R. Stuart Tobias University of Minnesota Minneapolis 55455
I
Raman Spectroscopy -in h0rgkiC Chemistry Applications
I n the past few years, the most extensive uses of Raman spectroscopy have been in the study of inorganic system, while the technique has beenused rarely by organic chemists. Infrared spectroscopy has served the organic chemist well, since his compounds are often liquids or are soluble in solvents like CC14 and CS2 which have very simple infrared spectra. I n addition, most of the vibrations of organic molecules which are of interest have frequencies higher than 650 cm-', and relatively inexpensive spectrometers with rock salt prisms may be used to record the spectra. The situation is not nearly so favorable for the inorganic chemist. Water is an excellent solvent for many inorganic compounds, and while i t has a simple molecular structure the very large changes in the dipole moment accompanying the vibrations of the molecule lead to extremely intense absorption in the infrared. This characteristic rules out water as a suitable solvent for general infared spectroscopy. On the other hand, the polariaability of the water molecule changes little during its vibrations. This leads to very weak Raman scattering and makes water a particularly good solvent for Raman spectroscopy. Most metal-ligand vibrations occur with frequencies lower than 650 cm-', and a very well equipped infrared laboratory is required if such vibrations are to be studied. I n contrast, Raman spectra generally can be obtained a t 200 em-' as easily as a t 2000 cm-'. I n addition, simple techniques have been developed in recent years for the study of powdered solids as well as liquids and solutions. These techniques are beginning to rival the mull and alkali halide disc methods of infrared spectroscopy. Often it is very useful to he able to study the same compound in the crystalline state where the structure may be known from x-ray studies and also in aqueous solution. Many labile complex ions undergo structural changes upon dissolution, and Raman spectroscopy provides a powerful tool for determining whether or not the ion has the same structure in the crystal and the solution. I n the discussion below, some specific examples of the uses of Raman spectroscopy in the study of inorganic systems will be considered. For convenience, the applications of Raman spectroscopy can be broken down into two groups: (a) those involving primarily the measurement of Raman frequencies and (b) those Editor's Note Part I of this paper, discussing the essential theory of Raman Spectrosea~,appeared in the January issue of mls JOURNAL (43, 2, 1966). The numbers of figures and literature cited in this Dawr follow eonseeutivelv those in Part I.
70
/
Journal o f Chemical Education
involving primarily the measurement of the intensity of the Raman scattering. Each of these will he discussed separately. Applicalions Depending Upon Frequency Measurements
Even though infrared spectroscopy has been used much more extensively than Raman in recent years, it has always been recognized that it is extremely desirable to have the Raman as well as the infrared spectrum of a compound in any thorough study of the structure and dynamics of the molecule. This is because, as we noted in Part I earlier, only those vibrations which cause the molecular dipole moment to change give rise to absorption of infrared radiation, while only those which cause the molecular polarisability to vary lead to Raman scattering. We shall not be concerned here with these standard applications of Raman spectroscopy to inorganic system, since reviews of them have appeared in recent years (18, 19). Rather we shall consider more unusual uses and applications to systems where infrared spectroscopy is unsuitable. Structure o f Complexes in Aqueous Solution
Water is almost completely opaque to infrared radiation, therefore it is very difficult to study the structure of species in aqueous solution by infrared spectroscopy. Although chemists have devoted a great deal of time to the study of the stoichiometry of "complexes" in aqueous solution,' very little direct experimental evidence bearing on the structure of the species formed has been available. I n the cases where such information is available, i t generally has been for transition metal ions and was obtained from visible-ultraviolet spectra interpreted by ligand field theory. Many labile complexes which exist in aqueous solution cannot be obtained in the crystalline state, so solid-state information is not always helpful. Information on the structure of solutes is especially necessary for an understanding of the mechanisms of their reactions in solution. One type of ion which has been studied extensively in aqueous solution is that formed by aquation of organometallic halides ($0). The appearance of a The material for this article is taken in part from lectures at the Summer Institute for College Tachem in Advanced Inorganic Chemistry at the University of Minnesota, June 1965. This conference was made possible by a. grant from the National Science Foundation which has also supported some of the author's research in this ares, grants NSF GP-653 and GP-5022. For example see S I L L ~ N L., G., AND MARTELL, A. E., ''St* bility Constants of Metal-Ian Complexes," Special Publication No. 17, The Chemical Society, London, 1964.
