An Undergraduate Project Involving an Unknown Metal Complex Graeme H. Searlei and Graham S. Bull University of Adelaide, Adelaide, South Australia. Australia Donald A. House University of Canterbury, Christchurch, New Zealand We have recently described several project experiments for undergraduate students involving the complete characterization of unknown metal complexes (1-3). Either the complexes are synthesized by students, in which case the comnonents are inferred from the svnthesis. ~.or the com" pounds are provided, with the components specified. From the results of analvses for all the comnonents in a narticular complex, and of chemical reactions and physical'measurements on the complex, students have to determine the formulation and molecular structure. T h e compounds used in such experiments should he readily obtainable, and for challenging projects in which a number of analyses and measurements are necessary to solve the structures the complexes should have four or more constituents. T h e diversities of structures and properties of com~lexesreauired that nroiects he tailored to the individual compoun&, so that k p r o v i d e students with outline directions for carrvine out snecified determinations and measurements. For the students' exoerience to he heuristic it is desirable to provide different un&owns projects around a class, and we now describe a further project based on the well-known chromium(II1) complex Reinecke's salt, (NH4)Cr(NH3)2(NCS)4] - X H ~ OWith . ~ five components, this complex allows a variety of analyses and measurements, and this-article describes 10 such experiments. Not all of these individual experiments are necessary to completely characterize the complex, hut the purpose of the present paper is to demonstrate the variety of separate measurements possible within an unknown compound project, rather than t o present a compact and precise experiment. We prescribe the project for our advanced students (final year BSc) with all 10 experiments included, and with students collaborating in pairs this full project requires two laboratory days. ~~
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Synthesis of the Complex, and Its Properties Caution: Chromates are carcinogenic (at long exposure), formaldehyde is a skin and respiratory irritant, hydrogen peroxide is a strong oxidant and is corrosive to the skin, and concentrated nitric acid is oxidizing and corrosive. These reagents should therefore he handled with appropriate care. The complex can he readily synthesized by fusing ammonium thiocyanate with ammonium dichromate (4-8), and it is also available commercially from many suppliers. The crude compound should be recrystallized from hot water at 65-70 "C. Solubility is about 30 g per 100 mL. After filtering the hot solution, the filtrate should be cooled slowly. The resulting pink-red crystals are filtered off, washed with a little ice-water only, and air-dried to constant weight. Solubility of the compound in ethanol and acetone precludes the use of these in the washing and drying process. The principal byproduct removed on the hot filtering is the less soluble guanidinium salt of the "reineckate" anion, known as Morland's salt. Many of the properties of Reinecke's salt are compiled in Gmelin (9).The compound is moderately soluble in water, in which it is stable up to about65 'C, hut above 70 'C it decomposesrapidly with the formation of a clear blue color and HCN. It is light sensitive, so should he handled in subdued light and stored in the dark (10). It
forms sparingly soluble salts with primary and secondary mines, amino acids and complex cations, and has frequently been of value as a oreeioitant well . . for these since the salts usuallv, crvstallize , ~~. ~ ill14).Thsuoluhilityin water indicates thecompound to branrlrcrrolyre and this is alao apparent from the conductance and ion-exchange experiments below. Reinecke's salt has long heen formulated and assumed as the monohydrate, hut our work in developing this student experiment shows this hydrate content to he questionable. The product recrys: tallized from water contains two forms of crystals, as observed by Reinecke and subsequentworkers (9):shiny, pale scarlet-red plates, and more intensely colored red cuhes or rhombic dodecahedra. In our recrystallized materials these seemed to be in roughly equal proportions, and we found the plates to be small ill-defined aggregates. AnX-ray crystal structureanalysis on the cubes (15)gave the stoichiometry as ~~Q~~-(NH~)[C~(NH~)Z(NCS)~].(~/~)H~O, and the structure also showed that the thiocyanates are N-handed to ehromium and that the coordinated ammonia are trans. Werner and Richter ( 9 ) suggested that the cubes and the plates were different hvdrates. slthoueh their sueeested formulations as anhvdrous and "monohvdrate. resoeetivelv. are at variance with the ervstal structure r e d t and alao wlrh our prewnt work Our wntrr analy.~, of several samples of the t~mpoundhave grven thr hydrate ronrenr as abuut 0.3 mol proportion, which suggests that the plates may be anhyThe commercial yrducLr are mostly listed with the formulation 1-hydrate, and minimum assays are typically given as 9 5 9fi% (ex Crl. 