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1972 1875Hexanitrometaliates. Part 111.l Electron Transfer SpectraBy John C. Barnes," and (in part) (Miss) C. S. Duncan and R. D. Peacock, Chemistry Department, TheUniversity, Dundee DD1 4HNIn the complexes A,A'M(NO,), the region 20-30 x 1 O3 cm-l shows a variety of transitions depending on A.A'and M, in addition to (d-d) transitions of M. If the M-N bond i s weak and M is not oxidizable or reducible theinternal transitions of NO,- occur, hardly perturbed from those in NaNO,. If M is CeI", BPI, or PbII electrontransfer [(NO,-), f- M I occurs. When A or A' include TI1, AS1, or PbI1 and M is optically reducible (CO~~',CoII, FeII, NiII, or CuII) one transition (M 4- A,A') i s observed. [M 4- (NO,-)J transitions are seen whenM is optically reducible. In Cs,CdCd(NO,),doped with CoIZ1 and Pb1I, which both occupy the M site, no (CoIII +- PbII) electron transfer is observed.Electron transfer transitions (NO,- f- A,A') are not observed.THE class of complexes A2A'M(N02)6 can be preparedwith an unusually wide range of A,A' and M.2 Stable,well characterized compounds are available with the Asite occupied by K+, Rb", Cs+, Ag+, or Tl+; the A'site occupied by Na+, Ag+, T1+, Ca2+, Ba2+, Pb2+, Cd2+,or Hg2+; and the M site occupied by Cd2+, Hg2+, Ln3+,Bi3+, &positive or tripositive transition metal ions.Notall the possible, electrically neutral, compounds havebeen reported but the range can be extended by doping.For example, pure Pb(N0,):- and MII(NO,),~- com-pounds are unknown but both can be prepared in thewhite Cs,CdCd(NO,), host lattice.The compounds A2A'M(N02), have a face-centredcubic structure 293 or are tetragonal with only small dis-tortions from the cubic ~ t r u c t u r e .~ , ~ M is surrounded bythe N atoms of six NO2- ions. The site symmetry isideally Th but both static and dynamic distortions havebeen A occupies a site of Td symmetrysurrounded by twelve 0 atoms from twelve differentNO,- ions. A' occupies a site of Th symmetry sur-rounded by twelve 0 atoms belonging to six NO2- ions.In the present paper the electronic spectra (5-30 xlo3 cm-l) of a range of these complexes are reported(Table 1). Where appropriate the spectra show (d-d)transitions but the discussion which follows will berestricted to the electron transfer transitions.EXPERIMENTALPure compounds were prepared by methods described inthe literature 5 9 7 modified where appropriate to give caesiumor thallium salts. All preparations involving cobalt(r1)Part 11, J.C. Barnes, Nature, 1971, 234, 99.R. W. G. Wyckoff, ' Crystal Structures,' Interscience, 1965,J . A. Bertrand and D. A. Carpenter, Inorg. Chem., 1966, 5,4 J. A. Bertrand, D. A. Carpenter, and A. R. Kalyanarama,vol. 3, p. 376.514.Inorg. Chim. A d a , 1971, 5, 113.or iron(I1) were carried out under oxygen-free nitrogen t oavoid contamination with oxidized products.TABLE 1Spectra of A,A'M(NO,),, band maxima in lo3 cm-lAcscsT1 csc sKKc sc sTIc sc sTI cscsTIc s cscscscscscsA'NaCdCdBaPbBaPbBaBaPbBaBaPbBaNaCd i%+ NaCd + TICd + NaAgHgCd + Agn.1LaYCdCdFeFec ocoHgc oNiNiNic u c uc uBiCd + PbCd + CoIIICd + CoIIICd + CoIIICd + CoIII + PbHg + Pb24, 28.5, 30.524, 28-7, 30.023, 28.5, 30.024, 29.0, 34.022.5, 27-7, 31.022, 25.5, 81.620.5, 25-5, 30.38.6, (13), 16.7, 23.0, 26.0, 308.8, (13), (16*5), (19*2),23.0, 26.0, 308.8, (13), (16*0), 20,(22.5), 25, 30(10.9), 13.6, 21.3, 30(10.8), 13.7, 21.7, (26-6), 30(10.8), 13.5, 20.3, (27-5), 3016.2, (21.5), 24.5, 29.416.4, 19, 24.5, 29.516-4, (19), 24.5, 29.522.8, 27.8, 30.521.7, 26, 29.5, 3421.0, 26.0, 30.0, (33)(21*4), 