J. lnorg. Nucl. Chem., 1966, Vol. 28, pp. 447 to 454. Pergamon Press Ltd. Printed in Northern Ireland

THE STABILITIES OF THE 9-MOLYBDOMANGANATE (IV) AND 9-MOLYBDONICKELATE (IV) IONS

L. C. W. BAKER Georgetown University, Washington 7, D.C.

and

T. J. R. WEAKLEY Queen's College, Dundee, Scotland

(Received 15 June 1965)

Abstract--The isostructural hetero-9-molybdat¢ ions are stable monomers at high concentrations in saturated aqueous Na~SO~. They are only slightly solvated. Some decomposition occurs in more dilute solutions. (MnMogOsIP- is decomposed above pH 5; (NiMosOsiP-, above pH 5.5. Unstable H6(MnMo0Oa2)hasbeenpreparedinaqucoussolution. Successivc pK's are close together. Comments are made on the spectra of the two complexes.

THE hetero-9-molybdate anions have the general formula [MMogOa2] e-. The Ni (IV) and Mn (IV) complexes were first reported by HALL, (1) together with a Co (IV) complex whose existence has since been disproved. (2) The [MnM09032] 6- ion is always formed in any at tempt to prepare molybdomangana tes (III) or (IV). (2'3'4) The a m m o n i u m salts are i somorphous ; they crystallize as plain rhombohedra and also as rhombohedra with addit ional faces conferring left- and right-handedness. An X-ray structure determination (5) has shown that the [MnMogO32P- ion has D3 symmetry and is based on a M n O e octahedron. The nickel complex is diamagnetic, (e) as should be the case for a d 6 ion in a strong-field octahedral environment. The spectra o f bo th complexes have been reported. (4'7"s) The present work was undertaken to investigate the stabilities o f these complex ions towards dissociation, hydrolysis and racemization.

E X P E R I M E N T A L

The ammonium salts were prepared by a modification of Hall's method, (1) using ammonium peroxydisulphate or (for the manganese complex only) hydrogen peroxide as oxidant. A large excess of oxidant was undesirable in the preparation of the nickel complex, which is attacked by the decom- position products of S2Od- ion. The best yields were obtained by adding a solution of MnSO4 or NiSO4, containing the oxidant, to a slight excess of 10 ~ (NH4),MoTO,4 solution at 95°; the mixture was boiled and stirred for five minutes, and quickly filtered and cooled. The orange-red (Mn) or red-black (Ni) crystals were recrystallized three times from hot water (70°); [NiMogOsd e- ion could, however, only be freed from small amounts of cationic nickel by passing a solution through a cation- exchange column in the sodium form. The oxidation states of the betero-atoms were confirmed by

(t) R. D. HALL, J. Amer. Chem. Soc. 29, 692 (1907). (2) G. A. TSlGDINOS, Ph.D. thesis, Boston University (1961). (s) p. Soucvo, Y and A. TCI~aC~AN, Ann. Chem. 1, 249 (1946). (4) p. SoucrLcv and R. SCH.~,L, Analyt. Chim. Acta 3, 1 (1949). (5) D . P . SHOEMAKER, J . L . T. WAUGFI and L. PAULr~G, Acta Cryst. 7, 438 (1954). (s) p. RAY, A. Bm~DtrPa and B. SARMA, J. Indian Chem. Soc. 25, 51 (1948). (7) y. SmMugA, H. ITO and R. TSUCHmA, J. Chem. Soc. Japan 75, 560 (1954). (s) C. K. JOROENSEN, Acta Chem. Scand. 12, 1539 (1958).

447

448 L . C . W . BAKER and T. J. R. WEAKLEY

reducing the complex with an excess of oxalic acid or of acidic As (III) solution and back-titrating. Solutions of the sodium salts, and of 9-molybdomanganic acid, were prepared by ion exchange, ~D~

The ammonium salt was recovered from the acid in low yield by adding dilute ammonia to pH 4.5, and saturating with ammonium nitrate. Its X-ray powder pattern was identical with that of a sample prepared as described above.