'l'hlr imnlies wriabilitv in commsirion. and this is must likelv to he in terms of variable Later, perhaps free as well as hydrati, raneine In our recrvstallizations. air" "uo . to about 2 mol .orooortion. . drying to constant weight has been our criterion far dryness, when any unbound water should he minimized. Information Provided Samples of recrystallized compound, about 6 g, are provided to students (in pairs). Because of its photosensitivity, students are instructed to handle the compound in subdued light as much as po~sihle.~ The components are specified as Cr, NCS-, NH3, and NHlt and hydrate water, and the formula weight is stated to be in the range 300-500. From this information, some deductions can be made about the formulationof the complex. From students' knowledge of the chemistry of chromium in its various oxidation states, Cr(II1) should he deduced as the most likely state in this complex with thiocyanate, ammonia, and water as possible ligands. Cr(I1) would he the only other possibility with these types of ligands, although this state is strongly reducing. The presence of NHnt suggests that the complex is probably an anion. The total charges of the Cr"+ and NHnt components must be balanced bv the charees of the NCS- anions. so that the minimum ratio NCS-: Cr should be :I riftheoridarim state wereCr'.,, end i t follows from the formula wright range given that the cumplex must be mononuclear. Author to whom correspondence should be addressed. Thecomplex wasdiscovered by Morland in 1861 and wascharacterized by Reinecke in 1863. In room light the compound does not undergo any obvious change, but in strong sunlight it changes to violet over several hours and becomes moist. The photolytic reaction is aquation with NCSrelease in the primary step (lo),, Volume 66
Number 7
July 1989
605
The formula weight is required far calculating the results of the ion-exchange, conductance, and magnetic susceptibility measurements, and since the formula weight is derived from the collective analysesfor all the components, it is advantageous that the analyses be completed first. Otherwise, or in the event of an unsuccessful or uncertain analysis, an approximate formula weight would have to he used, and the results of these other measurements adjusted subsequently if necessary.
Analyses for the Components Analysis for Chromium Chromium is determined spectrophotometrically after oxidation to chromate by hydrogen peroxide in approximately 0.1 M sodium hydroxide solution (16, 17). A standard solution of the complex is prepared by dissolving an accurately weighed amount (about 0.15 g) in water and making the solution up to 250 mL in a volumetric flask. Duplicate 5-mL aliquots of this solution are then transferred to 50mL conical flasks, and to each flask NaOH (2 pellets), water (about 10 mL), and Hz02 (6 drops of 30%solution) are added. The flasks are heated over asteam bath for 30-40 min until the excess peroxide has been decomposed and clear yellow solutions result. After cooling, the solutions are transferred quantitatively to 50-mL volumetric flasks and diluted to volume. The absorbances are then measured a t the maximum 372 nm. (Absorbance is about 0.8.) Chromate standard solutions (duplicates) are prepared from K2Cr207(shout 0.30 g of AR accurately weighed) and NaOH (2 g), and made up to 500 mL. Aliauots are then diluted 25 times (10-mL aliouots ulus 1 e NaOH ma& up to 250 ml.), and the nbaorbanrcs arc mcawred at>:) nm. (Ahsorhance is ahuut 0.79.) The chromium roncrnrrnrion in the sdution of the unknown i~ cslrulated from the propurtimslity of chromate concentration to absorbance, with allowances made for the dilution^.^ Alternatively, the chromium can he determined by atomic ahsorption at 357.9 nm using an air-acetylene flame, working range 28 ppm, with solution standards prepared from KzCrz07 with added H2SO4. TO prepare the complex solution an approximate formula weight has t o be used.
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Analysis for Total Ammonia The ammonia is determined hy distillation from an alkaline solution of the complex. The distillate is absorbed in H3B03 solution, and the resulting H2B03- is titrated with standard HC1(18,19). The apparatus is basically a Kjeldahl distillation setup consisting of a 100-mL round-hottom flask with ground glass joint (standard taper 14/20), a custom-built plain distillation column 10 em long (ST 19/22 joint) with combined splash head (ST 14/20), Liebig condenser (ST 14/20), and plain Long bend receiver adapter (ST 141 20). - ~ , A weighed amount of complex (about 0.10 g) is introduced into the flask, and 40 mL of 5% NaOH and antibumping granules are added. The apparatus is assembled with the receiver tube dipping into about 50 mL of 2% H3B03 in a squat 100-mL measuring cylinder. The mixture is distilled with a steady flame for 8 min, with care taken to avoid sucking back in the later stages. The H3B03 solution with washines of the is transferred to a conical flask. and (toeether . condenser and receiwrl is titrsted with standard 0.04 M HCI using hromorresol green indicator. (Calculated tlter for I-hydrate is 21.2 mL.) I t should he realized that this analysis gives total ammonia, ingave cluding that from NHlf. A determination on [CO(NH~)SCI]CIZ 98.9% recovery of the calculated ammonia.
