Lanthanide complexes with bis(2′-quinolyl)-2,6-pyridine

J. inorg, nucl. Chem., 1976, Vol. 38, pp. 25%263. Pergamon Press. Printed in Great Britain

LANTHANIDE COMPLEXES WITH BIS(2'-QUINOLYL)-2,6-PYRIDINE S. GURRIERI, S. MUSUMECI, E. RIZZARELLI and A. SEMINARAt Istituto di Chimica Generale, Universit~ di Catania, viale A. Doria 8, 95125 Catania, Italy (Received 14 April 1975) Abstract--Complexes of lanthanide (III) nitrates, thiocyanates and perchlorates with the tridendate ligand, bis(2'quinolyl)-2,6-pyridine are reported. The molar conductivity values in acetonitrile-benzene mixture (80-20) and IR data indicate that the nitrate and thiocyanate complexes are non electrolytes and the perchlorate 1: 3 electrolytes. The electronic spectra of the Pr, Nd, Ho, and Er complexes show enhancement of intensities for the hypersensitive bands; moreover, except in the case of the ErIn complexes, where a very slight red shift is observed, other complexes show the expected nephelauxetic effect. with 15 ml of ethyl orthoformate and refluxed for 6hr with stirring; a solution of th6 ligand (2 mmoles) in hot benzene (70 ml) was added. The complex precipitated gradually over a period of 0.5 hr; it was isolated by filtration, washed with hot benzene and dried in vacuo for a week at room temperature. Measurements. The 1R spectra were obtained with a PerkinElmer 257 spectrophotometer for the 4000-625cm ~ spectral range, and with a Beckman IR 20A spectrophotometer for the 650-250 cm-' spectral range, in Nujol mulls or KBr pellets. The integrated absorption intensity (A, M-~ cm-2) of the CN stretching band of thiocyanate complexes was determined by Ramsay's method of direct integration[6]. Electronic absorption spectra were obtained with an Optica CF4NI spectrophotometer equipped with 5 cm quartz cells; the diffuse reflectance spectra (25-10 kK range) were obtained with the same instrument equipped with a single-beam standard attachment. The electrical conductivities were measured with an Amel impedance bridge type 951 on the same solutions used for the electronic spectra, at 25-+0.1°C, with a conventional closed type cell. The thermal analysis was performed with a Stanton HT thermobalance or a Mettler thermoanalyzer on 50 mg of the complex, in air, using a linear heating rate of I°C rain-~.

INTRODUCTION NUMEROUS complexes of lanthanide(IIl) cations with nitrogen-donor ligands have been prepared [1]. Rigorously anhydrous materials or high thermodynamic stability are conditions generally required to prevent hydroxide precipitation or to avoid water molecules strongly bonded as aquo-cations. Complexes with polyamines have been prepared only using non aqueous solvents and anhydrous lanthanide salts as starting materials, while complexes with weakly basic ligands, such as 1,10-phenanthroline or 2,2'-dipyridyl have been isolated either with anhydrous salts and solvents or from 95% ethanol; interactions of lanthanide(III) cations with weakly basic donor species in aqueous solvents have been also observed, but only with potentially tetradentate nitrogen-donor ligands [2]. The bis(2'-quinolyl)-2,6-pyridine (DQP), I, is a heterocyclic ligand containing three nitrogen atoms; it is similar to 2,2',2"-terpyridyl, but has two quinolinic rings attached to the central pyridine ring. Complexes of some transition metals with this ligand have been previously reported [3, 4]. It has been observed that such complexes are distorted from Oh symmetry, similar to the corresponding 2,2',2"-terpyridyl complexes. In the present paper we report the synthesis and characterization of the DQP complexes of some lanthanide (III) salts.

EXPERIMENTAL Materials. The ligand bis(2'-quinolyl)-2,6-pyridinewas prepared as previously described [3, 4]. Schuchardt hydrated lanthanide(III) nitrates and oxides (except Pm) were employed as received and used for the preparations of the other salts; the lanthanide(III) perchlorates and thiocyanates were prepared as previously reported [5]. Synthesis of complexes. The hydrated lanthanide(IIl) salts (1 mmole) were dissolved in 10 ml of hot anhydrous ethanol mixed

tAuthor to whom all correspondence should be addressed.

