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Transition metal acetatesD. A. EDWARDSN D R. N. H A Y W A R DSchool of Chem istry and Chernical Engitzeering, Bath U~ liu ers ify f Te cl~nology, ath, Sonierset, England

Received May 3, 1968Som e anhy drous transition metal acetates (Mn(II), Co(II), C u(II), Ni(II), Zn(II), Ag(I), Mo(II), Ce(I II),La(II1)) have been prepared and their infrared spectra measured in the solid state. Th e infrared spectrahave been related to established mode s of bonding of the acetate group to metals. Thermal decom posi-tions of the anhydrous acetates have been investigated by thermogravimetric analysis; magnetic mo me ntsand visible spectra have been measured.

Canadian Journal of Chemistry, 46 , 3443 (1968)

IntroductionRao and co-workers (I) have recently reportedinfrared and thermal decomposition data on

anhydrous acetates of the lanthanides, lead(I1and IV) and copper(I1). We have also obtainedresults upon some of these compounds togetherwith data on several other transition metalanhydrous acetates. Our infrared results pointto the unreliability of using the v,,,~(COO)-vSy,(COO) frequency separat ion to determine themode of coordination of the acetate group to themetal, although such a method has been usedfrequently in the past (2). Visible reflectancespectroscopy and magnetic measurements havebeen used, where appropriate, to show thecoordination number of the metal in the anhy-drous acetates and t o place the acetate group inthe spectrochemical and nephelauxetic series.

ExperimentalThe anhydrous acetates of Mn(II), Co(II), Ni(II),Cu(II), Zn(II), La(III), and Ce(II1) were prepared byrefluxing the commercially available hydrates with a largeexcess of a 3 :2 (vol./vol.) acetic acid - acetic anhydride

T A B L E IAnalytical results

% Metal % Acetate--ompound Found Calcd . Fou nd Calcd .

mixture. After cooling, the insoluble products werefiltered, washed with the solvent mixture followed bysodium-dried ether, and finally pumped under vacuumfor 2 h at room temperature. Nickel acetate was found tohave 4-mole of associated acetic acid after this prepa rationand pu mping, this acid being removed by heating at 200"under vacu um. The othe r acetates were not similarlysolvated. Molybdenum(I1) acetate was prepared from thecarbonyl (3) and silver(1) acetate was commerciallyavailable. Analytical results are given in Ta bl e I.Infrared spectra were recorded as Nujol, hexachloro-butad iene, and fluorlube mulls using a Perkin-Elmer 237grating spectrophotometer.Visible spectra were recorded using a Un icam S P 500spectrophotometer, the diffuse reflectance spectra beingobtained using the standard attachment.Magnetic measurements were determined b y the Gouymethod a t room temperatu re .

T A B L E I1Infrared frequen cies (cm-')-.

Acetate VSY,(COO) coo) AV

Thermogravimetric studies were carried out using aS tan to n TR 1 thermogravimetric balance heating at 4"per minute over the range room temperature to 1000.Produ cts remaining at th e end of such decompositionswere analyzed for metal content and shown to be theoxides mentioned in Table IV.

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3444 CANADIAN JOURNAL OF CHEMISTRY. VOL. 46, 1968Results and Discussion

I~g rar ed pectraThe free acetate ion, symmetry C,,, has 15infrared active fundamentals, all of which, exceptthe v6 torsional mode, have been observed (4, 5).Four modes of coordination to metals (1-4) arepossible (2b), but since the symmetry of the freeion is low no great differences in the infraredspectrum would be expected for each type. Thev3 (v,,,(COO)) and v, (V,,~,(COO)) frequencies,and their difference, Av, for the acetates studiedhere, are shown in Table 11.

It has been suggested (2) that where the ace-tate is a unidentate ligand (2) a divergence ofV,,~,,(COO) and vsY,(COO), compared with thefree ion, is expected due to a decrease in theequivalence of the C-0 bonds. Thus theV a S y m ( C O O ) - v S y , ( C 0 0 ) s e p a r a t i o n f o r[CO(NH,),OCOCH,]~+ is 223 cm-' (2a) com-pared with 153 cm-' for sodium acetate of type(1). Of the compounds studied here, the tetra-hydrated nickel and cobalt acetates are known tobe octahedral and the corresponding manganesecompound is expected to be the same, the metalsbeing coordinated with four oxygens from thewater molecules and two oxygen atoms from uni-dentate acetate groups (6). A large V,,~,(COO)-vSym(COO)eparation is, however, not observedin the infrared spectrum; the small separationfound could be due to hydrogen bonding with thewater molecules making the C-0 bonds moreequivalent than expected for unidentate acetatecoordination. In the corresponding anhydrousacetates the mode of coordination of the acetategroup to the metal must change since the visiblespectra and magnetic measurements show theacetates to be still octahedral and hence poly-meric. Since overall there are only two acetatesper metal atom both oxygens of the acetate groupare expected to be involved in the coordinationand the acetates must bond to more than onemetal atom by bridging. The V,,~,(COO)-vsym(COO) eparation, however, is unexpectedly