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Analysis for Thiocyanate The following procedureis the reverst. ofthe usual Vulhard proredure ,201. T o weighed amounts of the complex (about U.17 g,duplirates1 in conical flasks. water and NaOH I10 mL of 1 M I are added. The solutions are heated just to boiling to decompose the complex with the formation of green c r ( 0 H ) ~After . cooling to room temperature, H N 0 8 is added (10 mL of 2 M), followed by Fe(II1) indi~ator.~ The liberated NCS- is then titrated with standard 0.05 M AgN03 solution to the disappearance of the deep red color of [Fe(NCS)I2+. At the endpoint, which is sharp, the turbid solution containing the precipitetks of AgNCS and some undissolved Cr(0H)z appears pale gray-green. (Calculated titer far I-hydrate is 38.4 mL.)
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Journal of Chemical Education
Analysis for Ammonium Ion This analysis is based on the reaction of ammonium ion with formaldehyde (21,22):
A 2% solution of formaldehyde is prepared by adding 6 mL of AR 36% solution to 100 mL water. The solution is neutralized to phenolphthalein by the addition of a few drops of NaOH solution. Weighed amounts of compound (about 0.4 g, duplicates) are transferred to 250-mL beakers. T o each, formaldehyde is added (20 mL of the prepared solution), and the solution is left to stand for 5 min. The volume is adjusted to about 80 mL, and the liberated acid is titrated potentiometrically with standard 0.1 M NaOH using glass and calomel electrodes in a pH titration. (Calculated titer for 1hydrate is 11.3 mL.) Analysis for Hydrate Water The hydrate water content is determined from the weight loss of a weighed sample after heating at 103 'C for 3 h. The Lightly ground sample (about 0.8 g) is spread on a small weighed Petri dish. Prior to each weiehiue .. the dish (olus samole) should he cooled in a desiccator for 8 min, and the weighing rl;ould then he made rapidly as the sample gains weight in the atmosphere ( f eight lvsr fuund O.Ul4 g. 1.87.) As a supplementary assignment, a plot of percent weight loss versus time could be determined from weighing8 made at hourly (andsubsequently longer) intervals, from which the two processes of dehydration and decomposition should be evident. The results for the above analyses should he reported in the following ways:
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(1) % weightlweight. These figures calculated for the 1-hydrate compound are Cr 14.7, total (NH3 NHlt) 14.4, NCS- 65.6, NHlt 5.1, and H1O 5.1, hut for lower hydration the calculated fxures for Cr, NHs, NCS-, and NHlt are higher. Our own results have been Cr 14.5 by spectrophotometry and 14.7 by atomic absorption, total (NH3 NHlf) 15.2, NCS- 66.7, NHlt 5.0, and Hz0 1.8. (2) Mdes of nmponent (except water) per gram of compound. Fmm there values the mole ratios ofthe components should be calculated, and the emoiriral formula e m then be deduced based on Cr = 1 by rounding the ratios to integral values. An accurate value for the formula weight of the anhydrous complex can then he obtained. This can he used to calculate the mole proportion of water, and the actual formula weight can then he ohtained by adding this water contribution. This formula weight is required for calculating the results of the other measurements below.