(26) and (CHs)8nClz (?25). Spectra for the two solutions are illustrated in Fignre 9 for thc region where thr metal-carbon skeletal vibrations are expected to appear. 'rhe studies on ions like [(CH,),TI]+ indicate that, in thr absence of covalent cation-watcr bonding, the vibrations of the organa-metallic moiety may be treated in terms of a 3 atom model to a good approximation. Thr methyl groups are taken as single masses. For :L linear (CH3)&I skeleton, the polariaability should change as shown in Figure 10 for CO*, while if thc (:-M-C skeleton is bent appreciably the polarizability should change as shown in Figure 11 for SOz (as was discussed in Part I). For (CH& Sn(1V) there is but one Raman line with appreciable intensity strongly suggesting the linear structure, while there are four lines with (CH&Ge(IV) indicating an angular configuration. Three of these arise from the expected skeletal vibrations of the type shown in Figure 11, while the fourth is caused by stretching of rather covalent G-0 bonds. Apparently even in strongly acid solutions, the dimethylgermanium halides are hydrolyzed to (CH&Ge(OH)%, while the analogous tin compounds give the dipositive aquocation [(CHa)2Sn(OH2).]+2.
Many Raman investigations have been made of the tcndenq of different ligands to displace water molrcr~lcsfwm the first coordination sphere of metal ions i n solution. Frequeutly, rather symmetrical ligands lilw pcrchlorate, sulfate, or nitrate are used as probes. Coordination of these anions to the metal ion results in a lowering of the anion symmetry and a more complex Raman spectrum for the anion vibrations. Such studies confirm the expectation that perchlorate ion has very little tendency to coordinate to metal ions in aqueous solutions ($7, $8). On the other hand, similar studies show that nitrate (28-90) ions frequently and sulfate (50) ions in a t least one case are bound in the first coordination sphere of metal ions. Figure 13 shows a Raman spectrum of In(NO& solution which exhibits lines due to both free and complexed nitrate ion. As noted above, if the metal-oxygen bonds in aqnocomplexes involve significant electron sharing, t.hr
Syrnrnetricai Stretching,v,
Bending ,v,
Antsymmetrical Stretching,l& Figure 11. Polarirobility changes during the vibratiam of ruifur dioxide leioggeratedl.
72 / Journal of Chernicol Education
Figure 12. Roman spectrum of saturated lnlNOa)s together with the spectrum of 1 M NaNOs for comporiron. The odditionol liner in the InINO& spectrum are due to coordinated nitrote.
(86) and (CHJzSnCI2 (86). Spectra for the two solutions are illustrated in Figure 9 for the region where the metal-carbon skeletal vibrations are expect,ed to appear. The studies on ions like [(CH3)ZTL]+indicate that, in thc absence of covalent catiou-wat.er bonding, thevibrations of the organo-metallic moiety may be treated in terms of a 3 atom model to a good approximation. Thc methyl groups are taken as single masses. For :L linear (CH3)&1 skeleton, the polariaability should change as shown in Figure 10 for CO*, while if thc C-M-C skeleton is bent appreciably the polarizability should change as shown in Figure 11 for SOz (as was discussed in Part I). For (CH& Sn(1V) there is but one Raman line with appreciable intensity strongly suggesting the linear structure, while there are four lines with (CH&Ge(IV) indicating an angular configuration. Three of thesc arise from the expected skeletal vibrations of the type shown in Figure 11, while the fourth is caused by stretching of rather covalent Ge--0 bonds. Apparently even in strongly acid solutions, the dimethylgermanium halides are hydrolyzed to (CH3)2Ge(OH)2, while the analogous tin compounds give the dipositive aquocation [(CH&Sn(OH2).If2.