Solutions for viscosity measurements were concentrated in cellophane bags in an air current at room temperature. Viscosities were measured with an Ostwald viscometer at 25 ° -4- 0.1°; kinetic energy and density corrections were applied.

Transition-point temperatures in fused NasSO~.10H~O were measured to +0.001 ° with a Beck- mann thermometer in a vacuum-jacketed 20 × 3 cm tube. ~'x°~ Mixtures were prepared by adding 15.0 g anhydrous Na~SO~ to 20 ml portions of solutions of the sodium salts, analysed for molyb- denum and the hetero-element.

Instruments employed included a North American Philips recording X-ray powder diffractometer; Beckmann DU and Unicam SP.700 spectrophotometers for solution work; a Hilger and Watts H.700 spectrophotometer with powder reflectance attachment; and a Leeds and Northrop 7662 pH meter. Potentiometric titrations were performed as described elsewhere. ~m

Attempts were made to resolve both the complex anions. The methods tried were: (1) Selective absorption on optically-active quartz. ~m (2) Selectively reducing one enantiomorph by D-tartrate and D-antimonyl tartrate. (3) Inducing a first order asymmetric transformation cxs~ in the presence of D-camphorsulphonate

ion. (4) Growing large single asymmetric crystals of the ammonium salts. (5) Forming diastereoisomers with optically active cations (o- and L-1:2-diammonium propane,

D- and L-~-phenylethylammonium, D-Coena 8+, cinchoninium). These ions, however, form very insoluble salts with heteropolyanions, from which it is difficult to displace the active cation.

The optical rotation of each test solution (about 10 -8 M) was examined at several wavelengths between 400 m/~ and 650 in#, using a modified Beckmann DU spectrophotometer similar to that described by KmSCHNER. tt4~ Rotations of 0"004 ° could be detected.

RESULTS

Crys ta l i s o m o r p h i s m and chemical analysis suggests tha t the he te ro-9-molybda te ions are i sos t ructura l . This is conf i rmed by the ident ica l l ine pos i t ions and very s imi lar intensi t ies in the X- ray powder d i ag rams o f the a m m o n i u m salts.

Bo th complexes are reduced by iodide , arsenite, oxa la te or sulphite in neut ra l or sl ightly acidic solut ion. M n ~+ and [NiMo6024H6] 4- ions are formed. The la t ter was p rec ip i t a ted as the a m m o n i u m salt, t2~ which was identif ied by its X- ray powder pa t te rn . The Ni (IV) complex also oxidizes hydrogen peroxide and (in acid medium) water . The kinetics o f the la t ter process are being studied. Consequent ly , 9 -molyb- donickel ic acid canno t be p repa red ; bu t the very soluble 9 -mo lybdoma nga n i c acid is sufficiently Stable for an aqueous so lu t ion to be p repa red by ion-exchange.

~g~ L. C. W. BAKER, B. LOEV and T. P. McCUTCHEON, J. Amer. Chem. Soc. 72, 2374 (1950). tl0~ L. C. W. BAKER, M. T. POPE and G. A. TSmDrNOS, Abstracts, Div. of Inorg. Chem., Amer. Chem.

Soc. meeting, Boston, April 1959. cxt~ L. C W. BAKER, G. A. GALLAGrmR and P. T. McCuTcrmora, J. Amer. Chem. Soc. 75, 2493 (1953);

L. C. W. BA~:ER, G. FOSaZR, W. TAN, F. SCHNOLNICK and T. P. McCtrrcHEON, Ibid. 77, 2136 (1955).

c~2~ See references given by WmKms and WmLt~MS in LEWIS and WmKrNs' Modern Coordination Chemistry, p. 205, Interscience, New York (1960).

ltnl F. BASOLO and R. B. PEARSON, Mechanisms of Inorganic Reactions, p. 285, J. Wiley, New York (1958).