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Other Measurements Displacement of H+ from Cation-Exchange Resin Duplicate columns of the strong-acid cation-exchange resin Dowex50W-X2,50-loomesh, H t form (1.2 diam. X 5 cmof wet hed resin) are set up. Complete conversion to H I form is essential, and is effected by using 6 M HCI, then washing the column with water until the effluent is approximately neutral (pH > 4). A solution containing a weighed amount of complex (about 0.4 g) is passed slowly through each column, and the effluent is collected in a beaker from the time of application. The column is then washed until the H+ is essentially all removed (pH > 3, tested with pH strips), and the total effluent is titrated patentiometrically with standard 0.1 M NaOH using glass and calomel electrodes in a pH titration. (Calculated titer for 1-hydrate is 11.3 mL.) Students should deduce precisely the information that this determination gives about the constitution of the complex. Displacement of CI- by Anion Exchange A column of the strongly basic anion-exchange resin Amherlite CG-400,100-200 mesh, CI- form (1.2 diam. X 6 cm of wet bed resin) The extinction coefficient for chromate is not required. om it can be calculated a s about 4840 mol-' am3 cm-'. Use of this extinction coefficientvalue would ooviale preparation of the standard cnromate solution. The indicator is 10% Fe(N03)3.9H20acidified with HNO., used in 1 mL portions. The solution should be acidified only when cool, or NCS- may decompose and the [Fe(NCS)I2+indicatorcolor may be bleached (20).
is p r e ~ a r e dComplete .~ conversion to Cl- form is effected hy the passage of 1M NazC03, then 3 M HCI. The column is washed with water7 until the effluent is free of CIF (tested with 0.05 M AgN03 acidified with HNOa in small test tuhes). A solution of the complex in water (ahout 0.25 g in 80 mL) is passed slowly through the column, and the effluent is collected in a beaker from the time of application. The column is washed with water until the C1- is essentially all removed (tested as above). To the total effluent HN03 isadded (10mL of 2 M HN03, and the C1is titrated potentiometricslly with standard 0.05 M AgN03(calomel electrode, connected to the solution via a KN08 salt bridge, and silver electrode). (Calculated titer for 1-hydrate is 14.1 mL.) Students should deduce the information that this determination gives about the constitution of the complex. The complex cannot he removed from the resin hy conventional ion exchange, since the complex has a very high affinity for the quaternary ammonium groups on the resin and it effectively precipitates within the resin. Reemeration of the resin can he effected hv decomposing the complexby passage of 2 M NaOH at 50 'C, then M HCI, m three cycles. The column is washed with water until the effluent is free of Cl-. Conductance in Water The molar conductance in water at infinite dilution Am0 is determined by measuring the conductances A, of several solutions of different concentrations at 25'C and extrapolating to zero concentration (1, 2). From this value the number of charge units per formula weight is deduced, given that each charge unit contributes about 70 ohm-' cm2mol-' to Am0 s t 25 "C. Magnetic Susceptibility The magnetic susceptibility is determined and pen calculated. The numher of unpaired electrons per formula weight and hence the oxidation state of the metal are deduced. The diamaenetie correction can he taken as 200 X 10-6 cgs units. Visible Spectrum The visible spectrum in water should he measured (using ahaut 0.1 g in 50 mL water). Crystal field bands are at 392 nm ( 6 93 mol-I dm3em-') and 520 nm ( 6 106 mol-' dm3 em-') (10). Concentrated HN03(4 drops) is added to the solution in the cell, and the spectrum is then re-recorded. From this, students should deduce whether the complex is labile or inert, and relate this to the ehemistryof chromium in its various possible oxidation states. The lowest energy absorption hands for the parent hegaligand complexes are: [Cr(NHa)#+ 464 nm (e 30 mol-I dm3 cm-I); [Cr(NCS)6]3-(N-bonded)565 nm (c 160 mol-'dm3 cm-'). From this data, and using Jorgensen's "Rule of Average Environment" (23), the energy of the first band of the "unknown" mixed complex can he ~alculated.