Many Raman investigations have been made of the t,ondcncy of different ligands to displace water mole(:ulrs from the first coordination sphere of metal ions i l l solution. Frequently, rather syn~metricalligands lilit. pcrchloratc, sulfate, or nitrate are used as probes. Coordination of these anions to the metal ion results in a lowering of the anion symmetry and a more complex Raman spectrum for the anion vibrations. Such studies confirm the expectation that perchlorate ion has very little tendency to coordinate to metal ions in aqueous solutions (27, 28). On the other hand, similar studies show that nitrate (28-30) ions frequently and sulfate (SO) ions in a t least one case are bound in the first coordination sphere of metal ions. Figure 12 shows a Raman spectrum of In(l\TO& solution which exhibits lines due to both free and complexed nitrate ion. As noted above, if the metal-oxygen bonds in aqnommplexes involve significant electron sharing, tlir
Symmetrical Stretching,~,
Bending .v2
A - WAVENUMBERS Figure 11.
Polarirobility changes during t h e vibrations of sulfur dioxide
lexoggeratedb
72 / Journal of Chemical Educotion
Figure 12. Roman spectrum of rotvroted lnlNOsla together with t h e qectrurn of 1 M NoNOs for comparison. The additional liner in the In(NOdt spectrum are due to coordinated nitrate.
iutensity of the Raman lines caused by metal-watcr oxygen bond stretching should be appreciable. Such interactions have been reported for R!Ig+2,C U + ~Znf2, , IIg+%, Ga+3,In+3,and Tl+3(31,82). It has been known for many years that halide ions often form thermodynamically stable complexes with metal ions in aqueous solution. I n several cases it has been possible to deduce the structure of these complexes from their Raman ~ p e c t r a . ~Thus while the complexes [SnC13]- and [SnBr8]- formed from an ion with the d'Os2 electronic configuration, Sn+2,might be expected to have a trigonal planar structure to minimize chloride ion repulsions, the spectra indicate that the ions are pyramidal (58). This supports other evidence that tin(I1) behaves like a nonspherical ion with respect to the packing of ligands about i t (34). I n these particular experiments, the anionic tin complex was extracted from the aqueous solution into ether to obtain better spectra. A similar technique has been used by Creighton and Lippincott to obtain spectra for the linear species [CuCI,]- and [CuBrz]- (35). The spectra of mercury(I1) iodide solutions indicate appreciable association via single iodide bridges with t,he formation of angular [Hg-I-Hg]C3 as well as [HgI] + (probably [I-Hg-OH2]+). Bromide solntions contain only [HgBr]+, while both [HgCl]+ and linear HgCL are reported for chloride solutions (36). Many simple tetrahedral and octahedral halide complexes have been examined recently in aqueous solution, and some of these are listed in Tahles 1 and 2.3 The spectra of very concentrated aqueous solutions of Table 1. Metal-Ligand Bond Stretching Force Constants (mdynes-A-') for Some lsoelectronic Molecules and Ions Obtained from Roman Spectra
'There is same question about the assignment of o tetrahdral structure to this ion in aqueous solution; see bhe discussion C. O., and SPIRO, T. G., Imrg. Chem., iu the text and QUICKBIL~ 5, 2232 (1966). a The C-31-C skeletons of these are all linear. The force constants were calculated hy ttrenting the met,hyl groups xs single m~rsw,:mi ihe tkelelnl vihr:tt.iansare likc tl~oseil1ustr:tlcd inPigure 6 .
