chem. 2, 340 (1956). 16. D. M. ADAMS. Metal-ligand and related vibrations. Edward Arnold Ltd., London Chap. 6. p K. NAKAMOTO. Infrared spectra of inorganic and coordination compounds. John Wiley and Sons, New York p W. BILTZ and C. MAU. Z. Anorg. Chem. 148, 170 (1925). 19. E. TASSILLY. Ann. Chirn. Phys. 17, 38 (1899). 20. N. B. S. Circular No. 500 (1951).