26.2, 29.5, (33)21.0, 26.0, 29.5, (34)(22*0), 25.4, 29.5, (34)21-0, 26.0, 29.5, (34)Doped samples were prepared by adding ca.5 mol yo oflead(r1) nitrate or sodium hexanitrocobaltate(r1r) to solu-tions of cadmium nitrate or mercury(I1) nitrate in saturatedsodium nitrite. The complex was precipitated by addingcaesium nitrate solution to which had been added, if re-quired, thallium(1) nitrate or silver nitrate. The productswere well formed cubes of edge 10-30 pin, which appearedhomogeneous under the microscope and in X-ray powderH. Elliot, B. J. Hathaway, and R. C. Slade, Inorg. Chern.,1966, 5, 669.D. L. Cullen and E. C. Lingafelter, Inorg. Chem., 1971, 10,1265.A. Ferrari, B. Alessandro, and C. Colla, Gazzetta, 1935, 65,7971876 J.C.S. Daltonpatterns. Further details of the doped samples listed inTable 1 will be presented separately as part of a widerstudy of doped A,A‘M(NOa)6 compounds.8Diffuse reflectance spectra were recorded at room tempera-ture using a Unicam SP 735 spectrophotometer. Purecompounds were diluted with alumina but the doped sampleswere recorded without dilution.The spectra of a numberof the samples were also recorded at 77 K. No more than amarginal improvement in resolution was obtained.DISCUSSIONBarnes and Peacock 9 have interpreted the spectra ofCs,NaLn(NO,), by assuming only weak interactionbetween M and (NO2-),. The region 20-50 x lo3 cm-lshows the characteristic spectrum of nitrite ion, onlyslightly perturbed from that of sodium nitrite [Table2(a)]. Electron transfer transitions appear a t lowenergy if M is readily reducible or readily oxidizable, asin yellow Cs,NaCe (NO,) 6.9TABLE 2(a) Spectrum of NO2-Energy/103 cm-l E3B1 + lA1, (bl* + al*) 22.5 10-2lB1 + lA1, (b,* f-- al*) 27 28lA, f-- lA,, (b, - b,) 46 5000Data and notation from K.L. McEwen, J . Chem. Phys.,1961, 34, 547; and W. C. Allen and R. N. Dixon, Trans.Faraday SOC., 1969, 65, 1168.(b) Predicted spectrum for M(N0,)P- with strong M-N inter-action: K. G. Caulton and R. F. Fenske, INorg. Chem., 1967,6,5623eg f-- al4,eu 20-23 ForbiddenSt, 4+-- 44, 24-26 Two allowed3eg + 44, 40-50 Two allowedlA2 + lA1, (b,: a,) 34 9Energy/ 1 O3 cm-1CM - W 2 - ) I 3 1“NO,-), - M3[&I + (NOZ-)~]Caulton and FenskelO discussed the case of strongM-(NO,-), interaction. They predicted that the energylevels of nitrite ion will be so altered that the transitionscorresponding to Table 2(a) would occur only at veryhigh energy.The 20-50 x 103 cm-l region would con-tain the electron transfer transitions listed in Table 2(b).The lowest energy allowed band is expected to be a metalto ligand, [(NO,-), f- MI transition (5t, + 4tg).These workers interpreted the spectra of some cobalt-(111), cobalt(n), iron(n), and rhodium(II1) complexes interms of their molecular orbital scheme.In either model the introduction of oxidizable ions tothe A or A’ sites raises the possibility of electron transferfrom these ions to nitrite or to M.With other ligands it is sometimes possible to correlateelectron transfer spectra with the appropriate half-cellpotentials for the metal i0ns.ll-1~ It has been observed* J.C. Barnes and C. S. Duncan, to be presented.9 J. C. BarnesandR. D. Peacock, J . Chem. SOC. ( A ) , 1971,558.lo K. G. Caulton and R. F. Fenske, Inorg. Chem., 1967, 6, 562.l1 I?. S. Dainton, J . Chem. SOC., 1952, 1533.l2 J. C. Barnes and P. Day, J . Chem. SOC., 1964, 3886.l3 J. H. Miles, J . Inorg. Nuclear Chem., 1965, 27, 1595.l 4 L. G. Nugent, R. D. Baybarz, L. J. Burnett, and J. L. Ryan,J . Inorg. Nuclear Chem., in the press.that a difference of 1 eV between corresponding