RESULTS AND DISCUSSION For all the complexes an invariable lanthanide(III) cation-DQP ratio of 1:1 was found. All complexes were analysed, but only the complexes of the representative lanthanide(III) cations (Ln = La, Pr, Nd, Sin, Gd, Dy, Ho, Er and Yb) were used for conductivity, thermal and spectral (i.r. and visible) studies. The complete list, analytical data and some properties of these complexes are reported in Table 1. The compounds are indefinitely stable in a dry atmosphere; they are soluble in acetonitrile and light alcohols; addition of benzene increases the solubility of the complex. Thermal and elemental analysis indicate that the nitrate complexes, except the thulium and ytterbium complexes, are anhydrous and of the type Ln(DQP) (NO3)3; the thulium and ytterbium complexes are monohydrated; the thiocyanate and perchlorate complexes are all hydrated and of the type Ln(DQP)X3. nHzO (n = 2 for X = NCS; n = 3 for X = ClO4). We were uncapable to attach more than one molecule of ligand to the lanthanide(III) cations; the cation coordinates the anionic groups or water molecules rather than a further DQP molecule. This behaviour, unlike the

259

S. GURRIERIet al.

260

Table 1. Analyticaldata found and (calcd.)and some propertiesof DQP complexeswith lanthanide(llI)salts General

Ln

% C

~ n

% N

M a~

% Ln

M.IO 3

ti Ocb)

formulatlon Ln(DQP)(NO3I 3 La

42,19(41,96)

2,29(2,30)

12,73(12,77)

21,19(21,10)

1~65

250

Pr

42,27(41,84)

2,25(2,29)

12,68(12,73)

21,70(21,34)

17,6

1,73

250

Nd 41,82 (41,63)

2,28 (2,28)

12,57(12,66)

21,72(21,73)

19,8

1,90

255

Sm

2,19(2,26)

12,46(12,55)

23,O3(22,45)

22,5

1,50

250

40,82(41,25)

18,3

Gd

40,78(40,83)

2,24(2,23)

12,38(12,42)

23,29(23,24)

27,9

1,80

260

Dy

40,60(40,51)

2,21(2,22)

12,25(12,32)

23,94(23,83)

30,3

1,42

265

HO

40,O2(40,37)

2,18(2,21)

11,98(12,18)

24,55.(24,10)

28,5

1,60

270

Er

40,45(40,23)

2,19(2,20)

12,18(12,24)

24,41{24,36)

32,5

1,65

260

(H20) Yb

39,48(38,88)

2,45(2,41)

11,67(11,83)

24,73(24,36)

30,0

1,78

270 (80) c)

46,OO(45,75)

2,79(2,81)

12,30(12,31)

20,44(20,35)

20,5

1,48

250 (130)

Pr

45,80(45,62)

2,77(2,80)

12,32(12,28)

20,33(20,58)

18,7

1,53

250 (130}

Nd

45,61(45,40)

2,77(2,78)

12,43(12,22)

21,O5(20,97)

20,3

1,65

245 (135)

Sm 44,49(45,OO)

2,B0 (2,76)

11,88(12,11)

22,O3(21,66)

24,0

1,48

250 (12&)

Gd

44,81(44,55)

2,74(2,73)

11,93(11,99)

22,40(22,43)

24,8

1,46

240 (135)

DF

44,18(44,22)

2,71(2,71)

11,84(11,90)

23,11(23,O1)

26,3

1,50

255 (13Q)

Ho

44,51(44,O7)

2,64(2,70)

11~77(11,86)

23,46(23,28)

38,O

1,65

270 (130)

Er

44,O7(43,93)

2,70(2,69)

11,77(11,82)

23,59(23,53)

39,6

1,70

260 (130)

¥b

42,70(43,57)

2,56(2,67)

11,61(11,73)

24,80(24,34)

40,2

1,42

260 (130)

Ln(DQP)(CIO4) 3 La

33,85(33,50)

2,60(2,57)

5,16(5,IO)

17,O5(16,84)

216

1,52

260 (IO5)

33,60(33,42)

2,54(2,56)

5,10(5,O8)

17,18~17,O4)

225

1,47

250 (110}

Nd

33,20(33,28)

2,50(2,55)

5,O3(5,O6)

17,50(17,38)

215

1,45

260 (115)

Sm

33,55(33,O4)

2,56(2,53)

5,10(5,O3)

18,18(17,98)

235

1,40

255 (110)

Gd

33,O5(32,77)

2,46(2,51)

5,O5(4,98)

18,83(18,65)

230

1,50

255 (110)

Dy

32,75(32,57)

2,48(2,50)

4,98(4,95)

19,20(19,16)

240

1,62

265 (115)

HO

32,55(32,47)

2,50(2,49)

