Tetragonal nickel(II) complexes with singlet and triplet ground states: Complexes with 2-isopropyl imidazole

L inorg, nucL Chem., 1976, Vol. 38, pp. 1891-1896. Pergamon Press. Printed in Great Britain

TETRAGONAL NICKEL(II) COMPLEXES WITH SINGLET AND TRIPLET GROUND STATES: COMPLEXES WITH 2-ISOPROPYL IMIDAZOLE PANCHANAN PUJARIt and KAILASH C. DASH* Department of Chemistry, Utkal University, Vani Vihar, Bhubaneswar 751004, India

(Received 25 October 1975) Ahstraet--2-Isopropylimidazole (L) reacts with nickel(II) salts to yield complexes of the formulae NiL,XdX = CI , Br , I-, SCN , SeCN , NO3 , C104-, PF6 or NiLXCIO, (X = CI-, Br-, I , SCN- ). The structure of these complexes is rationalised on the basis of spectral, magnetic and conductivity measurements as well as their analytical data. All the compounds possess tetragonally distorted octahedral structure (D,h). The extent of tetragonality varies with the nature of the axial ligands. Complexes with X = SCN-, SeCN , NO3 are tetragonally distorted to only a small extent giving a triplet ~A2 ground state resulting in the formation of paramagnetic complexes with two unpaired electrons. Strong tetragonal distortion leading to a singlet 'E ground state and consequent diamagnetism is observed when X = Cl , Br , I , CIO4 . The complexes NiL,(PF6)z and NiL(SCN)(C10,) have intermediate magnetic moments in the range 2.2-2.3 B.M. and thus have a singlet ground state with a thermally accessible spin-triplet level. Thermal decomposition of NiL,X2 (X = CI-, Br-, NOs ) give strongly tetragonal, diamagnetic NiLC12 and weakly tetragonal, paramagnetic NiL2Br2 and NiLdNO~)2 complexes. The coordination number in all the cases is found to be six. INTRODUCTION THE 5-MEMBERED heterocyclic ring, imidazole, as a histidine moiety, functions as a potential unidentate ligand (or also as a bridging ligand as in imidazolato bridges) toward transition metal ions in a variety of biologically important molecules including ironheme systems, vitamin B~2 and its derivatives and several metaUoproteins. Imidazole contains both a pyridine-like imine nitrogen and a secondary amine nitrogen and thus behaves as an ambidentate ligand towards transition metals [1]. Transition metals, such as Mn(II), Fe(II), Co(II), Ni(II) etc. form complexes of the type M(imid)6X2 [2, 3] with unsubstituted imidazole. The coordination behaviour of substituted imidazoles has been studied to some extent [4-10]. We have been studying the coordination behaviour and the influence upon the ligand field properties, of substituents placed in position 2, adjacent to the donor atom, of the five-membered imidazole ring. Work in this laboratory[10] and in others[4,9] showed that 2-methyl imidazole forms paramagnetic Ni(2-Meimid)4X: (X = C1, Br, 1) complexes. Due to steric effects, 2-methyl imidazole is not known to form M(2-Meimid)6X2 complexes, as is found for unsubstituted and 1-substituted imidazoles. A preliminary communication from this laboratory[ll] reported the formation of a 6-coordinated, tetragonal, diamagnetic complex of 2-isopropyl imidazole, NiL4C12. We now report detailed studies of complex formation between 2-isopropylimidazole (Fig. 1) and several nickel(II) salts, which form tetragonal complexes of the type

H Fig. 1. 2-Isopropyl imidazole (L). NiL4Xz (X = (21, Br, I, SCN, SeCN, NO3, ClO4, PF6) and NiL4XCIO4 ( X = C I , Br, I, SCN) all possessing D4, symmetry and show that on varying the two axial ligands, +B.J.B. College, Bhubaneswar.

the extent of tetragonality changes resulting in consequent changes in magnetic behaviour of the complexes. No work on 2-isopropylimidazole as a ligand has been reported earlier.