~x4~ S. Kmscm~R, A. J. SONr, mSSA, D. C. BrIATNAOAR and D. MoYu, Abstracts, Div. of Inorg. Chem., Amer. Chem. Soc. meeting, New York, Sept. 1960.

The stabilities of the 9-molybdomanganate (IV) and 9-molybdonickelate (IV) ions 449

The use of fused-hydrate cryoscopy to ascertain the formula weights of hetero- polymolybdates has been described previously, tx°~ Figure 1 summarizes the results obtained in this work. The molarities are those of the aqueous solutions to which anhydrous Na2SO 4 was added (previous section). The limiting value of AT/c (1.93 degree-litre-mole -x) obtained by extrapolation from the higher concentrations is that found for sodium salts of known formula weight. The increase in AT/c at low concentrations is comparable with the experimental error and probably exaggerates the degree of dissociation of the complex anions.

2.2

2.0

1.8

1'6

o ________.<r- ~ o -

° - - q - ~ - - ~ - - ~ ° ~ .

o h4n eNi

r r I ~ r 0 0 '02 ' 0"()4 0 ' 06 0"08 0"10

c (molar)

FIG, 1 . - - F u s e d N a ~ S O 4 - 1 0 H 2 0 c r y o s c o p y ; s o d i u m h e t e r o - 9 - m o l y b d a t e s .

The pH titration curves for the Mn complex are shown in Fig. 2. The initial pH for the ammonium salt cannot be ascribed entirely to hydrolysis of the cation, but must imply that some attack on the anion by solvent has occurred. The species first formed cannot be identified, but the overall equation for hydrolysis would be:

7MnMogOa2 ~ + 6H20 -+ 7MnO2 + 9Mo70~46- + 12H +

(this reaction is reversible; if freshly-precipitated MnO2 is digested with hot para- molybdate, the [MnMogOz2] 6- ion is formed). The plateau in each titration curve shows that the anion is largely decomposed above pH 5. The end point at 12 OH- per anion is that expected for complete degradation to MnOz and MoO4 ~-.

The neutralization curve for the acid varies slightly from one preparation to another. Evidently some decomposition takes place. Two typical curves are shown; these indicate that successive dissociation constants are all in the range 10-2-10 -z M.

The first end point is observed after the addition of 5-5.5 (not 6) equivalents of base,

possibly because degradation of the anion sets in before neutralization is complete. The titration curve for (NH~)rNiMogO32 (not shown) is similar to that for the

manganese compound, except that base attack occurs around pH 5.2-5.5. Viscosity data are shown in Fig. 3. ~Tsp is the fractional increase in viscosity caused

by the solute. For 6-molybdocobaltic acid ~1~) the equation ~16~

~Tsp = Ac 1/~ + Bc

t15~ L. C . W. BAKER a n d M. T. POPE, d. Phys. Chem. 63, 2083 (1959).

~16~ G. JONES a n d M. DOLE, d. Amer. Chem. Soc. 51, 2950 (1924).

12

450 L . C . W . BAKES and T. J. R. WV, AKLeY

f l o

8

pH

6

2

o ~ ; ~'~ ~6 20 0 H-~Mn

Fxo. 2 . - -pH titrations. (1) (NH4)s[MnMosOn], 1"10 x 10 -s M. (2) free acid, 3.73 X 10 -s M; (3) free acid, 2.91 × 10 -s M.

1.o

0.8

0-6

~sp

0.4

0.2

"

i i i

0.1 0.2 0.3 Molarity

FIG. 3.--Viscosity measurements. Ordinate: specific viscosity. (1) He[IVlnMo.Oss]; (2) sodium salts; (3) limiting slope, curve 2.

The stabilities of the 9-molybdomanganate (IV) and 9-molybdonickelate (IV) ions 451

holds good, with A equal to zero; but the data for sodium 9-molybdomanganate and 9-molybdonickelate, and for 9-molybdomanganic acid, cannot be fitted to this equation. The acid is somewhat decomposed; but the limiting slope of the plot of ~?sp against c for the sodium salts can be used to estimate the hydrodynamic volume of the anions. We assume that at low concentrations

~Tsp = B c

where B, according to GURNEY, t17) is the sum of contributions from individual ions.