4,90(4,94)

19,50(19,39)

239

1,80

280 (115)

Er

32,50(32,38)

2,46(2,48)

4,90(4,93)

19,55(19,61)

252

1,60

270 (115)

¥b

32,40(32,17)

2,45(2,46)

4,95(4,89)

20,28(20,15)

280

1,50

280 (110)

Ln(DQP)(NCS) 3 (H20) 2

(n20) 3

La

Pr

a) In a e e t o n i t r l l e - b e n | e n e s o l u t i o n (80-20), a t 25 ~ O,1 °C; b) temperature of d e c o m p o a i t i o n

beg~nning!

c) temperature of w a t e r - l o s s

2,2'-dipyridyl,1,10-phenanthroline or 2,2',2"-terpyridyllanthanide complexes [1], is ascribed mainly to stereochernical considerations. It has been noted that in the bis(2,2',2"terpyridyl) complexes of transition metals appreciable distortion from Oh symmetry occurs; the molecular conformation of this ligand makes unequal the metalnitrogen bond lengths and the angle formed by the metal and the two outer nitrogen atoms less than 180°[7-9]; moreover, the geometry of this ligand causes the loss of planarity, when it is forced to coordinate to a metal cation[9]. In the tridentate imine DQP complexes it is also necessary to point out the steric repulsive effect of the two quinoline ring hydrogen atoms attached cis to the quinolyl nitrogens [3] which probably cause an increase in the average metal-ligand bond distances.

Thermal analysis Water molecules were endothermically eliminated from the hydrated complexes in the 110-1700 range, in air, for thiocyanates and perchlorates and in the 80-1000 range for thulium and ytterbium nitrates. The following step for all

beginning.

the complexes lies in the 240--280° range; no appreciable difference was observed for complexes with different anionic groups but only a common slight increase in the decomposition temperatures from light to heavy lanthanide complexes (Table 1). The thermal behaviour for the water-loss process may be due to the coordination of water molecules in the hydrated complexes, except thulium and ytterbium nitrates.

Electrolytic conductivity The conductivity measurements on the DQP complexes of thiocyanate and nitrate lanthanide(III) cations in solution in acetonitrile-benzene mixtures (80-20) indicate that these complexes are essentially non conducting (Table 1). Thus coordination of all three thiocyanate or nitrate groups to the lanthanide(III) cations occurs in these complexes, although steric crowding causes a higher degree of dissociation with the smaller ions; the two series of complexes exhibit a slight increase in conductivity from the lanthanum to the ytterbium complexes. The molar conductivity values obtained for the perchlorate complexes in acetonitrile-benzene mixtures vary between

261

Lanthanidecomplexeswithbis(2'-quinolyl)-2,6-pyridine 210 and 280ohm-' cm:. Although it is not possible to compare these values with those previously reported for lanthanide(III) perchlorate complexes owing to the addition of benzene to the acetonitrile (this addition decreases appreciably the conductivity), however they are of the same order of those obtained for hydrated lanthanide perchlorates under the same experimental conditions. Anomalous conductivity values for lanthanide(IlI) perchlorate complexes have been previously observed and attributed to ion association rather than to coordination of the perchlorate groups to lanthanide cation[10-12]. On the basis of these results we must conclude that the three perchlorate groups in DQP complexes are ionic in character.

IR spectra The IR spectra were examined principally to determine whether and in which way the DQP ligand and the nitrate, thiocyanate or perchlorate groups are coordinated to the metal ion. The IR spectra of the complexes of each series are fundamentally identical and no significant difference was observed between KBr pellets and Nujol mulls spectra. The vibrations which undergo significant changes, indicating ligand coordination, are those exhibited by the free ligand in the 1650-1500, 1000-700 and 450-350cm-' spectral ranges. The two ring vibrational modes observed at 1620 and 1595 cm ~in the spectrum of the free ligand are moved to lower frequencies (5-10 cm 1, A,,) in the spectra of the complexes indicating that all three nitrogen atoms of the DQP molecule are coordinated to the lanthanide cations; this behaviour differs from that generally observed for transition metal complexes with heterocyclic imine ligands and is probably due to the appreciable distortion undergone by molecule itself owing to the coordination. The band at 1505 cm-' in the spectrum of the free ligand splits clearly in the spectra of the complexes, the two components lying in the 1510-1480cm-' spectral range; the two skeletal bands observed in the 405-385 cm ' range shift toward higher frequencies (15-25 cm ', A~,). In the 850-700 cm-t spectral range several bands due to skeletal and CH deformation vibrational modes are shown by DQP; they are also present in the complexes spectra, but the bands exhibited by the ligand at 850 and 820 cm-' are generally split. This behaviour resembles that observed for the complexes of d and f transition metals