Table 2. Metal-Ligond Bond Stretching Force Constants (mdyner-k') for Some Octahedral Halide Complexes Obtained from Roman Spectra Ion
k
Reference
BeCI, and BeBh are consistent with the presence in solution of polynuclear complexes involving di-p-halo bridges (37). Spectra of aqueous solutions of (CF,),Hg in the presence of high concentrations of chloride, bromide, or iodide ion fail to reveal any evidence for the existence of complexes where mercury has a coordination number greater than 2 (88). A four coordinate mercury complex, [(CFJZH~CIZI-~ had been suggested to exist in these solutions containing high concentrations of chloride ion. The products formed by the dissolution of metallic tantalum in aqueous H F have been identified as octahedral [TaFB]- and [TaF71-? together with another, unidentified ion (59). Spectra of solutions of K,NbOFs H 2 0are consistent with the existence of an octahedral oxopentafluoroniobate(V) ion, [NbOFs]-2, but not with the hydrated form [Nh(OH)zFs]-2. I n the presence of high concentrations of hydrofluoric acid, the characteristic spectrum of an octahedral ion [NbFe]-, is also observed (40). I n general, most ionic inorganic compounds are insoluble in solvents used for infrared spectroscopy. Studies on the vibrations of ions have, therefore, often been made by infrared absorption measurements on mulls or alkali halide discs containing the crystalline compound. Often the strong forces in the crystal lattice lead to complications in the spectra. Many ionic compounds are soluble in water, however, and so Raman spectra may be obtained which are representative of the "free" ion in the absence of effects due to thc crystal lattice. Among the ions and molecules for which spectra have been obtained by measurements on aqueous solutions are, in addition to those listed above, t,etrahedral V04-8 (55), [AI(OH)&, [Zn(OH)r]-Z ( 5 6 ) , %(OH)&(57); octahedral [Sn(OH)e]-2 (51); and [(CH3),Sn(OH),]-2(58). 1l:uly of the very st,ahlr cyanide complcxes n3w1 havc Ixvn st,~~(lied in sdution, for examplr, [Ag(CX)I]-, I ( - ( 9 , G O ; [Cu(CN),]-?, [Ag(CN)n]-', (ii2); [Zn(CN)I]-2, [Cd(CN)&', [Hg(CN)4]-"G1, [CU(CX)~]-~, and [lk(CN)sl-3 (62). These results a m Some of the earlier work has been reviewed briefly hy MATEJ. P., J. I n ~ l y & . Nuclear Chem. 8 , 3 3 (1958). These force constants were calculated using simple valence forces witho~~t any interaction force constant. The (we of other force fields will give slight,ly ditievent vnlttes; however the farrr with resp+~.l, constants retain essenti:tlly the r:me relative YILIIIP* lo m e :bnolher. These aolatnnl.; are lhwcfore -;elisl:trl~,ryfw conqxixing similar mr,lemles m d iuns. IEU,
Volume 44, Number 2, February 1967 / 73
he compared with the rather extensive data obtained from infrared spectra on solid cyanide complexes. Whilc thermodynamic measurements on dilut,c solitt.ious cannot distinguish bctween hydrat,ed and notihydrated species, Raman spectra can often be used for this purpose. The spectra of aqueous solutions of boric acid, H3B03, strongly iudicate that the anion formed by dissociation of the acid is tetrahedral [B(OH)&]-, isoelectronic with [BF4]-, rather than [H2B03]- (63). This infers that the undissociated acid is probably strongly hydrated to give four oxygens coordinated to boron. An infrared and Raman investigation of solutions of vanadyl chloride-VOC12, fluoride-VOFz, and perchlorate-VO(C104)z indicates the presence of an unprotonated V0+= (aq) ion rather than V ( o H ) ~ + ~ ( a q(64). ) I n this case, water is sufficiently transparent to infrared radiation a t 1000 em-' so that the V 4 stretching vibration can be observed in the infrared absorption spectrum of thin films of the solution as well as in the Raman spectrum. Raman spectra of alkaline germanium(1V) solutions indicate that the dinegative germanate anion is tetrahedral [Ge02(OH)~]-2(65) rather that the octahedral hydrated form [Ge(OH)el-2 as it sometimes has been written. Information on Chemicol Bonding
The most intense Raman line in the spectrum of a. simple tetrahedral ML4 or octahedral MLs molecule or ion almost always results from the vibration in which :dl of the bond lengths increase and decrease in phase with one another and where there are no changes in the bond angles. These vibrations are illustrated in 1~'igure 13. In this so-called "breathing" vibration,
Figure 13. Totally r y m m e l r t a l ribrotionr of simple tetrohedrol and octahedml compleier.
