Pertanika 3(2), 148-154 (1980)

Thermochemical measurement of theligand field splitting energies for hexaaquo complexes ofMn (II), Ni (II) and Zn (II) ions in the Tutton's salts.

M. BADRI, K. A. CROUSE and W. IBRAHIMDepartment of Chemistry, Faculty of Science and Environmental Studies,

Universiti Pertanian Malaysia, Serdang, Selangor, lVIalaysia.

Key words: Ligand field splitting energy - of hexaaquomanganese(II) - of hexaaquonickel(II) - of hexa­aquozinc(II) ;Tutton's salt - ligand field splitting energies - heat of solution j

Heat of solution - of Tutton's salts.

RINGKASAN

Beberapa hidrat garam Tutton Mn(II), Ni(II) dan Zn(II) yang bertenaga tinggi dan juga berhablurbiasa telah disediakan dan haba larutannya disukat. Nilai beza tenaga pada garam kontang telah diberikansebagai kJ/mol heksahidrat dan dianggapkan sebagai lODq bagi kompleks-kompleks tersebut. Nilai-nilaiini telah dibandingkan dengan tenaga pemecahan medan ligand bagi kompleks-kompleks itu dalam garamsulfat. Sisihan tenaga ini telah ditapsirkan sebagai akibat keujudan ion-ion ammonium dalam kekisi yangmungkin telah merendahkan ketumpatan elektron dalam molekul-molekul air kodinatan.

SUMMARY

Various high-energy modification and crystalline hydrates of Tutton's salts containing Mn(II), Ni(II)and Zn(II) have been prepared and their heats of solution measured. The energy difference of the anhydroussalts given in kJ/mole of hexahydrate have been assigned as 10Dqs for the complexes. The values obtainedhave been compared with the ligand field splitting energies of the complexes in the sulphate salts. The discre­pancies were explained in terms of the presence of ammonium ions in the lattice which may have reduced theelectron density on the coordinated water molecules.

INTRODUCTION

A number of investigations of the thermo­chemical measurement of the ligand field split­ting energy have been reported (Jamieson et al.,1965, Badri and Jamieson, 1970, 1977, 1979).For the aquo complexes, the technique involvesthe measurement of the heats of solution ofvarious crystalline hydrates and their corres­ponding high-energy modifications which wereprepared by dehydration in vacuo. In each case,the difference in their heats of solution at themonohydrate composition was expressed in kilo­calories per mole of the heptahydrate. Thevalues obtained agree quite well with 10Dq'sfound by the spectroscopic method. This agree­ment suggests the validity of the technique.

The present paper shall report on the recentmeasurements for the hexaaquo complexes in

148

the double salts of general formula ( H4)2S04'MS04 . 6H20 where M = Mn(II), Ni(II) andZn(II). The results obtained for the caseswhere M = Fe(II), Co(II) and Cu(II) havebeen reported elsewhere (Badri and Hussein,1979) but a brief mention shall be made for thepurpose of clarity and comparison.

According to their crystal structures, thewater molecules in the salts are coordinated tothe central metal ions (Huckel, 1950) to formdistorted octahedral complexes. For M = Fe(II)and Cu(II), (Bowers and Owen, 1955, Mont­gomery and Lingafelter, 1966, Webb et al.,1965) the average M-OH2 bond lengths. areshorter compared to the average M-OH2 bondlengths found in the sulphate salts FeS04 . 7H20and CUS04' 5H20 (Baur, 1964, Beevers andLipson, 1934). Thus one would expect that

M. BADRI, K. A. CROUSE AND W. IBRAHIM

lODq's for the complexes in the double salts tobe larger. Similarly, for M = Ni(II) and Zn(II),(Montgomery and Lingafelter, 1964, 1963) theaverage metal ligand bond lengths in the doublesalts are longer compared to those in the sulphatesalts (Beevers and Lipson, 1934, 1932). Thusone would expect the lODq's to be smaller.Unfortunately, for M = Mn(II) and Co(II),no detailed structures are available to us forcomparison.