EXPERIMENTAL

Materials 2-Isopropyl imidazole was supplied by BASF (West Germany). Nickel chloride hexahydrate (BDH) was used as such. NiBr2 was prepared by the reaction of nickel carbonate and HBr. NiI~, Ni(SCN)2 and Ni(SeCN)2 were prepared by metathetic reactions of Ni(NOs)2.6H20 and KI, KSCN and KSeCN respectively in ethanolic medium. In each case, the precipitated KNO~ was filtered off and the filtrate concentrated by slow evaporation. Solutions of NiXCIO4 (X = CI, Br, I, SCN) were conveniently prepared in situ by mixing equimolar amount of Ni(CIO~)2'6H20 and NiX~.6H20 (X = C1, Br, I, SCN) in ethanol. Triethylorthoformate was used for dehydration. All the complexes were prepared more than once and characterised in each preparation, giving reproducible results. Ni(2-Isopropyl imidazolehX2 (X = C1, Br, I, SCN, SeCN, C104). These complexes were prepared by one general procedure. The corresponding nickel(II) salts were dissolved in the minimum volume of ethanol and triethylorthoformate and then added to the ligand in the same solvent mixture with continuous stirring. The stoichiometric ratio of metal to ligand taken was I : 4. The mixture was refluxed for 2 hr and the solvent then concentrated to 1]3rd its volume. Slow cooling of the solution afforded the desired orange-yellow, blue or green compounds. Ni(2-Isopropyl imidazole)4(PF6)2. Ni(PF6)2 was prepared by metathetic reaction of NiCI2 and AgPF~ in EtOH, filtering off the precipitated AgC1 and concentrating the volume to some extent. To this Ni(PF6)2 soln, the required quantity of triethylorthoformate was added. The resulting soln was added to a stoichiometric amount of ligand in the same solvent mixture. The mixture was refluxed for 1 hr, then the volume reduced and cooled. On addition of light petroleum (40-60°) and on keeping in an ice-chest a stable yellow-green solid was obtained, which was collected on a flit, washed with ether and dried in vacuo. Ni(2-Isopropylimidazole),XClO, (X = CI, Br, I, SCN). These were prepared as above taking NiXCIO4 solution and the ligand in the molar ratio 1 : 4. Ni(2-Isopropylimidazole)4(N03)2. This complex was prepared by taking Ni(NOs)~.6H20 in ethanol and triethylorthoformate and adding to the ligand in the same solvent. The mixture was refluxed,

1891

PANCHANAN PUJARI and KAILASHC. DASH

1892

the volume reduced and then ether added when the green compound separated out. Ni(2-Isopropylimidazole)C12. This brown complex was prepared by heating the correspondingtetrakis complex,NiL4C12,at 120°C to constant weight. The percentageloss observed was 58.37 (theoretical percentage loss 57.98). Ni(2-Isopropylimidazole)2Brv This blue complexwas obtained as above by thermal decompositionof the tetrakis complex.The percentage loss observed was 33.05 (theoretical 33.45). Ni(2-Isopropylimidazole)~(NO~)v This green complex was obtained as above by thermal decomposition of the dinitrato tetrakis (2-isopropylimidazole)ulckel(II) complex at 120°C. The percentage loss observed was 35.80 (theoretical 35.35).

Analyses Nickel, halogenand thiocyanate estimationwere carried out as described earlier[10]. Carbon, hydrogen and nitrogen were determined in the microanalyticallaboratory, I.I.T., Kanpur. Characterisation measurement. The electrolytic conductance, magnetic and spectral data were obtained as described previously[10]. Electronic mull spectra and IR spectra in KBr phase were obtained from I.I.T., Kanpur. RESULTS AND DISCUSSION The new complexes formed are listed in Table 1, along with their colours, m.ps and elemental analyses. Variations in metal-ligand ratio did not alter the stoichiometry of the complex formed, the tetrakis (2isopropylimidazole) complex being obtained always. All the compounds are quite stable in air and are fairly soluble in common organic solvents like ethanol, methanol, acetone, acetonitrile, DMF and even chloroform. The coordination number and the stereochemistry of these complexes were determined on the basis of analytical data, conductivity in nonaqueous solvents, I g spectra, electronic spectra (in solution and in solid state) and magnetic measurements. Electrolytic conductance measurements. The conductance measurements in acetone, acetonitrile, methanol and DMF showed similar trends in each of these solvents. However, the interpretation of conductivity data is not very straightforward. Of all the NiL~X~ complexes

reported in this paper the complexes where X = CI-, SCN- and NO3- have a molar conductivity value typical of nonelectrolytes (Table 2). On going from Cl- to Br- and I- and from SCN- to SeCN- the conductivity value increases steadily. These values as well as the colour changes of these complexes on dissolution in organic solvents clearly indicate that extensive solvation occurs in all these solvents. For example, in the case of the NiL4(SeCN)= complex, the observed molar conductivity is much less than that expected for a 1:1 electrolyte, but higher than that for a nonelectrolyte and the NiL4I= complex appears to be a 1 : 1 electrolyte. These conductivity values exclude the possibility of ion-pair formation and only indicate that solvolysis becomes more pronounced on going from CI- to I- and SCN to SeCN . Therefore, these compounds are all non-electrolytes, but undergo ready solvation thus increasing the molar conductivity values. The NiL(CIO4)2 complex has a conductivity value somewhat smaller than that expected for a 1 : 2 electrolyte and similarly the conductivity values for NiL4X(CIO4) are somewhat smaller than that expected for a 1:1 electrolyte. However, in the latter series of complexes, due to solvolysis, the conductivity value increases from X = Clto Br- and I-. The IR data (see below) show that the ClO4 ions are probably involved in very weak-coordination. Although the conductivity of NiL(PF6)2 corresponds to a 1 : 2 electrolyte in acetone, the IR data shows that the PF6ions are also weakly coordinated. In all other cases, the ligand, 2-isopropylimidazole and the anions are all truly coordinated to the central nickel(II) ion. The NiLC12, NiL~Br2 and NiL2(NO3)~ complexes are found to be essentially non-electrolytes. Electronic spectra. The electronic spectral data recorded in acetone solution and as Nujol mulls (or reflectance spectra) are reported in Table 3. The spectra in solution were compared with the solid state spectra (mull or reflectance) in order to ascertain if any drastic changes in structure occur in solution either through replacement by solvent or through expansion of the coordination

Table 1. Colour,m.p. and analyticaldata of nickel(II)complexes* Sl No.

