摘要:
J. CHEM. SOC. DALTON TRANS. 1985 1281Mossbauer Study of the cis-trans Isomers of Tin(iv) Complexes. SomeConsiderations about the Sign of the Electric-field GradientDavid Tudela and Vicente FernhndetDepartamento de Quimica lnorganica, Facultad de Ciencias, Universidad Autonoma de Madrid, SpainJesus D. TorneroDepartamento de Electroquimica, Facultad de Ciencias, Universidad Autonoma de Madrid, SpainMossbauer spectra are reported for the cis-trans isomers of SnCI,L, [ L = NN-dimethylacetamide(dma), NN-dimethylformamide (dmf), dimethyl sulphoxide (dmso), or tetrahydrothiophen (tht)]and SnBr,(dmf),. The following octahedral partial quadrupole splitting values were obtained:tht = +0.09, dmso = +0.14, dmf = +0.18, and dma = +0.20 mm s-'. A correlation has been foundbetween the quadrupole splitting and the values for v(Sn-CI) (from i.r. and Raman spectra) fortrans- Sn C I, L,.Mossbauer spectroscopy has been widely used in studies on thestereochemistry of tin tetrahalide complexes.'-4 In one case,SnCl,[(n-C,H,),SO,], despite the fact that conditions regardingits formation and structure are not well defined, two forms havebeen isolated with quadrupole splittings approximating to a 2 : 1ratio as expected for a cis-trans pair.' Ruzicka and Merbachreported the synthesis of SnCl,(tht), (tht = tetrahydro-thiophen) with a trans geometry in dichloromethane and a cisgeometry in n-pentane; however, 35Cl n.q.r.studies have onlyprovided evidence of the trans In this paper we reportour studies, using Mossbauer spectroscopy, on the cis andtrans isomers of SnCl,(tht),, in which we found small butperceptible differences between the two.We also report aMossbauer study of the cis-trans isomer pairs of SnCl,L, [L =NN-dimethylformamide (dmf), NN-dimethylacetamide (dma),or dimethyl sulphoxide (dmso)] and SnBr,(dmf),.'Experimentalcis-SnCl,(tht), was prepared by dropwise addition, withconstant stirring, of a 10% solution of tht (10% excess) in n-pentane to a 10% solution of SnCl, in the same solvent cooledto -5 "C. The precipitated white complex was washed with n-pentane and dried in uucuo. Constant and thorough stirring isessential because any local excess of ligand during the additioncould lead to mixing of the cis and trans isomers. If a solutionof SnCl, is added to a solution of tht, the final product is thetrans isomer.Both isomers were characterized by i.r. andRaman spectroscopy.6 When SnCl, was added to a solution ofdrna in CHCI, a mixture of cis- and trans-SnCl,(dma), wasformed. The cis isomer was isolated by vacuum sublimation.trans-SnCl,(dmso), was obtained by keeping a freshly-prepared sample of its cis isomer for 17 d at 22 OC," andsubsequently heating it to 135 "C for 20 h. The remainingcompounds were prepared by previously reported methods andwere characterized by i.r. and Raman spectros~opy.~ In allpreparations the usual precautions were taken to avoidmoisture.The i.r. spectra were recorded on a Nicolet 5DX FT-IRspectrometer in the range 4 000-200 cm-', using Nujol mullsbetween CsI windows.The Raman spectra were recorded in aJarrell-Ash spectrophotometer, model 25-300, using an Ar +laser (4 880 A). The Mossbauer spectra were obtained withpolycrystalline samples at liquid N, temperature. The constantacceleration spectrometer, velocity calibration, temperaturecontrol, and computation procedures have been describedpreviously." A standard source of Ca"'Sn0, was used intransmission geometry, filtering the y-radiation with a Pd (50pm) foil filter. The isomer shifts are referred to BaSnO, at roomtemperature. The sample thickness in every case was 11 mg ofnatural tin per cm2 and the maximum statistical error in theexperimental points was 0.2%. The