C) ~

50 40 30 20

Fro. 4.--Spectrum of(NH4)dMnMogOss]. Abscissa, frequency (103 cm-1). (1) powder reflectance; ordinate, absorbance (arbitrary units). (2) solution; ordinate, log e.

In this instance, B = 2.0 litre-mole -I. For Na +, B = 0.086 litre-mole-1; whence for the heteropolyanions B = 1.5 litre-mole -1. It has been shown cxS,~s~ that for ions of this type the Einstein viscosity equation may then be applied to obtain the hydro- dynamic volume. For the hetero-9-molybdate ions this volume is approximately 100 A a, corresponding to a diameter of 12-12.5/~.

The solution and diffuse reflectance spectra (Figs. 4 and 5) indicate that the species present in the crystals largely persist in aqueous solution. The complete spectrum of the manganese complex, showing the "ruby" band at 14,300 em -1 (700 nap) has not

,1" R. W. GURm~Y, Ionic Processes in Solution, McGraw-Hill, New York (1953). ~ts~ T. Ktmucs~v, A. M. SAROV.SON and B. D. W~'T, d. Phys. Chem. 61, 1567 (1957).

452 L.C.W. BAKER and T. J. R. WrAKLEY

been published before. The log ~ values in the ultra-violet are only approximate, as both complex anions show slight deviations from Beer's law (both in pure water and in buffered solution), the sign of the deviation depending on the wavelength. The maximum near 25,500 cm -x is seen when concentrated solutions are examined in 0-1 cm ceils, but is masked in dilute solutions by the steeply-increasing absorbance in the ultra-violet.

L , ~0 ' 3'0 ~ ' 5 0 4 20

FIG. 5.--Spectrum of (NH4)s[NiMogOs2]. Abscissa, frequency (10 s cm-1). (1) powder reflectance; ordinate, absorbance (arbitrary units). (2) solution; ordinate, log ~.

The form of the spectrum of the Ni complex in the visible region is little affected by the pH in sodium acetate-acetic acid buffers, but the intensity falls off with time as water is oxidized by the Ni (IV). In contrast, the form of the spectrum of the Mn complex depends markedly on the pH (Fig. 6) but shows little change with time. At higher pH's only a shoulder is seen, and hydrous MnOz is slowly deposited.

The spectrum of H6[MnMogO32] is similar to that of the ammonium salt at pH 2.9. As the pH is raised, by the cautious addition of sodium acetate solution, the absorbance increases until MnO~ appears, but the characteristic double peak is not seen.

Attempts to resolve the complexes were all unsuccessful. In experiments where the test solution contained an optically-active species with a normal dispersion curve there was no sign of a superimposed Cotton effect that could be associated with the

The stabilites of the 9-molybdomanganate (IV) and 9-molybdonickelate (IV) ions 453

absorption bands of the complexes in the visible region. The low concentrations-- about 10 -8 M--of the strongly-absorbing complexes must be remembered.

0 . 5

0 - 4

0 . 3

0 . 2

0.1

O ~ ~ 400 500 600 rn/~

FIG. 6.--Spectrum, (NH4)8[MnMogOs2]; 1.19 x 10 -3 M in acetate buffers. Ordinate: optical density, pH: (1) 4-29 and 4"64; (2) 3"96 and 4.91; (3) 3.78; (4) 3.51; (5) 3.30;

(6) 2.90.

D I S C U S S I O N

The effective diameter estimated for the anions in solution is only a little greater than that of the bare anion (I 1-2-11.5 A) which approximates to a sphere. Evidently the hetero-9-molybdate ions, with a -6 charge but a low surface charge density, are only slightly solvated, as are the 6-molybdocobaltatd TM and 12-tungstosilicate t18~ ions.