only the ligands L move. Since there is no change in the dipole moment during this type of vibration, it does not lead to the absorption of infrared radiation; and in general the frequency must be obtained from the Raman spectrum. With the assumption of simple valence forces, t,his frequency uniquely determines the RI-L bond stretching force c o n ~ t a n t . ~Since this force constant is a measure of the stiffness of the metalhgand bond, the values provide information on trends in the bonding of similar molecules. Values of the metal-ligand bond stretching force constants for several scquences of isoelectrouic ions and moleoules are listed in Table 1. As was pointed out by Woodward (,$I), there is a general tendency for these force constants to increase as the effective nuclear charge of the central atom acting on the valence 74 / Journal of Chemical Education
electrons increases. I n the isoelectronic sequences whcre only the cc~ltralatom is changing, thin leads to air in(.rease it1 the force constants with the central at,im atomic numbcr. I t can be seen that the oxy-anions behave somewhat differently. First, the force constants are much larger reflecting the central atomoxygen s bonding.' Secondly, in the cases where K bonding is appreciable, the force constants change but little with the central atom atomic number in an isoelectronic sequence, presumably due to compensating changes in the rr and s contributions to the borvls. For the uontrar~sition metals, the bouds generally become less resistant to stretching down a given family. Considering the tetrahedral and octahedral halo complexes in Tables 1and 2, it is seen that only mercury(I1) is anomalous. Mercury(I1) is again anomalous in (CH&Hg where the bonds are much stiffer than would be expected, and this may result from some involvement of the mercury 5 d orbitals in the bonding. With thc transition metals where the d subshell is incomplet,e, the bonds tend to become stiffer with the heavier members of a given family as is shown by the data for the tetrahedral oxy-anions in Table I and the octahedrd halo complexes in Table 2. Studies on Colored Compounds
I n the initial discussion, it was assumed that the exciting frequency was chosen so as not to coincidc with any resonant absorption frequency of the rnolecnl(~ or ion, for example that due to an electronic transition. The blue line, 4358 A (22938 em-') or less often t,hv green line of the mercury arc, 5461 (18307 cm-I), is generally used to excite the spectra. Since many transition metal compounds absorb in this region, they have been examined only rarely by Raman spectroscopy. The d electron transitions which make it difficultto obtain a Raman spectrum provide, in themselves, much information on the molecular sterochemistry when interpreted in terms of ligand field theory. There is, therefore, not so great a need for thc Raman spectra of these compounds. I n a few detailed studies of molecular vibrations, Raman spectra have been obtained for colored transition metal compounds by using special light sources like the sodium arc. Among the molecules studied is ferrocene, (s- C6HJ2Fe(66). Helium and other rarc gas lamps have been used to investigate colored ions like tetrahedral [CrOl]-z (67) and square planar [PtClrl-' and [AuCLI- (68). Recently CW lasers have been used to excite Rarnan spectra (69-7I), and they promise to provide strictly monochromatic radiation sources. Thus far the HeNe gas laser operating a t 6328 A has been used mainly (69-71), although the argon ion la~er~appears to provide ten frequencies from 4545 to 5257 A (72). Other gas lasers will certainly become available in the future.
' For a dismssiorl of the calerh.tion of force constants, one oi Lhe more advanced texts on vibrational spectroscopy should br eonsnlted, e.g., "Infrrtred and Ramrtn Spectra. of Polyatomic C., 1). \'an Nostrand Co., Inc., PrinceIMolea~les," I~ENZSERG, ton, N. J., 1945. I t ha.? been knowu for some Lime that these oxyanions exhibit partial double bond character; see, for example, PXULING, L., "The Nature of the Chemical Bond" (3rd ed.) Cornell University Press, Ithaca, N.Y., 1960, chapter 9.