transi-tions in ML, and M’L, is associated with a difference of1 V in the half-cell potentials of M and M’. For therelationship to hold the potentials used must be thosefor the oxidation or reduction of ML, and M‘L,.This isparticularly important for systems such as FeIII/FeII,Co1I1/CoI1, and CuI1/Cu1 where the potential varies widelywith the environment of the metal ion. No potentialshave been reported for M(NO,),,- complexes, because ofthe instability of these complexes in solution and becausenitrite ion itself can be oxidized or reduced in aqueoussolution. There is no justification for seeking more thana qualitative correlation between half-cell potentials inother media and the electron transfer spectra of hexa-nitrometallates of transition metals. A semi-quantita-tive relationship is observed (see below) for the com-plexes of bismuth(m), cerium(III), lead@), and thallium-(I); the oxidation potentials of these ions are lesssensitive to changes of environment.The best alternative correlation for electron transferspectra requires electron affinities and ionization poten-tials for nitrite ion and for the metal ions in the A,A’M-(NO,), ~omp1exes.l~ No data are available and theneed for a strong interaction model and a weak inter-action model for bonding in these complexes suggeststhat the electron affinity and ionization potential for(NO,-), will vary markedly with M.At the present time the assignment of the spectra ofA,A’M(NO,), must rely very largely on the changes ob-served as A,A’ and M are changed. The spectra will bediscussed in groups.No change of Oxidation State Possible for A,A’ OY M.-The spectra of CS,NaY(NO,),, Cs,CdCd(NO,),, and Cs,-HgHg(NO,), are all similar to that of CS2NaLa(N02)6,9[Figure l(a)].The transitions are those of NO,- essen-tially unperturbed by complex formation. The bandat 23 x lo3 cm-l shows some vibrational structure ineach case. The additional intensity seen in this singletto triplet (T +- S) transition compared with sodiumnitrite may be due to a spin-orbit coupling mech-anism involving the heavy M atom.16-18An Oxidizable Metal Ion Occupies the M Site.-Both theweak interaction and the strong interaction modelspredict a transition [(NO,-), +MJ at low energy.Apart from those containing transition metal ions (seebelow), the best characterized complexes of this class arethe yellow Cs,NaCe(NO,), and A,A’Bi(NO2),. Thelatter show intense absorption bands at 22.8 and 27.8 xlo3 cm-l [Figure l(b)].No pure A,A’Pb(NO,), complexes have yet been re-ported but lead(I1) can be introduced into the Cs,CdCd-(NO,),, Cs,HgHg(NO,),, and Cs,NaLa(NO,), host lattices.These doped samples show bands at 21-22 and 26 x lo31967, 10, 247.Phys., 1968, 48, 4694.Phys., 1968, 49, 4925.Phys., 1969, 50, 2777.15 M.R. Robin and P. Day, Adv. Inorg. Chem. Radiochewz.,l8 H. J. Maria, A. T. Armstrong, and S. P. McGlynn, J . Chem.17 H. J. Maria, A. Wahlborg, and S. P. McGlynn, J . Chem.18 H. J. Maria, A. T. Armstrong, and S. P. McGlynn, J . Chern1972cm-l which are not present in lead-free samples. Theintensity of these bands increases with increasing leadconcentration. These spectra suggest that lead occupiesthe N-co-ordinated M site in these host lattices.Thereis no absorption assignable to lead at 21 x 103 cm-l in20cxs608 21 2 0 26 201 0 - ~ 3 Em-’FIGURE 1 Reflectance spectra of hexanitrometallate com-plexes. Samples were diluted with A1,0, and ground uni-formly. (a) Concentration 39.2 mg NO,- per 100 mg sample.Cs,NaLa(NO,) !, CS,NaY(NO2)6-, Cs,CdCd(NO,) 6, NaNO, ;(b) Concentration 2.1 mg NO,- per 100 mg sample. Cs,NaLa-(NO,) 