~ From comparison with the observed spectrum,themore likely mode of thiocyanate coordination N-bonded or S-bonded should he decided. Students rhould demonstrate how all the experimental result.: can bc used to obtarn or ~onftrmthe formulatmn uf the complex. Results and Discussion From the moles of the components (except water) per gram of compound, the mole ratios should he calculated as multiples of the chromium value, then rounded (allowing for experimental errors) t o give Cr:total ( N H 3 NH4$):NCS-:NHa+ = 1:3:4:1. T h e correct empirical formula, and hence the formula weight (336.4 for the anhydrous complex) should thus come from the collective results of the analyses. Even with errors of 5% in any of the analyses, i.e., up to 10% error in the mole ratio of any pair of components, the correct formula will he obtained. The water proportion is determined directly from the weight loss on heating a t 100 "C. A weight loss versus time plot showed that the water is loosely bound and is removed readily a t 100 OC (by 3 h), and that this is followed by a further gradual weight loss (days) due to decomposition and which is accomoanied bv a eradual color chance " toward purple. Although these processes overlap, the water content can be estimated hv this orocedure to within f0.3% wlw or *0.1 mol proportion. The water analysis results for three different samples recrystallized by us and for a commercialsample (British Drug
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Houses, used as supplied) were similar and gave the composition as 0.3 mol proportion hydrate. If the cube form of the complex is (2/3):hydrate, this overall water result for our samples suggests that the plate form may be anydrous. Somewhat variable hydrate content might be expected depending on the proportions of the two hydrates in different samples. A claim of incomplete hydrate water removal by Kafhammer and Eck (9) was probably based on an incorrect assumption of a l-hydrate composition and failure to recognize the decomposition. Of the other analyses only the ammonia determination had an accuracy sufficient to give an indication of the water content; our samples gave ammonia results corresponding to a lower water proportion than l-hydrate. A dehydrated and partly decomposed sample gave a similar ammonia result to that from the unheated compound, suggesting that the thermal decomposition involves ammonia loss. T h e possible variation in the hydrate water content does not diminish the success of the experiment since the water proportion does not hear on the constitution of the complex species itself, and in any case the other analyses and measurements (apart from ammonia) are not sensitive to this low proportion of water (1.8%). Other deductions from the analyses are that ammonium is t h e m l y cation, thecharge halance requires the chromium as Cr(ITI), and that the coordination numher is six, which in the usual for Cr(Il1). On all these grounds, the molecular lormuThe la can be proposed as (NH,)ICr(NH2)2(NCS)41.xH20. water is hydrate rather t h a n coordinated water, since its removal occurs under moderate heating and results in no obvious color chanee. " T h e other measurements provide confirmation of this deduced molecular structure. Should any of the analvses he unsuccessful or uncertain, these furtger data (calculated using a n approximate formula weight) should assist completion of the empirical formula. In the ion-exchange experiments, the colored species passes through the cation-exchange resin hut is retained on the anion-exchance resin, which shows that this colored species is the complix anion. Each of the ion-exchange experiments gives fundamentally the moles of charge of the particular ion Der mass of samole taken. since each mole of H+ (or C1-) displaced from the cation-exchange resin (or anionexchange resin) must correspond to one unit of charge on the cation (or the anion) of the complex compound. These figures from the two experiments should clearly be the same because of charge balance. T h e inverses of these figures give the mass of compound per mole of cation or anion charge, and in general the (gram) formula weight is a multiple of this. I n the present compound, the determined mass per mole of ionic charge is in fact the actual (gram) formula weight since i t is within the formula weight range in the information provided. This also shows t h a t i h e compound is a 1:l electrolyte, since a 2:1 ur 2 2 elertrolyte would give the actual formula weieht to he double the abwe. and outside the range quoted. f h e s e deductions provide acheck on the. formulation of the compound as deduced from the analyses. Typical formula weights thus obtained have been 349 from the cation-exchange experiment, and 357 from anion-exchange. Alternativelv. ~ .the. standard resin Amberlite IRA-400.CI- form. can be used, but with more ditliculty because the complei applied a; a less tight oand on this coarser beaded resin. Column dimensions should be 1.2 diam. X 13 cm. Aoout 0.23 g of compound should be used, and the solution should be applied slowly. Any channelling or uneven application can be corrected by stirring the top part of the resin with a thin stainless steel rod. Removal of COPbubbles may require washing the resin from the column and reoackina it. 'The energy of the first band of a d3 octahedral complex is the crystal field splitling A. Energles in wavenumbers are ootazned by: wavenumbers cm-' = 1O7lwavelengthin nanometers. ~~