with 2,2'-dipyridyl and 1,10-phenanthroline and is indicative of cation ring-nitrogen bonding[13, 14]. In Table 2 are reported the frequencies (cm t) of the vibrational modes of the nitrate, thiocyanate and perchlorate groups for the DQP-lanthanide complexes. The spectra of some lanthanide(III) perchlorate complexes with DQP exhibit the C104 v3 vibration clearly split into three components lying in the 1150-1080cm-' spectral range; furthermore the Vl vibration is activated and is present in the i.r. spectrum as a medium band. The splitting of the band indicates that distortion of the perchlorate group from T~ symmetry is occurring in the complexes. This behaviour may be due to solid state effects rather than to weak coordination. No significant splitting of the v3 vibration observed in acetonitrile solution and the electrical conductivity values indicate that all three C104 groups are ionic in character. The i.r. spectra of the nitrate complexes show that the three nitrate groups are coordinated to the lanthanide(III) cations. The spectra of the lighter lanthanide complexes exhibit two weak bands in the 1800-1700cm ' range which is used by some authors to differentiate between mono and bidentate nitrate groups[15, 16]. The magnitude of the separation of the bands is about 30-40 cm ' and is typical of the bidentate nitrate group. Mixtures of monoand bidentate groups seem to be present, on the other hand, in the heavier lanthanide(III) complexes; the two combination bands in the spectra of these complexes give a weak broad band with more than two peaks in the 1760-1710 cm -~ range[16]. In the spectra of the thiocyanate complexes the CN stretching vibration appears as a composite band, exhibiting two or more components, in the 2100--2000 cm ~range. Splitting of the CN vibration band has been previously observed for lanthanide thiocyanate complexes and attributed to solid state effects[5, 17]. According to Mitchell and Williams[18] the frequency of the CN stretching band, if it lies in the 2050-2040 c m ' spectral region, may be interpreted as indicating coordination through the nitrogen atom. This conclusion is supported by the appearance at 490-485 cm ' of the NCS deformation band, also indicative of N-bonding[19J. The values of integrated absorption intensity for the CN stretching band of about 8-11 x 104 (M ' cm 2) for the single NCS group, the spectra in acetonitrile solution, still confirm the N-coordination of the NCS group and clearly indicate the hard character of the lanthanide(IIl) ions [20].

Table 2. Complexesof DQP with lanthanide(IlI)cations:IR data for nitrate,thiocyanateand perchlorategroups Ln(DQP)(NO3) 3

~3

~5

v6

(Ln = L a - S m

)

1765-6Ow

~2 +95

1735-3Ow

~ 2 + ~6

15OOs.br

128Os

IO25m

82Om

745m

720w

(Ln = G d - Y b

)

1755-5Ow

1740-15w.br

149Os.br

1295s

IO3Om

815s

745m

Ln(DQP) (NCS)3(H20)2 ( Ln = Pr,Nd

( Ln = H o , E r

)

)

( Ln = G d , E r , Y b ( Ln = H o , D y

)

a) T h e CS s t r e t c h i n g in t h e

~(CS) a)

900-750

~(NCS)

--

485m

208Ow

206Ow

2045vs

--

485m

2OSOvs

--

480m

~3 1115s

~ IO80vs

~

945m

( Td )

64Osh,

625s

1145sh

1115sh

IO80vs

945w

625s

1145sh

1115s

I085vs

945w

625s

band

is n o t o b s e r v e d

c m -I r e g i o n .

owing

to the n u m e r o u s

bands

( C2V

( 138Ow,D3h

204Os

115Om

)

~2

2080~

3

)

v~

210Ow

207Om

Ln(DQP)(CIO4)3(H20) ( Ln = L a - N d

~(CN)

)

( Ln = G d - O y , Y b

ul

exibited

by

ligand

S. GumttERlet al.

262

static field and changes in the symmetry around the lanthanide ion[23--28]. Spectral data for the complexes of some ianthanide(III) salts with DQP are reported in Table 3 and Fig. 1, for