6 , CsNaCe (NO,) 6, CsNaBi(N0,)Cs,PbNi(NO,), where lead occupies the O-co-ordinatedA’ site. Also solution complexes of lead(I1) and nitrite,which are N-co-ordinated, absorb a t 22.8 x lo3 cm3.16J7(The 26 x lo3 cm-l region was obscured by the largeexcess of nitrite present in solution.)The bands at 21-23 x 103 cm-l in the cerium(m), bis-muth(IiI), and lead(I1) complexes are 10-100 fold moreintense than those in pure Cs,NaLa(NO,), or Cs,CdCd-(NO,),.The vibrational structure observed on the(T f- S ) transition in the host lattices is not seen whenthe oxidizable metal ions are present (Figure 1).The assignment of the 21-22 and 26-27 x 103 cm-lbands in Cd(N0,),3-, Pb(N0,),4-, and Bi(N02)63- toelectron transfer [(NO,), f- MI, that is the promotionof an s (orf) electron from M to the 5t, orbital of (N02-)6,is consistent with the very similar oxidation potentials ofthese metal ions. Extrapolation from these potentialsindicates the corresponding transition in Sn(N02),4-would be expected ca. 11 x lo3 cm-l (Figure 2). Whenelectron transfer transitions are predicted at such lowenergies the result is often a complete redox reactionrather than an electron transfer absorption followed byreversion to the original ground state. A familiar ex-ample is Cu2+ + 21-- CuI + &I2.No hexanitro-complexes of tin(I1) have been characterized. Additionof a tin(I1) salt to a nitrite solution produces immediateeffervescence. It seems likely that the initial step in thisreaction involves reduction of nitrite by tin(I1).The oxidation potentials of thallium(1) and silver(1)suggest [(NO,-), f- transitions at ca. 20 and26 x lo3 cm-l respectively (Figure 2). Both thesemetals form N-bonded nitrite complexes in solution butsince singly charged ions cannot occupy the M site inIs C. K. Jprrgensen, ‘ Oxidation Numbers and OxidationStates,’ Springer, Berlin, 1969, p.129.1877solid A,A’M(NO& they are restricted to the O-co-ordinated sites in these compounds. McGlynne and co-workers l t ~ ~ ~ have reported that nitrite solutions ofthallium(I), silver(I), and cadmium(I1) show absorptionmaxima a t 21.9 x 103 cm-l ( E 0-64), 22.2 x lo3 cm-l( E very low), and 23.4 x 103 cm-l ( E 0.02) respectively.They assign these transitions to the (T + S) absorp-tion of nitrite enhanced by spin-orbit coupling with themetal. Luminescence measurements have confirmedthat the (Tt-S) transition occurs at 21-23 x lo3cm-l in these complexe~.~~J~ The present authorssuggest that the greater intensity of absorption in thethallium solutions is associated with the [(NO,-) f- MItransition fortuitously occurring at similar energy to the(T + S) absorption.Solution spectra of silver nitritecomplexes have not been examined in the 26 x 103 cm-lregion, where the [(NO,-) f- Ag] transition is expectedto occur.An alternative assignment describes the transitions at22 and 26 x lo3 cm-l in nitrite complexes of bismuth-(111), lead(II), and thallium(1) as the triplet and singlet(s$ + s2) transitions. The present authors reject thisassignment. In well-characterized (s$ f- s2) spectrathe singlet transition is found 11-15 x lo3 cm-lhigher in energy than the triplet. The two bands in thenitrite complexes are separated by only 4 x lo3 cm-l.Again, when (sp + s2) transitions occur at low energythey are found in the order Bi < Pb <;Tl, spread overj/ 125 t /‘Ag/0.0 1.0 2.cVoltsFIGURE 2 Correlation of [(NO,-), + MI electron transferband energy with the oxidation potential of M.The dottedline has the slope of 8300 cm-1 per volt derived from studieson other systemssome 10 x 103 cm-l. In the nitrite