reproducibility of theparameters was f 0.02 mm s-' for 6 and A and k0.03 mm s-'for r.After recording the Mossbauer spectra, samples werecollected and checked by Raman spectroscopy to ensure thatthey had not undergone any changes.Results and DiscussionThe Mossbauer spectra of the cis and trans isomers are shownin Figure 1 whilst their correspoqding parameters are given inTable 1. Those spectra that do not display a resolvedquadrupole doublet were first fitted as a single line. From theresults for x2 and r (see Table l), the spectra were taken to bequadrupole doublets. On the other hand, reasonable parametersfor cis-SnCl,(tht), were obtained from a single-line fitting (seeTable 1). The possible existence of a quadrupole doublet wasalso considered because the quadrupole splitting of its transisomer shows that the partial quadrupole splitting (p.q.s.)I2 oftht does not equal zero.An abnormal line asymmetry appearedduring the fitting process which made us consider a symmetricalquadrupole splitting (Table 1). In this case, the error for A ispossibly larger than & 0.02 mm s-'.The Mossbauer spectra of cis-SnCl,L, (L = dmf, dmso,or dma) and trans-SnCl,(tht), have been previously repor-ted,1,3,4,13-17 and they present an isomer shift that is in goodagreement with that measured in our study. For the quadrupolesplitting, spectra were mainly fitted as a single line. Values for Aother than zero have been reported for SnCl,L, (L = dmso ordma)I3*l and agree reasonably with our measurements. Fromour results, however, (see Table 1) it would appear evident, inthe case of SnCl,(dmf),, that the values of 0.734.75 mm s-lattributed to the quadrupole splitting'3*15 are for the transisomer.Although Davanzo and Gushikem' have found that cis-SnCl,L, (L = dmf or dmso) complexes undergo a spontaneousisomerization at room temperature, in 15 and 30 d respectively,we have been unable to confirm the fact by i.r.and Ramanspectroscopy. We have recorded the Mossbauer spectra ofsamples of cis-SnCl,(dmf), which had been kept at 22 "C for 19d and 11 weeks respectively, and in both cases we obtainedparameters corresponding to the cis isomer. From these data,certain details in the Experimental section, and the existingcontroversy regarding ~is-SnCl,(tht),,~** we believe that somefactors, as yet not fully understood, govern the formation o1282 J.CHEM. SOC. DALTON TRANS. 19851230 250 270 290 310B (Sn - CL) Icm-1hv)C3w .^C alCcVelocitylmm s-1Figure 1. Mossbauer spectra of (a) SnCl,(tht),, (b) SnBr,(dmf),, (c)SnCl,(dmso),, (d) SnCl,(dmf),, and (e) SnCl,(dma),. The upperspectrum of each pair corresponds to the cis isomereither isomer and the transformation from one to the other.Studies of these compounds should always include verificationof the compound's stereochemistry. Raman spectroscopy isuseful for this p ~ r p o s e . ~ , ~From Table 1 it is evident that values for the isomer shift ofthe cis and trans compounds of each species are the same withinFigure 2. Relationship between T(Sn-CI1) and Athe margin of experimental error.Such behaviour differs fromthat generally observed for Sn'' organometallic compounds; theisomer shift of the cis compounds is usually lower than that ofthe trans compound^.'