The spectrum for the complex [Fe(H20)6]2+in the double salt ferrous ammonium sulphatehas been reported (Cotton and Meyer, 1964).The value for lODq was given as 9.6 X 103cm-1

which agrees very well with that found by thecalorimetric method (Badri and Hussein, 1979).The theomochemical technique was also appliedto the double salts of Cu(II) and Co(II). Eventhough the values obtained were of the rightorder of magnitude, the relationship of theirheat of solution data with the percent watercontent of the salts was found to differ signi­ficantly. It was found that in each case of thecrystalline hydrates of cupric ammonium sul­phate and cobaltous ammonium sulphate, theheats of solution fall on a single straight line ingoing from the hexahydrate to the respectiveanhydrous salts. These results, together withthe present ones, have led us to reconsider theinterpretation of the heat of solution data forthe ferrous ammonium sulphate and the heatsof transition between the high-energy and low­energy modifications of the hydrates of the doublesalts. The differences in the 10Dq's obtainedfor the complexes in the double salts and thesulphate salts shall be discussed in terms of thepresence of the ammonium ions in the outercoordination sphere.

EXPERIMENTAL

The starting materials used were the hexa­hydrates of the double salts. (NH4)2S04.ZnS04'6H20 was supplied by B.D.H., (NH4hS04'NiS04.6H20 was supplied by Hayashi PureChemical Industries, and (NH4)2S04.MnS04'6H40 was supplied by Wako Pure ChemicalIndustries. The water content of each salt waschecked by thermogravimetric analysis. Theresults agree well with the theoretical valuesexcept for the manganese(II) salt where theexperimental value is found to be 26.00% com­pared to the theoretical value of 27.60%. Theexperimental values were used for the purposeof subsequent calculations.

149

For the preparation of the lower crystallinehydrates, the hexahydrates were heated in anoven at various temperatures for various timeintervals. The high-energy modification hydrateswere prepared by dehydrating the hexahydratein vacuo at a pressure of less than I X 10-4 torrfor various time intervals. The percentage ofwater in each product was calculated by the lossin weight. The heat of solution was measuredimmediately.

The calorimeter used for the measurementof the heats of solution was made up of a plasticbucket of diameter 22 cm. A small plastic bottlewas cut to house a 300 ml silvered Dewer flask.The bottle was glued to the centre of the bucketand the space in between was filled with decanoicacid (m.p.31°C). It was then covered with awooden cover with two small holes made toaccommodate thermocouple reference junctions.The Dewer flask was fitted with a cover withholes to accommodate the hot junctions ofthermocouples, a stirrer, a 5-ohm heater madeof chromel wire and a glass funnel for sampledelivery. About 200 ml of distilled water wasused as solvent. The whole assembly wasimmersed in a water bath thermostatted at 31°C.The thermocouples were connected to a micro­voltmeter with an output to a recorder.

RESULTS

The heats of solution obtained are tabulatedin Tables I, 2 and 3 for double salts of Mn(II),Ni(II) and Zn(II) respectively and are plottedas a function of percent water in the salts inFigs. I, 2 and 3. In each case, the heats ofsolution for the crystalline hydrates fall on asingle straight line BA in going from the hexa­hydrate to the anhydrous salt. Similarly, theheats of solution for the high energy modificationhydrates fall on a higher heat of solution line BC.

Mn(II)Using the method of least squares, the

equations for the lines BA and BC in Fig. 1have been found to be

YBA = 139 - 8.95x

YBC = 245 - 13.0x.

From the line BA, the heat of solution for thehexahydrate of manganese(II) ammonium sul­phate has been estimated to be -149 Jig, at D.The line DE has been drawn parallel to the lineBe with the equation

YDE = 251 - 13.0x

LIGAND FIELD SPLITTING ENERGIES

TABLE 1

Heats of solution for the hydrates ofmanganese ammonium sulphate

Heats of solution for the hydrates ofnickle ammonium sulphate

Percent waterin salt

o0.092.183.775.377.60

12.0014.6718.5326.000.03.387.47

10.6613.7717.1717.96

Percent waterin salt

0.02.652.923.654.434.695.295.446.366.827.939.05

10.5914.7515.0315.1916.5817.6019.4223.5525.5127.1027.1927.343.464.72

13.7117.4120.1720.4123.0825.2426.67

Heat of solutionJig

149135115102

88.867.331.822.128.194.6

242203152110

63.120.511.5

TABLE 2

Heat of solutionJig

265228236214207205191191175172165156131

61.154.462.439.731.611.654.683.4

-105- 92.5-106

329301155

87.939.942.122.569.5

- 91.5

Method ofPreparation

heat at 122°e110°C

88°C88°e83°e83°e83°e83°e70°C

hexahydratevacuum

Method ofpreparation

heat at 226°e226°e226°e2000 e159°C159°e167°e153°e167°e167°e159°C153°e153°e132°e159°e136°e136°e132°e136°e1200 e1200 e1100e1200 e

hexahydratevacuum

150

The line DE represents the highest possible heatof solution line for high-energy modificationhydrates of manganese ammonium sulphate withthe hexahydrate as the starting material. Thusthe highest possible heat of transition for theanhydrous salt is 112 Jig which is equivalent to3.66 X 1Q3cm-1 per mole of hexahydrate.