complexes the orderis Tl, Pb < Bi, with a spread of only 2.5 x lo3 cm-1.Ligand effects in (sp + s2) spectra are similar to thosedescribed by the nephelauxetic s e r i e ~ . ~ ~ ’ ~ ~ The (s? f-s2) transitions in N-co-ordinated nitrite complexes should2O J. A. Duffy and M. D. Ingram, J . Chew. Phys., 1971, 54,4431878 J.C.S. Daltonlie above those of the chloro-complex, that is above30.5 x 103 cm-l (Bi), 36-8 x lo3 cm-l (Pb), or 40.5 x103 cm-l (Tl).These bands are not observed. Theymay be concealed by ligand absorption above 30 xlo3 cm-l.A OY A' Oxidizable, M not Red.ucible.--Tl,CdCd(NO,),and CS,AgLa(NO,), have spectra very similar to thoseof CsCdCd(NO,), and Cs,NaLa(NO,),. There is noadditional absorption which could be assigned to electrontransfer from thallium(1) or silver(1) to the oxygen atomsof nitrite. The spectra of Cs,PbM(NO,), differ fromthose of Cs,BaM(NO,), (M is CorrJ NiII, or CuII) only inthe existence of one extra band. This band is assignedto electron transfer (M f- Pb) and not (NO,- Pb)because the energy of the transition varies with M (seebelow).A or A' Oxidizable, M Reducible.-The most probableassignment of any additional features present in thespectrum of, for example, Tl,BaM(N0,)6 which do notoccur in Cs,BaM(NO,), is electron transfer (M + TI).50 r35 25 ylkK 15FIGURE 3 Spectra of Co(N02),3- complexes in Cs,CdCd(NO,),A Cs,CdCd(NO,), host lattice; B Host lattice doped withCo(N0,),3- and Na+; C host lattice doped with CO(NO,),~-and T1+.D host lattice doped with CO(NO,),~- and Ag+.,411 samples were diluted with A1,03. Curves B, C, and D aredisplaced vertically for clarityThere are relatively few descriptions of electron trans-fer between ions of different metals.15y21 Jgrgensen 22923found that the spectra of Tl2Irc1, and Ag,IrC1,, andsimilar salts of OSC~,~-, and OSB~,~- and ReCl,,-, showan additional absorption band lying 4-6 x lo3 cm-lbelow the first allowed transition in the correspondingpotassium salts.In each case the band in the silver saltoccurred cn. 1000 cm-l below that of the thallium salt.Similar observations have been made on a series ofchromates.22 These additional bands were assigned toelectron transfer (complex + T1, Ag). The acceptingorbital was largely but not completely localized on themetal atom of the complex. Braterman% observed asimilar process in transition metal ferrocyanides, whereelectron transfer occurred from Fe(CN),4- to MI1 forcobalt, nickel, and copper but not for zinc or manganese.Figure 3 shows the spectra of samples of Cs,CdCd-(NO,), doped with Co(N0,),3- and with either Naf orAgf or TI+. An additional band appears in the silverand thallium containing samples, lying 3.4 x lo3 cm-l(Tl) and 5-0 x 103 cm-l (Ag) lower in energy than theG. C.Allen and N. S . Hush, Progr. Inorg. Chern., 1967, 8,367.first allowed transition of CO(NO,),~-. These bandsinust represent transitions (CoIII t- T1, Ag). Onlyone transition is possible, electron transfer to low spinCoIII must give the excited state of CoII, tW6e,l (2Eg).Cs,PbM(NO,), and Tl,BaM(NO,), each show only oneabsorption band below 30 x lo3 cm-l which is not presentin Cs,BaM(NO,), (M = NiII, CoII, and CuII). K,PbFe-(NO,), shows one band not present in K2BaFe(N0,),.For each of these hexanitrometallates only one (M +A,A') transition is possible. The ground state of ,4,A'(s2) loses an electron to give the excited state (sl) and theconfigurations for M are :Ground stateFeII, t,,,CoII, tW6eg1N P , t,,seg2CuII, t2,,eg3Excited stateFeI, t,,%,l (2E,)NiI, twseg3 (,E,)CuI, t2,%,4 (lAlg)COI, t,,ee,2 (3A2,)For a given M the transitions