greater than in the hydrated unidentate acetatcases. We are unable to state if all the acetatgroups in the octahedral polymers are bridgin(4) or whether some are chelating (3), since in thtwo cases we have studied where all the acetatgroups are known to be bridging, copper(I1) (7and molybdenum(l1) (8), a very similar degree ov,,,,(COO)-v,ym(COO) separation is found. Fobridging or chelating symmetrical acetate structures both COO stretching frequencies are shiftein the same direction on changing the metal, sthe separation will not be greatly different fromthat of the free ion.Tlie structure of dihydrated zinc acetate iknown (9) to be octahedral, the coordination tthe metal consisting of two oxygen atoms fromthe two water molecules and four oxygens fromtwo chelating acetate groups. We find, again, thinfrared spectrum cannot be regarded as diagnostic of such an acetate coordination. Anhydrous zinc acetate shows an almost identicav,,ym(COO)-v,,,(COO) separation, so the acetatcould be tetrahedral with just two chelatinacetate groups, but an octahedral coordinatioachieved by bridging and shared acetates cannobe ruled out.Our results for anhydrous lanthanum(II1acetate are similar t o those published by Raoet a(I ) except we find a lower vSym(COO)requencyWe have also recorded the infrared spectrum ocerium(II1) acetate (10) which Rao and coworkers.did not include in their study. Rao anco-workers (1) suggested that the acetate groupin lanthanum(II1) acetate were unidentate, buthe results, in our opinion, do not require thisand indeed, in view of the high formal positivcharge on the metal and the favoring of higcoordination numbers by the lanthanide elementschelation or bridging acetate groups would seemmore likely.Thus we are of the opinion that the use of thinfrared v,,,,(COO)-vSym(C00) separat ion as means of assessing the mode of coordination oacetate groups to metals must only be used witextreme caution and cannot be . regarded auniversally valid.Visible SpectraDiffuse reflectance visible spectra have beemeasured for anhydrous cobalt(II), nickel(II)and copper(I1) acetates, the results and assignments being given in Table 111. The spectra of thcobalt and nickel salts are typical of octahedra

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i EDWARDS AND HAYWARD: TRANSITION METAL ACETATESTABLE 111

Visible spectra and magnetic momentsVisible spectra

p (B.M.) at -cetate room temp. Method Peak (cm-') Assignment

1.39DiamagneticDiamagnetic2.300.25

D.R."

D.R.

Sol.D.R.Sol.

0D.R.diffuse reflectance.bSol. solution i n acetic acid - acetic anhydride mixturtcoordination of these metals. The splitting of theband centered at 18 900 cm- ' for cobalt(I1)acetate has been noticed for many other octa-hedral cobalt(11) systems and attributed (1 l ) tovarious sources. In the present example, theshoulder could be tentatively assigned (1 1) to the'T,,(H) + 4T?,(F) transition, this having gainedintensity by m~xing ith the spin-allowed transi-tion. From these assignments for cobalt(I1)acetate 10 Dq, the Racah parameter B and thenephelauxetic ratio P3, have been calculatedusing the method of Underhill and Billing (12).10 Dq has been found to be 9250 cm- ' nd B 790cm-'. Using the free gaseous ion B value of 972cm-', p3 , is found to be 0.81.Calculations (12) of 10 Dq, B, and P,, fornickel(I1) acetate give the values 8420 cm-', 916cm-', and 0.89 respectively, using the free ion Bvalue of 1030 cm-'. These results place theacetate group close to water in both the spectro-chemical and nephelauxetic series (for water 10Dq = 8600 cm-' ; B = 925 cm-'; and P3, =0.88).Manganese(I1) acetate has only low intensityspin-forbidden bands in the visible region typicalof high-spin octahedral manganese(I1).Magnetic MeasurementsThe room temperature magnetic moments aregiven in Table I11 and are typical of octahedralcoordination for high-spin manganese(Il), high-

17 400 (sh)i 2 (sh)}17 400 (sh)i z (sh)}13 50014 60025 00013 70014 80025 00014 800 (br)14 300 (br)

spin cobalt(II), and nickel(I1). Manganese(I1)acetate has a moment virtuallv identical with the'spin-only' moment, there being no orbitalcontribution, but cobalt(I1) and nickel(I1) ace-tates have moments above the 'spin-only' valuesdue to orbital contributions, from the 4Tl,(F)ground state and 4Texcited states for cobalt, andonly from the 3T excited state and not from the3A,, ground state for nickel.The magnetic moment of copper(I1) acetateagrees with previous results (13) but the magneticmoment of dimeric molybdenum(I1) acetate (d4)is grossly reduced to 0.25 B.M., indicating a muchgreater metal-metal interaction than in the iso-structural copper acetate. This is in agreementwith X-ray results (8) which show a very shortMo-Mo bond length of 2.1 18L indicatingmultiple bonding, whereas the Cu-Cu bondlength in the dihydrate (7) is 2.6 8L in length,probably indicating weak 6 copper-copperbonding. The magnetic moment of cerium(II1)acetate is in the range expected for cerium(II1)compounds, ground state 'F,,,, whose magneticresults can be treated to a good approximation byL-S coupling and are essentially independent ofsurrounding ligands and stereochemistry.Thermal Decompositions (14)Thermogravimetric analysis (t.g.a.) results areshown in Table IV, the heating being carried outunder normal atmospheric conditions.