Fused Salts
is verv desirable to have s~ectrosconicmethods which h ~ w ronsi~lf!ral~Ic i i~~t,?rc,st, van bc applied to powdered samples. The Nujolln r e ~ ~ w ywrs, t tli~!rr11;~s 11alol:arbon1111111 ;ml ollmli halide disc tiv'huiques ;I,I,I% in the "st,ruaturr" of n ~ o l t r .sidt, ~ ~ systems. The knis uscd routinely y inorganic ohcmists for obtainiug tend t,o aggregate ink^ ~:lustcrsiu these melts becausc infrared spectra of compounds in the solid state. of the higll coulombic forces. I n many respects these Until recently few Raman studies have been made on ionic aggregates should be similar to the species present, powdered samples, although some measurements have in oonccntratcd aqueous electrolyte solutions. On tho been made of scattering from single crystals. Experiotlicr hand, it is generally casicr to treat quantitatively mentally, the diRculty stems from the large amount of t,hr properties of a molten electrolyte than it is to treat light a t the exciting frequency which is reflected directly a very concentrated electrolyte solution. Several int,o the Raman instrument from the crystal faces. 111 wviews have appeared which describe the uses of t,he presence of such intense light at the exciting f t ~ vit~rationalspectra in the study of ionic interact,ions in quency, it is exceedingly difficult to measure the vwy melts (75-75). low intensity Raman scattering. Just as inner-sphere complexing of metal ions in Two experimental procedures are used to reduce the aqueous solution can be detected by the splitting of iutensity of this reflected light. One method uses degenerate vibrational modes of symmetrical ligands, interference filters with very sharp absorption bands specific cation-anion interactions in melts can be identiintroduced between the sample and the Raman monofied in the same way. Particularly detailed studies chromator to attenuate this light. With grating monohave been made of molten nitrates. The changes in chromators, a premonochromator can he used as an the nitrate ion spectra in melts of alkali metal nitrates efficient filter. Several different srra.ngements arc (70, 77), silver nitrate (76-78), and thallium(1) nitrate used to support the powdered sample. It may h~ (79) have been interpreted in terms of simple changes pressed into a disc and either the forward or back in the polarizing power of the cation with size. With scattered light examined; or it may he held in a thin the possible exception of the silver uitrate melt, specific layer, for example, between two concentric glass cones cation-anion interactions with any degree of covalent and the light scattered a t 00' to the exciting light character seem to be absent from these fused salts. examined.= Similar studies on the interaction of the more highly Using multiple interference filters, good quality charged sulfate ion still fail to disclose any evidence of spectra have been obtained for crystalline perchloril: metal-ligand electron sharing (80). A comparison acid which indicate that molecular HC104 does exist in of the spectra of the melts with aqueous solutions of the the solid (87). I t had been suggested that the undissame sulfate reveals that the solvent, as expected, sociated acid mas unstable because of the reaction weakens the strong interionic forces. This results in sharper Raman lines for the aqueous sulfate more characteristic of a "free" ion. Vibrations of the tetrahedral anions Tc04- and R e W I n addition to melts of salts of simple oxyanions also have been studied using crystal powders and a which can be used to probe the strength of interionic double grating Raman spectrophotometer (47). Such forces, spectra of metal halide melts have been obtained data can be used to complement infrared spectra obfor several systems. Spectra of molten mercury(I1) tained with mulls or all~alihalide discs. Since it is halides reveal that the melts contain discrete, linear possible even with solid samples to determine Raman HgBh or HgCh molecules (81), the latter being esshifts down to ca. 150 cm-I or even less from the exciting sentially the same as the complex described above as frequency, this method is especially valuable for studies being observed in aqueous solution (56). If KC1 is of low frequency vibrations. Direct infrared ahsorpadded to the melt, the spectra indicate that [HgCLtion measurements a t these low frequencies are quit
+
+
'See the discussion in R o s s o ~ n ,J. C., ANn ROSSOTTI, H., " ~ Determination h ~ of Stability Constants," McGraew-HdI Book Go., Inc., Neew York, 1961, chapter 13. This i8 &fairly common phenomenon, and it can result from steric effects or from electronic effects such m are embodied in tire ectroneo+,ralit)-prineiple-n See I)ULINS, L-, up,? N ~ . ~,f ,+he , ~~~l , ~ , , , ~,,,,,y i , . ~ ~i:il.d od.) ~ ~ ~~ ~ , ,, i , ~, ~ ~ l~ i (l , ~ ,
Toble 3. Romon Frequencies of the Bromo Complexes of Zincllll Com~lex Assiened Freouencies (ern-') n t d St,rwclures [ZnBr] [ZnRr.]