The results of fused-hydrate cryoscopy, the spectral measurements, and the pH titrations together show that the complexes are stable at 0-1-0.01 M concentration in saturated Na~SO~ solution (although SO4 *- ion is very slightly basic); but that some decomposition occurs in more dilute solution. We regard deviations from Beer's law as arising from attack by solvent, rather than from ion-pairing, as salts of other highly-charged heteropolyanions (e.g. [CoW12040] 6-) obey Beer's law accurately. It is unfortunate that the immediate products of hydrolysis cannot be identified. It appears that the Ni (IV) complex, although more easily reduced, is more resistant to base hydrolysis than is the Mn complex. The latter is also decomposed in strongly acidic solution; 9-molybdomanganic acid always undergoes some decomposition during its preparation, with the formation of species which decompose directly to MnO2 as the pH is raised so that the [MnMogOa2] ~- ion cannot be reformed quantitatively.

454 L.C.W. BAKER and T. J. R. WF_AKLEY

The spectra of Ni (IV) complexes have been little studied, and more data are required for purposes of comparison before transitions can be identified with confi- dence. The absorption band at 17,600 cm -1 in the 9-molybdonickelate spectrum (Fig. 5) probably arises from the transition 1Alg to 1T2g.

The spectrum of the 9-molybdomanganate ion requires some comment. The maximum at 21,300 cm -1 (Fig. 4) has been assigned is) to the first spin-allowed transition for a d a ion in octahedral co-ordination, 4A2g to ~T2g. The maximum at 25,500-26,000 cm -1 may be the second spin-allowed band, excited state 4T1~ (F). If this is so, then a value of 370-420 cm -1 is obtained for the Racah parameter B, using the expressions quoted by JORGENSElq (19) for the energies of the spin-allowed transitions. The "neph- elauxelic ratio" fl is then 0.38 to 0.43 implying considerable covalency. The peak at 20,600 cm -1 has not yet been assigned. The Tanabe-Sugano diagram for d 3 suggests that it is the transition to 2T~g (which is within the configuration t2~, and should therefore give rise to a narrow band); but in that case, the means by which the spin-forbidden transition gains intensity is not obvious. The transitions to 2Eg and ~T~g at 14,300 and 15,100 cm -1 are very much weaker. The latter transitions appear very weakly at 14,500 and 14,900 cm -1 in the spectrum of 6-molybdochromate (III) ion, 129) but no subsidiary peak is seen near the 4T2~ band; this anion is also based on a MO6 group. It is therefore possible that the split peak at 21,000 cm -~ in the spectrum of 9-molybdomanganate results from the appreciable distortion of the MnOe group about the three-fold axis of the anion, which is indicated by the structural data. (5) Crystals of the ammonium hetero-9-molybdates are strongly dichroic, with colour changes from yellow to orange (Mn) and from red to almost black (Ni); maximum absorption occurs when the electric vector has a large component along the three-fold crystal axis. Polarized spectra at low temperatures should be informative.

The extinction coefficients for the spin-allowed bands of lowest frequency are 360 and 900 for the manganese and nickel complexes respectively. Even the spin-for- bidden band at 14,300 cm -~ in the spectrum of the manganate has an extinction co- efficient of about three. The nephelauxetic ratio, above, indicates that in the ground state the metal d-electrons are considerably delocalized and are partly concentrated on the polymolybdate "ligand"; we therefore suggest that the high intensities arise because the electronic transitions are not pure d-d ones but involve some charge transfer (from ligand to metal).

The failure to resolve the complexes and obtain ORD curves is disappointing, but may result from the fact that the large anions, though their point symmetry is Da, are nearly spherical (as a ball model reveals); therefore the enantiomorphs may interact very similarly with optically-active ions or adsorbents.

c1,~ C. K. JORGENSEN, in Progress in Inorganic Chemistry, Vol. 4, p. 96. Interscienc¢, New York (1962). tso~ T. J. R. WEAKImY, Unpublished work (1965).