^*'^ We have also observed that the transisomers present a greater resonance area than their corres-ponding cis isomers (see Table 1). This may be due to the greatersymmetry in the trans isomers which permits a more compactlattice.A1 though trans compounds have larger quadrupole splittingsthan their cis isomers, the AtrPns/Acis quotients deviate from theexpected value of 2 (Table 1). Two main contributions canexplain such behaviour. (i) The vibrational spectra of cis- andtrans-SnCl,L, (L = dmf, dmso, or dma) and SnBr,(dmf),9.20show an increase in the average values for v(Sn-L) and adecrease in the average values for v(Sn-X) (X = C1 or Br) whengoing from the cis to the trans isomers.This is in agreementwith a stronger Sn-L bond in trans isomers and implies agreater donor strength for the ligands, and hence a morenegative p.9.s. value l 2 when these co-ordinate in the transposition. Bearing in mind that these ligands have a positivep.9.s. value,22 (L),,' (the octahedral partial quadrupole splittingof L) would no longer be constant and would be greater in thecis than in the trans isomers, so that the Atrans/Acis quotientwould be less than 2. (ii) Important distortions are not foreseenfor the configuration of the trans compounds as predicted byZahrobsky's stereochemical model 23 and observed in single-crystal diffraction However, cis compoundsusually present distortions from regular geometry 27*28 whichcould contribute to the disagreement between experimental andcalculated quadrupole splittings.' In this respect, the valueobtained for Atrans/Acis (Table 1) would indicate considerabledistortion for cis-SnCl,(dmso),, which is confirmed by a X-raysingle-crystalBoth contributions could be related because distortions fromregular geometry modify the hybrid orbitals at the tin atom andhence affect the bonding properties.From a previous study by Bancroft et af.,'* a positive sign forV,, can be deduced for trans-SnCl,L, (L = dmso, dmf, or dma).Likewise, for trans-SnCl,(tht),, we predict a positive value forV,, (see Table 2).From these statements and the quadrupolesplitting values shown in Table 1 for trans-SnCl,L, (whereminimum distortions from regular geometry are assumed), wecan calculate the following p.9.s. values: tht = +0.09, dmso= +0.14, dmf = +0.18, and dma = +0.20 mm s-'. Thesevalues indicate donor strengths, against SnCl,, increasing in theorder dma < dmf < dmso < tht,I2 and are in agreement wit1283 J. CHEM. soc. DALTON TRANS. 1985Table 1. Mossbauer parametersCompoundtrans-SnCl,(t ht),cis-SnCl,(tht),trans-SnCl,(dmso),cis-SnCl,( dmso),trans-SnCl,(dmf) ,cis-SnCl,(dmf),rrans-SnBr,(dmf),cis-SnBr,(dmf),trans-SnCl,(dma),cis-SnCl,(dma),6/mm s-' A/mm s-l0.72 0.350.70 0.240.4 1 0.570.40 0.4 I0.38 0.730.39 0.530.66 0.830.66 0.440.38 0.780.38 0.45T,/rnm s-lapb0.90 (1.10)0.89d (0.98)0.92 (1.46)0.90 (1.15)0.870.90 (1.35)0.880.80(1.16)0.870.92 (1.23)TJmm s-la0.9 10.890.930.850.850.870.890.880.900.87Area trans/4r-l Ar is Area cis x2 '284 (776)256'(356)216 (3 376)301 (1 333)316228 (3 812)255219 (1 703)277265 (1 069)1.5 1.21.4 1.21.4 1.21.9 1.31.7 1.2a rl and r2 are computed full widths at half height at higher and lower energy respectively.r are given in parentheses for the single-line fitting. Forthe cubic compound [NH,],[SnCl,], r was 0.82 mm s-' at room temperature, under the same conditions of geometry and sample thickness. ' With191 degrees of freedom unless specifically indicated; x2 are given in parentheses (with 194 degrees of freedom) for the single-line fitting.Constrainedparameter (see text). With 193 degrees of freedom.Table 2. Quadrupole splitting-v(Sn-Cl) correlations in trans-SnCl,L,LOEt,thfdmadmfdmsoSMe,thtPPh,PMePh,AsEt,PEt,PhPBu,PEt,MeP(0)(NMe2)3C(O)(NMe,),v(Sn-CI)(E,)/cm-' P(Sn-Cl)/cm-' Ref.3 50 32 1 6344 314 6340 310 20335 308 2033 1 304 6328 302 20320 297 20327 304 6322 297 6306 286 2302 280 2307 280 2289 270 2292 274 2290 272 2235 219 e fA/mm s-' a Ref.(-)1.37 13(-)1.26 11(- w.78 b(-)0.73 b(-)0.69 15(-)0.57 b(-)0.51 15( - )0.40 13(-)0.35 b(+)0.31 c(+)0.57 2 + 0.90 d(+)Lo4 2(+)LO6 2 + 1.15 d + 4.28 ga Signs measured experimentally are given without parentheses, the rest are assigned by chemical analogy or a better fit on the straight line.Thiswork. 