Ni(II)

The equations for the lines BA and BC In

Fig. 2 are

YBA = 264 - 13.4x

YBC = 394 - 17.9x

The heat of solution for the crystalline anhydroussalt at A is 264 Jig and that for the high-energymodification anhydrous salt at C is 394 Jig.Thus the heat of transition at the compositionof the anhydrous salt is 130 Jig which is equi­valent to 4.30 X I03cm-! per mole of hexahydrate.

Zn(II)

The equations for the lines BA and BC III

Fig. 3 are

YBA = 167 - 9.83x

YBC = 263 - 13.3x

The heat of solution for the crystalline anhydroussalt at A is 167 Jig and that for its high-energymodification at C is 263 JIg. Thus the heat oftransition from C to A is 96 Jig anhydrous saltwhich is equivalent to 3.23 X 1Q3cm-1 per moleof hexahydrate.

DISCUSSION

It is evident that incorporation of ammoniumsulphate into the lattice of anhydrous metal(II)sulphate stabilizes the latter. The amount ofstabilization can be calculated from the enthalpychanges for the following reactions:

MS04·(NH4hS04 + aq ~ [M(H20)6]2+ (&Q)+ 2NHt(&Q) + 2S0~-(&Q) [1]

6. HI = heat of solution for the anhydrousdouble salt.

6. H2 = heat of solution for ammoniumsulphate

M. BADRI. K. A. CROUSE AND W. IBRAHIM

.6H3 = heat of solution for the anhydrousmetal sulphate.

By adding 3 to 2 and subtracting 1.

same. So the ligand field stabilization energyresulting should be about the same and thereforethe amount of the extra-stabilization in the doublesalt would be very small, if any. However evenin the cases of Mn(II) and Zn(II) this stabili­zation is considerable. It is well known thatthere is no ligand field stabilization energy inthese two systems. Thus it is more likely thatthe extra-stabilization is the result of electro­static interactions between ions in the anhydrousdouble salt.

TABLE 3

Heats of solution for the hydrates ofzinc ammonium suI phate

The various values of .6 H obtained this wayare given in Table 4. All systems show stabili­zation except for Fe(II) which shall be discussedlater.

o

In the anhydrous salt, it is possible thatthe metal(II) ions are surrounded by six oxygenatoms from the sulphate ions. Since the sulphateions are bigger than the metal ions, the crowdingmay result in a considerable repulsion. Ammo­nium ions in the double salt would act as shieldsto reduce the repulsion either by virtue of theirpositive charges or more likely by forming anextensive hydrogen bond system throughout thecrystal. Hydrogen bonding would reduce theintensity of the negative charge on the sulphateions and therefore the mutual repulsion wouldalso be reduced. The salt thus becomes morestable.

Fig. 1 Heats of solution for the hydrates ofmanganese ammonium sulphate.

10 20PERCENT WATER IN SALT

a: 200...ll.

...o

zQ...=>...Joen

o

100

-100

Method ofpreparation

heat at 123°C12FC111°C111°C

97°C97°C

111°C111°C

97°C111°C111°C

87°Chexahydrate

vacuum

168127

39.035.117.127.739.440.942.747.454.368.397.8

-264257238188167165138103

52.349.528.912.718.2

Heat of solutionJig

0.04.31

13.3913.9718.5719.2321.1021.4421.7622.0922.7124.4426.89

0.360.571.915.807.077.369.06

12.3916.0916.1817.9420.6720.96

Percent waterin salt

While a rigorous analysis is difficult to givehere because of lack of information about thecrystal structures, a qualitative picture could bepresented. This stabilization might be due tothe ligand field of the coordinated sulphate ionsin the crystal of the anhydrous salt. However,in both cases of metal(II) sulphates and thedouble salts, the coordinated species are the

Since the heat of solution for ammoniumsulphate is 9.91 X lO-J kJ{mol (Mellors, 1927)

.6H = .6H3 - 9.91 X 10-3 - .6H1

151

LIGAND FIELD SPLITTING ENERGIES

TABLE 4

Heats of solution for some anhydrous metal(lI) sulphates and double salts.