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Volume 66
Number 7 July 1989
807
If it is appreciated that ammonium is the only cation, the moles of cation charge per mass of sample used as obtained from the cation-exchanee exneriment ~ i v e sthe moles of NH4+per mass of cornpiex. Tius the % i / w of NH4+ can be calculated.for com~arisonwith the value from the formaldehyde method. The conductance measurements give Amo = 105-115 ohm-' cm2mol-', which indicates thatthe compound is a 1:l electrolyte. The magnetic susce~tibi~itv measurement gives f i e f [ = 3.9 BM, whieh shows that there are three unpaired electrons and therefore that the chromium is Cr(II1). The visible spectrum is unchanged on addition of acid, so that this cvmplex is inert, consistent with Cr(1II). By the "Rule of ~ v e r a g eEnvironment", the energy of the first d-d band for [Cr(NH3)2(NCS)41- is calculated as 19000 em-' or 526 nm, a i d since this agrees reasonably with the measured spectrum (520 nm) it is likely that the thiocyanates are Nbonded in the comnlex. Electronic and infrared snectra do not provide reliable means to demonstrate the geometric dispositions of the ligands in this complex (24). There is educational merit in providing students with elemental analvsis data (after their own analvtical determinations have been completed), to provide pradice in deducing the mole ratios of the components. The figures calculated for the anyhydrous complex are C 14.3, H 3.0, N 29.1, S 38.1%. From these, students should first deduce the atomic ratios as C:H:N:S = 1:2.5:1.75:1 (standardized to the lowest value C = l),or 4:10:7:4 in simplest integral terms. Allowing for the stoichiometries within NCS- and NH?. ". the comoonent ratios follow as NCS-:NHs:additional H = 4:3:1, where the additional H arises from NHa+. Students should compare their own experimental analysis results with these ratios and account for any divergences. Extended project experiments of this type provide a research-type experience, requiring students to adapt their previous experience with "standard" procedures or known
608
Journal of Chemical Education
compounds to a new and unknown situation. Students are able to appreciate how information obtained from a variety of techniques has to be integrated in the elucidation ofstructures of new inorganic compounds. This particular project provides experience with some less usual analytical procedures, and the usefulness of ion-exchange procedures for obtaining quantitative information about electrolyte charge and formula weight is also demonstrated. Acknowledgment Experimental assistance from Filomena Occhiodoro is gratefully acknowledged.
Literature Cited 1. Curtis,N.F.:Hsy,R.W.:House,D.A.;Searle,G.H. J.Chem.Edue.1986.63.699-901. 2. Bull. G. S.;Searle, G. H. J,Chom. E d u c 1986,63,90%904. 3. Tomkin8,I.B.;Searle,G.H.Edue.inChem.1987.24,14F146. 4. Dakin. H.D.Olg, Synth. 1935.15.74. 5. Palmer, W. G. Expcrimcnial Imwmic Chemrslry:Cambridge Uoiv: Cambridge, 1954:p 403. 6, khlessinger,G.G.InorgonicLoboraforyPr~r~~~mfiii;ChemicalPuhliahingCo.:Ncv York. 1962;PP 263-264. 7. Brauer, G.. Ed. Hondbook of Prrpomtive Inorganic Chemistry, 2nd ed.: Academic: New York, 1965;pp 13761377. 8. Ma.,. G.:. Rockett. B. W . Proctieo1 1narsonic Chemist?":von Nostrand Reinhold: London. 1972: p 56. 9. Gmelin, L. Hondbvch der anorgonischo Chemie, Smtem no. 52. Chromium, Supplement C: Verlag Chemie: Weinheim, 1965:pp 245-246. 10. Wegner, E. E.; Adamson, A. W . J. Am. Chem. Soc. 1966.88.394. 11. Ref% pp 245-252. 12. W n b r d , W. C.;Bauer, H. F.; Bailar, J. C. J. Am. Chem. Sm. 1960.82.2992. 13. Chatten, L. G.: Napper, A. C. Conod. J.Phorm. Sci. 1966,1,30. 1966.65,SWld. 14. Guasinvv.l.K.;Bapbanly,I.L.Chem.Abatr. 15. Takeuehi, Y.; Ssito Y. Bull. Chem. Sac. Jpn. 1957,30,319-325. 16. Haupt.G. W. J. Rea. N o 1 Bur. Stds.1952.48,414. 17. Poltmus, C.; King, E. L. J. Phy8. Chem. 1955,59,1208. 1s. voge1. A. I. A Tertook of Quonfiloli". 1norgonir Anolysia Ineluding Elemontory Instrumenlo1Anolyaia,3rd ed.;longman.: London. 1961;p 257. 19. House. D. A. Acta Cham. Scond. 1912.26,2W. 20. Ref 18,p 264. 21. ReL5, P 242. 22. Ref.8, p IS. 23. Sutton, D. Ekefronie Spectra of Tmnsilion Meld Complexes: MeGrau-Hill: Iandon, 1966,p 162. 24. N0rbury.A.H. Adu.Inorg. Chem.Rodimharn. 1975,17,231.
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