...
+
208 lZnBr11184? [ Z t ~ B r ~ l -172, ~ 61, 210, 82
System Reference
Tetrahedral Aqueous (107, 108)
...
205 I SF
206
... li", K3, 213, S1
172, GB, 208, SN
Aq\leo,ls
"Lqr~eous
Tetrahedral
(4s)
I X2
'I'ctrxhodml (10!1)
The simple salts ZnCL and ZnRr2 arc extremely soluble in water, and the Raman spectra of the solutions show that inner sphere complexing does occur. Thc spectm of very concentrated aqueous solutions of ZnCL and ZnBr* also have certain features in common mith thc spectra of the pure, fused salts and fused mixtures of the salts and the corresponding alkali halide. A summary of the data on these two similar systcm? is given in Tables 3 and 4. The relative intensities or the thrce sharp, intcnsc lines in the spcctra of the bromide solutions :it ca. 205, 186, and 172 em-' arc very clepc~dcntup011 the total bromide ion concentration and they increase in that order as the Af Br-:A[ Zn+?ratio is increased. There is general agreement that the last frcquer~cy172 cm-' is that of tetrahedral [ZnBr4]-2,since the solutious a t high M Br-:A! Zn+%ratios give a four line spcctrum characteristic of a tetrahedral ion. This spectrum is shown in Figure 14.# This tetrabromozincate(I1) ion is also knou~nto exist in crystals. The assignment of the Raman lincs of the con~plcxcs with fewer than four bromides is still somewhat uncrrtain. Since the intensity of the line a t 205 ern-' decreases and that a t 186 cm-' increases as Al Br-: ill Zn+2 increases, thc number of hromide ions in the cnmplex giving the 1% cm-' line must he greatcr than that in the complex giving the 205 em-' linc. I t is not clear whethcr these lines are caused by ZnRr2 and [ZnBr3]- or by [ZnBr]+ and ZnBr?. The assignments in the ZnCll system arc cvcn more uncertain. I n addition to the question of ~ h c t h c r [ZnCl]+ and ZnCL or ZnCla and [ZnC13]-are thc species with fewer than four chlorides causing thc ltaman scattering, it is uncertain whether the complex with four chlorides is tetrahedral tetrachlorozincatc(I1) or octahedral tetrachlorodiaquozincate(I1). The spectra of solutions with high d l C1-:dl Z I I + ~ratios do not show all four lines to he expectcd for the trt~r:ihcdr:d Toble 4. [ZnCl] IZnCI.1
+
[ZnCl~l[ZnCL]- 2 Other
System
Species
... 30.5 Linear 286 27.5, 79,306, 104
Tetrahedral ... Aqueous
Reference (109) S indicates solvent molecules.
i~onq~lcx.'I'hus :~lthonghthey are very useful, Ramall spcct,ra do not providc any panacea when it is a question of the composit,ion of a system mith several complexes. Recent,ly similar intcnsity meaeurements have been uscd to identify the thallium(1II) complexes [TICI]+", [TIClp]+, TICIS, and [TICId]- all in aqueous solutions (111). They also indicate the prcsencc of another romplcx a t high df CI- : Ill ratios, and this is
300
200
100
0
A - WAVENUMBERS Figure 14. Ramon spectrum of 2.5 M ZnBrr in the presence of 7.5 M NoBr. T h e Ipectrum h a g been reduced to o lineor baseline b y rvbtrocting t h e background xottering. T h e ~cottering species is tetrohedrol 12"Err] z .