MS04 (NH4hS04 . MS04 Stabilizationkllmole kllmole kllMole

------------ -----------M = (NH4+h 9.91 X 10-3

Mn 61.35 39.36 21.99

Fe 62.31 76.74 -14.43(39.79) (22.51)

Co 92.76 70.74 22.44

92.42 75.74 16.68

Cu 63.41 55.87 7.54

Zn 72.39 49.02 23.37

In all cases, except for Fe(II), the crystallinehydrates have their heats of solution falling ona single straight line in going from the hexa­hydrate to the anhydrous salt. This is contraryto what has been found for the hydrates ofmetal(II) sulphates (Jamieson et al., 1965) andthe hydrates of Fe(II) double salt (Badri andHussein, 1979). In each of these cases there isa change in the slope of the heat of solution linefor t he crystalline hydrates in going from thestarting material to the anhydrous salt. ForFe(II) double salt, an increase of the slope occursat the dihydrate composition whereas in the caseof metal(II) sulphate the increase occurs at themonohydrate composition. In the latter, theincrease in the slope was attributed to the factthat the water molecule in the monohydrate isanionic. The heat of solution line is a measureof the strength of the chemical bonds betweenthe water molecules and the ions in the salt,hence the change in the slope. Thus we canconclude that all the water molecules in thedouble salts are primarily held by coordination.Even though their crystal structures indicatethe possibility of hydrogen bonding, the extentis probably limited by other factors to be dis­cussed later.

In Table 4, the gtabilization energy due tothe formation of ferrous ammonium sulphatefrom iron(II) sulphate and ammonium sulphateis a negative quantity. However, if the heat ofsolution line for the crystalline materials wereextrapolated to the anhydrous composition, thevalue of heat of solution for the anhydrous saltis obtained as 39.79 kJ/mole (bracketed). Withthis, the stabilization energy would then be22.51 kJ/mole. This value conforms to the

152

other values obtained for the other double saltsand therefore perhaps it should be used for thecalculation of the ligand field splitting energyfor hexaaquoiron(II) in the double salt. Withthis, the recalculated lODq is 10.4 X 103cm-1

which is the same as those found for the complex

400 C

e:; 300a..

(/)

'"~::>o...,

200

z0~::>~

1000(/)

u.0

!i'" 0:r

-100

10 20PERCENT WATER IN SALT

Fig. 2 Heats of solution for the hydrates ofNickelammonium sulphate.

M. BADRI, K; A. CROUSE AND W. IBRAHIM

TABLE 5

10 20

PERCENT WATER IN SALT

Fig. 3 Heats of solution for the hydloates of Zincammonium sulphate.

in aqueous solution (Orgel, 1965) and m thesulphate salt (Jamieson et al., 1965).

-------Metal Sulphate salt Double salt

X103cm-l Xl03cm-1-------- ---------

Mn 7.7 3.66

Fe 10.4 10.4

Co 9.1 9.43

Ni 8.4 4.30

Cu 11.6 9.37

Zn 8.3 3.23

BADRI, M. and JAMIESON, J.W.S. (1977): Thermo­chemical measurement of the ligand field splittingenergies for hexaammine complexes of Mn(lI),Fe(lI) and Zn(Il). Can. J. Chern. 55, 3530-3535.

BADRI, M. and JAMIESON, J.W.S. (1979): Thermo­chem!cal measurement of the ligand .field splittingenergies for hexaaquocopper(Il) and hexaammine­copper(Il) ions. Can. J. Chern. 57, 1926-1931.

BADRI, M. and HUSSEIN, A. (1979): Thermochemicalmeasurement of the ligand field splitting energies

REFERENCES

Ligand field splitting energy for hexaaquocationsby calorimetric method

splitting of the d-orbitals, perhaps in the twosituations discussed, the effect is minimal. Thisis because in the hydrated form, the metal ionsin both situations are enveloped fully by theoctahedrally coordinated water molecules. Thepotential felt by the metal ions due to the sulphateions in the outer coordination sphere is probablytoo weak to transform it into a more sphericalor any higher-order symmetry.

Thus it is more likely that in the hydratedsalt, the presence of ammonium ions may havecaused the expansion of the lattice and in thismanner reduced the extent of hydrogen bondinCTbetween the coordinated water molecules andthe sulphate ions. Even though in terms ofcrystal energy this reduction is compensatedfor by the reformation of the hydrogen bondsbetween the ammonium ions and the sulphateions, the reduction of the hydrogen bonding onthe water molecules has the effect of reducingthe electron density on the oxygen atoms. Thiswould certainly result in the transformation ofthe water molecules into weaker ligands andhence a weaker ligand field. Thus 10Dq forhexaaquo complexes in the double salts shouldbe smaller compared to those in the sulphatesalts as reported here for Mn(II), Ni(II) andZn(II).