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CANADIAN JOURNAL OF CHEMISTRY. VOL 46. 1968TABLE IVTherrnogravimetric results

1st decomposition stage 2n d decomposition stage 3r d decomposition stage- %weight change %weight change %weight changeAcetate Compound Obsd. Calcd. Compound Obsd. Calcd. Compound Obsd. Calcd.-- --Mn(I1) MnO 56.8 59.1 M n z 0 3 52.8 54.4 Mn30 4 54.6 55.9Co(II) COO 54.8 57.7 c0203 50.5 53.1 c0304 51.9 54.6Ni(II).0.5CH3COOH Ni(CH,COO), 14.2 14.5 NiO 62.4 63.9Cu(I1) CuzO+ Cu 61.8 62.1 CuO 56.0 56.2ZnO 55.1 55.6MooJ 31.2 32.7La202C0, 41.5 41.5 La203 48.4 48.4CeO, 45.4 45.7- Ag 35.8 35.4 -- -aN o solid remains: MOOJvolatile.The t.g.a. of anhydrous lanthanum(II1) acetate Decompositions of zinc(I1) and molybdeagrees well with the results of Rao et al. (I), the num(I1) acetates lead to the formation of ZnOoxycarbonate, La20 ,C0, , being completely and MOO, at 350 and 280" respectively as solformed a t 480" and decomposing t o La 20 , at products, whereas from silver(1) acetate only th875". No evidence for the previously reported metal is produced by 280, there being no ev(15) oxyacetate was found. An oxycarbonate dence of oxide formation at lower temperaturesstage was not found in the t.g.a. of cerium(II1)acetate, the only product being the formation ofcerium(1V) oxide at 500". Both manganese@)and cobalt(I1) acetates decompose to the mon-oxides at ca. 350, which are then oxidizedcompletely to M 2 0 3stages by 600" followed byM3 04 stages at 950, although minor weight

change discrepancies for the individual stagesindicate that the decomposition stages overlap tosome degree, probably due to the heating rate ofthe apparatus. The hemi-solvate of nickel(I1)acetate loses the attached acetic acid completelyin air by 280" with the formation of the anhydrousacetate, but from the subsequent weight changethe acetate appears to decompose to a mixture ofthe metal and nickel(I1) oxide, the metal presentbeing finally oxidized to the oxide completely by600".Copper(I1) acetate starts to decompose at 230"to a mixture of copper metal and copper(1) oxide,both being completely oxidized to copper(I1)oxide by 535". No evidence for the formation ofcopper(1) acetate was found by this method, buta careful decomposition of copper(I1) acetate invacuo at 220" leads to the formation of a veryhygroscopic colorless sublimate of copper(1)acetate. Since little is known of the chemistry ofthis compound (16) it is, at present, being fullystudied.

1. K. C. PATIL,G. V. CHANDRASHEKHAR,. VGEORGE,nd C. N. R. RAO. Can. J. Chem. 46,257(1968).2. (a)K. NAKAMOTO,. FUJITA,. TANAKA,nd MKOBAYASHI.. Am. Chem. Soc. 79, 4904 (1957)(6 ) K. NAKAMOTO.nfrared spectra of lnorganland co-ordination compounds. John Wiley an d SonInc., New York. 1963.3. T. A. STEPHENSON,. BANNISTER,nd G. WILKINSJ. Chem. Soc. 2538 (1964).4. K. ITOHnd H. J. BERNSTEIN.an. J. Chem. 34170 (1956).5. K. J. WILMSHURST.J. Chem. Phys. 23,2463 (1955)6. J. N. VAN NIEKERK nd F. R. L. SCHOENING.ctCryst. 6, 609 (1953).7. J. N. VAN NIEKERKnd F. R. L. SCHOENING.ctCryst. 6, 227 (1953).8. D. LAWTONnd R. MASON.. Am. Chem. Soc. 87921 (1965).9. J. N. VAN NIEKERK,. R. L. SCHOENING,nd J. HTALBOT.cta Cryst. 6, 720 (1953).10. T. MOELLER,. GALASYN,nd J. XAVIER.. InorgNucl. Chem. 15, 25 9 (1960).11. A. B. P. LEVER nd D. OGDEN.. Chem. Soc. A2041 (1967).12. A. E. UNDERHILLnd D. E. BILLING. Nature, 21834 (1966).

13. M. KATO,H. B. JONASSEN,nd J. C. FANNChem. Rev. 64, 99 (1964).14. C. DUVAL.norganic therrnogravimetric analysi2nd ed. Elsevier Publ. Co., Inc., Amsterdam. 1963.15. V. N. MASKIMOVnd K. N. SEMENENKO.hNeorg. Khim. 3, 1468 (1958).16. A. ANGEL nd A. V. HARCOURT.. Chem. So1385 (1902).