Visible spectra The internal f - [ transitions of the lanthanide(III) cations are affected by the ligands on complex formation: shifts toward longer wavelength and splitting and enhancement of the intensity of certain bands are generally observed; these effects can provide useful information on the nature of the metal-ligand bonding and on the environment of the lanthanide ion. The nephelauxetic effect is in fact correlated with the covalency of the metal-ligand bonding and the Sinha's parameter (3) is usually supposed to be a measure of the covalency[21-23]; the hypersensitivity and splitting of some bands are attributed to the inhomogeneous electro-

example. The absorption spectra of the complexes of praseodymium and neodymium(Ill) salts with DQP show an appreciable red shift with respect to the aquo-cation of the 3H4-*3P2, 3p,, 3po' ID2 and of the 419/2-->4Gst2,2G7/2, 4S3n, 4F7/2, 4Fsn and 2H9/2 transitions respectively. Red shifts with smaller values for the g-parameter are also observed for the 518-.5G6, 5Fl, 5S2,5F4 and 5F5 transitions of the holmium(III) complexes. For the erbium(III)--

Table 3. Visible spectraof the complexesof some lanthanidecations with DQP ligand;the positionof the J-levels (in kK), the molarabsorptivity(in parenthesis)and the covalencyparameter(~)(') are reported

Ln(IIl)

nitrate Pr

perchlorate

nitrate

thiocyanate perchlorate

22,42(13,7)

22,37(12,8)

22,35(11,3)

22,37

22,37

22,42

3P I

21,16(4,8)

21,16(5,2)

21,14(5,5)

21,18

21,16

21,16

3p

20,64(5,0)

20,62(6,7)

20,66(4,4)

20,62

20,60

20,66

+0,70

+0,83

6 = +0,59

+0,67

+0,74

+0,56

2G7/2 ,

17,30 sh

17,30(8,5)

17,33(6,9)

17,21

17,33

17,36

4G5/2

17,19(18,4)

17,O9(17,15)

17,19(8,6)

17,O8

17,11

17,25

4S3/2 ,

13,55(6,6)

13,55(7,O)

13,55(5,9)

13,50

13,55

13,59

4F7/2

13,42(3,4)

13,37(1t,I)

13,42(8,3)

13,39

13,37

13,43

2a9/2 ,

12,52(6,0)

12,57 sh

12,56 sh

12,57

12,56

12,53

4F5/2

12,45 sh

12,45(13,5)

12,45(13,1)

12,47

12,45

12,44

6 = +O,61 HO

+O,71

+0,52

+0,82

+0,66

+0,44

5F1,5G 6

22,13(26,5)

22,13(29,6)

22,18(9,8)

22,08

22,11

5F4,5S 2

18,64(5,3)

18,53(8,9)

18,50(7,7)

18,53

18,57

18,57

15,55(4,8)

15,53(3,4)

15,48(7,O)

15,54

15,55

15,56

+0,48

+,O85

19,O6(4,8)

19,18(9,7) 19,10 eh

19,18(4,8) 19,O6(5,2)

19,06

19,18 19,1Osh

19,18 19,10 sh

15t31(2,O)

15,31(2t5)

15,29(2,1)

15,31

15,31

15,31

O,OO

O,OO

5F 5

6 = +0,23

Er

thiocyanate

3P 2

o

Nd

reflectance

solutionb)

J-level

2Hli/; ) {19,18 (10,3)

4F9/2

6 = +0,10

O,OO

+O,18

a) Calculated by Sinha's relationship 6 = •

• 1OO

+0,52 19,18

+O,10

+0,40

22,18

+0,39

(21), where B is the average value

B of the ratio

Vcomplex / Vaquo ; b) in acetonitrile-benzene (80-20) solution& c) in aqueous

solution the absorption maximum Is at 19,10 kK (eel315) and a shoulder is present at about 19,20 kK.

Lanthanide complexeswith bis(2'-quinolyl)-2,6-pyridine

/ \

6

263

complexes[21]. If direct proportionality of the 6parameter to the total e-donation is assumed, the smaller values obtained for the DQP complexes compared to those reported for the analogous mono-terpyridyl systems could be related to the greater ligand field strength arising from the terpyridyl ligand. These results, on the other hand, are in agreement with those previously obtained and reported for the DQP complexes of some transition metals [4].

/

<

Acknowledgement--This work has been financiallysupported by

National Council of Research (C.N.R., Italy).