from thallium(1) and fromlead@) are seen at very similar energies.The bandsoccur in the order CoII, CuII (19 x lo3 cm-l) < FeII(20.5 x 103 cm-l) < NiII (27 x lo3 cm-l).In the hexacyanoferrates 24 transitions assigned as[&I + Fe(CN),4-] were reported at 19.5 X lo3 cm-l(Cu") and 33.6 x lo3 cm-l (NiII). The data for ColIcannot be used for comparison because the cobalt ionis high spin in the ferrocyanide but low spin in Co(NO,),P-.When Cs,CdCd( NO,), was doped simultaneously withhexanitrocobaltate(m), with lead@), and with sodiumion the spectrum of the product showed only transitionsassigned to either CO(NO,),~- or to Pb(NO,)t-. Bothcobalt(II1) and lead(I1) occupy M sites in this host lattice.Electron transfer (CoIII +- PbII) would involve trans-fer of an electron by way of two nitrite ions and an inter-vening A (A') site.Using the classification of Robin and Day the lastexample represents a Class 1 system, for which nointeraction spectrum would be anticipated.The (M f-A,A') transitions through a single nitrite bridge aretypical of Class 2, and similar to the familiar example ofPrussian Blue.M I s a Transition Metal Ion, A and A' not Oxidizable.-The spectra of this class are by far the most difficult tointerpret. For CO(NO,),~- and CU(NO,),~- even thenumber of bands between 15 and 30 x lo3 cm-l is indispute.The spectra in Table 1 combine featuresobserved by other workers without agreeing in detailwith either of the published ~ p e c t r a . ~ ~ ~ ~ Since redoxpotentials cannot be used to confirm assignments thecomments given here must remain tentative.The lowest energy allowed transition in the schemepropounded by Caulton and Fenske lo represents elec-tron transfer [(NO,-), f- MI. It is a weakness of thisassignment that this suggests that cobalt (111) and copper-(11) should be more readily oxidized than reduced.Again, MnII is usually readily oxidized but MnII dopedinto Cs,CdCd(NO,), shows no intense absorption below35 x 103cm-l.22 C . K. Jrargensen, Acta Chew. Scand., 1963, 17, 1034.23 C. K. Jorgensen, Mol. Phys., 1961, 4, 235.24 P. S. Bratermann, J . Chem. SOG. ( A ) , 1966, 14711972 1879Since CorI1, Corr, FeII, Nirr, and Curl act as opticalelectron acceptors from TP, A&, or Pbrr it seems likelythat [M(3e,) +- transitions will occur a t lowenergy. If the A,A’M(NO,)B system behaves like A,IrC&,the lowest lying nitrite to metal transition will be found4-6 x lo3 cm-l higher in energy than the (M f- TlI)transition, that is ca. 25 x 103 cm-l for FeII, 24 x lo3cm-l for CoII, 26 x lo3 cm-l for ColIr, 32 x lo3 cm-l forX I x , and 24 x 103 cm-l for CdI. In each case there is aband close to the predicted energy.CONCLUSIONSThe weak interaction model has been shown to beconsistent with the spectra of La(N0,),3-, Cd(N0,),4-,and Hg(N0,),4-.The interpretations of the spectra of Bi(NO,):-,Pb(N0,),4-, and Ce(N0,)Z- as (NO,- f- M) electrontransfer and of bands in the spectra of thallium(I),silver(1) , and lead@) hexanitrometallates as ( M t -A,A’) electron transfer are consistent with either model.Doubt has been thrown on the assignment of thespectra of hexanitro-transition metal complexes bythe strong interaction model even in the case of hexa-nitritocobaltate(II1) where the spectrum is not con-sistent with weak interaction.There is need for a new model for these spectra inwhich the covalency of the M-N bond and the relativeelectron affinity and ionization potential of M are intro-duced as disposable parameters.[1/2410 Received, 16th December, 1971

ISSN:1477-9226    

DOI:10.1039/DT9720001875

出版商:RSC    

年代:1972

数据来源: RSC