Studies on Chemical Bonding Si111.c thc magnitude i d ( a ) and hcnw thc Ilamm intensity dcpends on the covalency of the bonds \vhioh are clistort~cd(lr~rillgthe vibration, it might be (:xpe(:t,ed that llaman int~cusiticscould give somc ill-
' Cornpam I he p n l l e r ~of~ the frequencies giveu lcn [ Z u l h l .', Vig~wv14, wilh t h < w 101. icI~xl~~d~.all CCI.,, Fignrc :1 (Part 1 I.
Romon Freouencies of the Chloro Com~lexesof Zincllll
>305 305
Linen]. 2 7 ~ i;l l )
Octahedral
[ZnIX(OFI~),I- 2 I'olyrnenc
... ;305 1,inc:u 2x2,
~2-1 lti
Tr:1 mhedl.nl ...
(ZUCI)~
:XN-400, Xd5, Z30,90
Aqmons
(110)
... ZOCI~S.~ +
. .. .. .
. .. . ..
%nCI?S2 2sn ? 2x2
. .. . ..
2V2, 85 280, 02
Pulynr~clear
I'olymeric
305
+
'L'ctmhedrsl complexes?
(ZnCId,
. .. 305
Ilncn 1.
Tetmhedr:d
266,' 1 1 1 1 [ZnCI,,]< 2 - " l '
...
l'olyme~~ie
(ZnCL).,
233
.2y1cons (107, 108)
hIelhanol-
Melt,
Melt ZnClz KC1
(83)
(83)
water
(111,112)
230, 'Jn
+
Volume 44, Number 2,
Melt (1131
Februory
1967 ' 77
formation on the nature of these chemical bonds. Qualitatively, it has been shown that ion pairs do not give appreciable Raman scattering (105). Some semiempirical relations have also been obtained betveen Raman intensities of simple molecules and different parameters including Pauling's covalent character of the appropriate bonds (114,115); however, a quantitative description of intensities st,ill remains n formidable problem ( I 1(i).
Courdi~>:~!.im Chemistry, Yiewm, 1964, p. 17. (44) SPIRO, T. C., Inorg. Cheru., 4, 731 (1865). (45) YEU, J . T. .\XI, ( ; \ v r w , W. I)., .I. A m . Chem. Soc., 70,
3463 (1049). (46) NIXON,J. .\so I'r.isr:, 11. A, .I. .1m. ('hrrx. Soc., 84, 4445 119l;21, . . (47) I l o s ~ i - ,II. IT., \ w I IRISIT,1). I,;., lnnrg. Chem.., 3, 1134
Summary
Several examples have bee11 givcii of situatio~iswherr Raman spectroscopy can make unique contributio~is to the study of inorganic systems. As commercial Raman spectrophotometers become more generally available, inorganic chemists will certainly use this structural tool more a d more. With compounds or nontransition elements, it is one of the few experimental metho& which can provide information on thr strwture of solute species in : L ~ I I C O I I Ssolutions. Acknowledgments
The author would lilcc to cxprc5s h ~ :~ppreciatioi~ s to Dr. .\Ialcolm G. Miles for helnful comments on thv malmscript and for obtaini~~g several of the spectra. Thanks are also due to Professor Henry A. Bent for many helpful comments and to Professor John Overend for many illuminating discnssions on moleculnr dynamics. Literature Cited (18) M ' o o ~ i i ' . ~ 1.. ~ ~.\., , l)tu~d.lims. f l , , d o t ~ ) 10, , 185, (l!M). (I!)) LONO,D. A., "An~malIleporls," \',L. A,, GARTON, G., A N D ROBERTS,H. I,., J . Chem. Suc., 3273 (1956). (86) \Voon\vnnn, I,. A,, GnEEvlvonl,, N. N., HALL,J. R., A N D ~
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