'Sh. Bashkirov, I. Ya. Kuramshin, A. S. Khramov, and A. N. Pudovik, Koord. Khim., 1980,6,537. D. Cunningham, M. J. Frazer, and J. D.Donaldson, J. Chem. Sac., Dalton Trans., 1972, 1647. ' C. W. Hobbs and R. S. Tobias, Znorg. Chem., 1970,9, 1037. I. R. Beattie, F. C. Stokes, andL. E. Alexander, J. Chem. Soc., Dalton Trans., 1973,465. R. V. Parish and R. H. Platt, Znorg. Chim. Acta, 1970,4, 65.vibrational spectral data (see later). Our relative order for p.9.s.values for dmso and dmf is contrary to that establishedpreviously,22 but is in agreement with the results found for thesesame ligands in trigonal-bipyramidal geometry in the apicalp~sition.~' It has also been shown that dmso is a stronger donorthan dmf with respect to SnCl,.3' It must be noted that theabove p.9.s.values have been obtained from a trans series ofcompounds with only a small variation in the type of ligands,the same kind of structure, and presumably minimal distortionsfrom regular geometry. For these reasons, they are probably notgood 'working values' for the prediction of the quadrupolesplitting for a more varied range of compounds, but they doreflect the donor properties of the ligands.Quadrupole Splitting- Vibrational Frequencies Correlation.-The force constant Sn-Cl for SnCl,L2 compounds decreases asthe Sn-L bond strength' ' increases. Since the p.9.s. value for aligand becomes more negative as the donor strength of theligand increases,I2 there must be a relationship betweenv(Sn-Cl) and A.Table 2 gives A values for various trans-SnC1,L2 complexes, together with the average values of thecorresponding frequencies for the Sn-CI stretching vibrationsfound in the i.r. and Raman spectra [?(Sn-Cl) = 4 ( 2 4 + A,, + Big)]. These values are plotted in Figure 2. The sixteen pointshave been fitted to a least-squares straight line (correlationcoefficient 0.993) with a gradient and intercept of - 5.56 x lop2mm s-' cm and 16.28 mm s-' respectively. This plot could helpin the prediction of the sign, and to a certain extent the value, ofA for trans-SnC1,L2 compounds, from their i.r. and Ramandata. Care should be taken regarding the prediction of the signfor A from ?(Sn-Cl) values around 293 cm-', by means of theregression line.Although a worse regression (r = 0.985) isobtained, there is also a linear correlation between A and the i.r.-active v(Sn-C1) (E,) (see Table 2) with a gradient of -4.79 xmm s-' cm and an intercept of 15.18 mm s-'.AcknowledgementsThe authors would like to thank Dr. Y. Gushikem for helpfulcomments and Professor J. M. Vara for the adaptation ofMossbauer spectra computer programs. We also thankCaroline Bintcliffe for correction of the text.References1 J. Philip, M. A. Mullins, and C. Curran, Inorg. Chem., 1968,7, 1895.2 D. Cunningham, M. J. Frazer, and J. D. Donaldson, J. Chem. SOC. A,1971,2049J. CHEM. SOC. DALTON TRANS. 19853 P. G. Harrison, B.C. Lane, and J. J. Zuckerman, Inorg. Chem., 1972,4 P. A. Yeats, J. 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Grimes, ‘ABibliography of X-Ray Crystal Structures of Tin Compounds,’International Tin Research Institute, Middlesex, Publication No. 588.29 A. Hanson and 0. Brunge, personal communication quoted in I.Lindqvist, ‘Inorganic Adduct Molecules of 0 x 0 Compounds,’Academic Press, New York, 1963.30 G. M. Bancroft, V. G. Kumar Das, T. K. Sham, and M. G. Clark, J.Chem. SOC., Dalton Trans., 1976,643.31 T . N. Srivastava, S. K. Tandon, and N. Bhakru, J. Inorg. Nucl. Chem.,1976,38,2311.Dalton Trans., 1974,2355.Dalton Trans., 1973, 1823.Petrosyan, Zh. Struct. Khim., 1977,18, 1103.Received 21st May 1984; Paper 4/82
ISSN:1477-9226
DOI:10.1039/DT9850001281
出版商:RSC
年代:1985
数据来源: RSC