C

::E<tG::<:>

200a:...A-

li>.....J~

0-,

100

z0;:~..J0I/)

a"-0

~...:r:

-100

The results for Cu(II) and Fe(II) point toa more fundamental difference in the electricalenvironments around the metal ions in thesulphate salt and the double salt. While thesymmetry of the ligand field can influence the

The values of 10Dq obtained for Mn(II),Ni(II) and Zn(II) in the double salt are con­siderably smaller than those obtained for thehexaaquocomplexes in the sulphate salts andaqueous solution even though they are of thesame order of magnitude. For ease of com­parison they are tabulated in Table 5. Thevalues for Fe(II), Cu(II) and Co(II) are quiteconsistent with those found in sulphate salts.In fact for Fe(II) and Cu(II) they should behigher because of shorter M-OH2 bond lengths.In the case of Co(II) we can thus assume thatthe bond lengths are shorter and similarly inthe present three cases the bond lengths arelonger compared to those present in the corres­ponding sulphate salts.

153

LIGAND FIELD SPLITTING ENERGIES

ORGEL, L.E. (1960): An introduction to transitionmetal chemistry: Ligand field theory. Methuen.London. p. 46.

MONTGOMERY, H. and LINGAFELTER, E.C. (1966):The crystal structure of Tutton's salt. III. Copperammonium sulphate hexahydrate. Acta Cryst.20, 659-662.

MELLORS, J .W. (1927): A comprehensive treatise oninorganic and theoretical chemistry. Longmans,Green and Co. New York. Toronto. Vol. II.p.664.

MUHAMMAD, B. and JAMIESON, J.W.S. (1970): Ligandfield splitting energies of the hexaammine cobalt(II)ion and the hexaammine nicke1(II) ion measuredby a calorimetric method. Can. J. Chern. 48,2177-2181.

E.C. (1963):salt. I. ZincActa Cryst.

MONTGOMERY, H. and LINGAFELTER,The crystal structure of Tutton'sammonium sulphate hexahydrate.17, 1295-1299.

MONTGOMERY, H. and LINGAFELTER, E.C. (1964):The crystal structure of Tutton's salt. II.Magnesium ammonium sulphate hexahydrate andnickel ammonium sulphate hexahydrate. Actacryst. 17, 1478-1479.

Rou, V.G. and KURKOTOVA, E.N. (1973): Cryst.Structure of Co(NH4h(S04h.6HzO. SovietPhys - Cryst. 17, 997-998.

WEBB, M.W., K'l.Y, M.F. and GRIMES, N.W. (1965):The Structure of ammonium copper sulphate,(NH4hCu(S04h·6HzO. Acta Cryst. 18, 740­742.

for hexaaquometal(II) ions in the Tutton's salts.Malaysian Institute of Chemistry Proceedings onChemical Research in Malaysia (in press).

BAUR, W.H. (1964): The crystal chemistry of salthydrate.III. The determination of crystal struc­ture of FeS04.7HzO (melanterite). Acta Cryst.17,1167-1174.

BEEVERS, C.A. and LIPSON, H. (1932): The crystalstructure of iS04·6HzO. Z. Krist. 83, 123-135.

BEEVERS, C.A. and LIPSON, H. (1934): Crystal struc­ture of CuS04·5HzO. Proc. Royal Soc. (London),A 146, 570-582.

BOWERS, K.D. and OWENS, J. (1955): Repts. Prog.Phys. 18, 304.

COTTON, F.A. and MEYERS, M.D. (1960): Magneticand spectral properties of the spin free 3d6 systemsiron(II) and cobalt(III) in cobalt(III) hexafluorideion: Probable observation of dynamic John-Tellereffects. J. Arnei' Chern. Soc. 32, 5023.

HUCKEL, W. (1950): Structural C~emistry of In­organic complexes. Vol. 2. Elsvler. New York.p.920.

JAMIESON, J.W.S., BROWN, G.R., GRUENER, D.W.,PEILUCK, R., and LA MONTAGNE, R.A. (1965):Possible calorimetric method for measurementof the ligand field splitting energies for the hexa­aquo cations of some transition elements. Can.J. Chern. 43, 2148-2156.

JAMIESON J.W.S., LA MONTAGNE, R.A., PATTERN,B.A.' and BROWN, G.R. (1965): High energymodification of the hydrates of zinc sulphateCan. J. Chern. 43, 3129-3132.

(Received 5 September 1980)

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