/ a 580

600

73o 750 ~, r}m

800

820

Fig. 1. Reflectance spectra of neodymium(III)-DQPcomplexes: (a) nitrate; (b) thiocyanate; (c) perchlorate. DQP complexes no red shift is observed (Table 3) for the 4115/2~ 2H./2 transition while the 4115/2~ 4F7/2, 4F9/2 transitions appear unaffected by complex formation or show a very slight red shifts. The reflectance spectra show similar red shifts but different average values for the ¶meter. All average values of the ¶meter are, however, positive both for solution and reflectance spectra, but smaller than unity and indicate weak covalent bonding. But the correlation of the Sinha's parameter with covalency is not simple; Sinha in fact has found for some complexes with strongly coordinated ligands, negative values for the ¶meter [22]. Changes in shape and intensity of the bands from those of the aquo-ions occur with the nitrate to perchlorate complexes. The 419/2-->4G5/2transition of the neodymium nitrate and thiocyanate complexes is found to be 3-fold increased in intensity compared to the aquo-ion, while the 5h~SG6, 5F1 transitions of the holmium nitrate and thiocyanate complexes increases in intensity about seven times. The band in the 19,20-19,10kK region of the spectra of the erbium(Ill) nitrate and thiocyanate complexes undergoes about a threefold increase. The ~Pj bands of the complexes of the praseodymium(III) salts show slight changes in intensity compared to the aquo-ion, but they are remarkably split. Smaller changes in intensity are found for the perchlorate complexes (Table 3). The three series of complexes must possess different atomic arrangements around the central cation. The arrangement moreover must be less symmetric in the nitrate and thiocyanate complexes than that in the perchlorate complexes. The ¶meter for the DQP complexes has been found to be smaller than for the mono-terpyridyl

REFERENCES

1. J. H. Forsberg, Coord. Chem. Rev. 10, 195 (1973). 2. D. A. Durham and F. A. Hart, J. lnorg. Nucl. Chem. 31. 145 (1969). 3. C. M. Harris, H. R. H. Patil and E. Sinn, lnorg. Chem. 8. I01 (1%9). 4. R. P. Bonomo, S. Musumeci, E. Rizzarelli and A. Seminara, Z Anorg. Allg. Chem. 414, 185 (1975). 5. G. Condorelli,A. Seminara and A. Musumeci,J. lnorg. Nucl. Chem. 36, 3763 (1974). 6. D. A. Ramsay, J. Am. Chem. Soc. 74. 72 (1952). 7. R. C. Stoufer, D. W. Smith, E. A. Clevengerand T. E. Norris, Inorg. Chem. 5, 1167 (1966). 8. D. V. Naik and W. R. Scheidt, Inorg. Chem. 12. 272 (1973). 9. W. Clegg and P. J. Wheatley, J. C. S. Dalton, 90 (1973). 10. Y. Gushikem, C. Airoldi and O. L. Alves, J. Inorg. NucL Chem. 35, 1159 (1973). 11. C. Airoldi and Y. Gushikem, J. Inorg. Nucl. Chem. 36, 1892 (1974). 12. O. L. Alves, Y. Gushikem and C. Airoldi, J. lnorg. Nucl. Chem. 36, 1079 (1974). 13. D. A. Baldwin,A. B. P. Lever and R. V. Parish, lnorg. Chem. 8, 107 (1%9).

14. V. Carassiti, A. Seminara and A. Seminara-Musumeci,Ann. Chim. (Roma) 54, 1025 (1964). 15. N. F. Curtis and Y. M. Curtis, lnorg. Chem. 4, 804 (1965). 16. A. B. P. Lever, E. Mantovani and B. S. Ramaswamy, Can. J. Chem. 49, 1957 (1971). 17. G. Vicentini, L. B. Zinner and L. Rothschild, lnorg. Chim. Acta 9, 213 (1974). 18. P. C. H. Mitchell and R. J. P. Williams,J. Chem. Soc. 1912 (1960). 19. A. Sabatini and 1. Bertini, lnorg. Chem. 4, 959 (1965). 20. R. G. Pearson, J. Am. Chem. Soc. 85, 3533 (1963). 21. S. P. Sinha, Spectrochim. Acta 22, 57 (1966). 22. S. P. Sinha, J. lnorg. Nucl. Chem. 33, 2205 (1971), 23. C. K. Jcrgensen, Prog. Inorg. Chem. 4, 73 (1962). 24. W. F. Krupler and J. B. Gruber, Phys. Rev. 139A,2008(1965). 25. B. R. Judd, J. chem. Phys. 42, 3797 (1965). 26. C. K. Jcrgensen and B. R. Judd, Molee. Phys. 8,281 (1964). 27. D. G. Karraker, Inorg. Chem. 7, 473 (1968). 28. D. G. Karraker, Inorg. Chem. 6, 1863 (1967).