Complex

Colour

1. NtLI+C12(a)

~. WiLb (SC~) 2

Orange yellow Ortmge yellow Orange yellow Blue

5. NtLb,(SeC~) 2

Grey

6. NILe(N03) 2

Light I'76 9.'75(9.~I) gre~ Orange >250 8.78(8.~I) yellow Yellow 205 '7.3`7(7.~3) green Orange 155 8.9'7(9.25) 5.k1(5.60) yellow Orange 160 8.90(8.65) 11.50(11.77) yellow Orange 185 8.05(8.08) 17.k2(1`7.50) yellow Yellow 165 8.85(8.93) 9.o'7(8.82) gree~ Brown 210 2~+.1+0( 2b,.k6 ) 29°'70(29.60) Blue 230 13.60(13)+0) 36.k.6(36.30) Green 185 lq. 1`7(lh-.56)

2. NILI+Br 2 3. NiI%I 2

'7. NtL4(CI~) 2 8. N iLb,(PF6)2 9. NiL~CI (CIO~) 10. NIL~Br(CIO~) 11. Ni~I (CIO~) 12. NIL~SC~(CIO 4) 13. NILC12 lk-. NtL2Br 2 15. NiL2(N03) 2

M.P.

(°C)

% Ni

% X

~H

125

10.73(10.29)

12.2~(12.k6)

k-9.?o(5o,J+8) 7.62(7.01)

19.k0(19.63)

182

8.62(8.90)

2)+.78(2~-. 25)

k2.72(k3.71)

5.72(6.07)

16.70(1'7.o0)

185

`7.66(`7.79)

33.66(33.7o)

3'7.68(38.22)

h-.92(5.31)

lk.80(lk.86)

190

9.62(9.5k)

19.23(18.86)

50.69(50.'72)

6.62(6.50)

22.'70(22.`76)

-

hA.21 (k,.k.02)

5.58(5.6~.)

19.9o(19.75)

}+6.01(k6.20)

6.1+0(6.61) 22.00(22.)+6)

925

-

Figures in the bracket indicate theoretical values. (a) Ref. 11.

~C

}+1.12(b,1.23)

5.85(5.72)

15.92(16.03)

36.31(37.48)

5.ho(5.07)

1k.k7(14.19)

~-5.50(h5.39)

6.1o(6.3o)

17.30(17.65)

~2.91(~2.k-2)

5.92(5.89)

1`7.oo(16.5o)

39.13(39.60)

5.2O(5.5O)

15.30(15.h3)

}+5.68(~-5.62)

5.8'7((6.o8)

19.96(19.18)

29.89(30.o0) 32.'76(32.81) 35.k.2(35.'73)

~ .o8(k.16) 11.k3(11.6`7) ~-.52(~-.55) 12.58(12.76) k..82(k.96) 20.70(2o.8)+)

Tetragonalnickel(II)complexeswith singletand triplet ground states

1893

Table 2. Conductivitydata in acetone and magneticmoments in solid state at room temperature Sl. No.

Complex

Cone.

(a)

(x I0"~)

mole-1

B.M.

2.06

Diamagnetic

1.02

50.80

Diamagnetic Diamagnetic

3.

NiL~I 2

1.07

167.30

h.

NIL~(SCN) 2

1.08

16.30

5.

NiL~(SeCN) 2

1.06

94.00

3.03

6. NiL~(N03) 2

1.01

16.63

2.98

7.

NiLk(ClO~) 2

1.1~

185.00

8.

NiL~(FF6) 2

0.~6

213.20

9.

NiL~CI(CIO~)

1.00

129.20

10.

NiL~Br(CIO~)

1.01

116.00

Diamagnetic

11.

NiL~!(CIO~)

1.11

182.70

Diamagnetic

12. NiL~(SCN)ClO k

1.01

9~.12

=.30

13.

NiLCI 2

1.07

7.37

Diamagnetic

Ik.

NiL2Br 2

1.23

~7.0k

3.00

0.91

2~.32

2,95

3.00

Diamagnetic

2.20 Diamagnetic

(a)

~ = 1 mol dm "3

(b)

Molar conductivities in D~F, MeOH end CH3CN give similar conclusions.

(c)

Values reported are average of at least four independent measurements st room temperature. Susceptibilities in each case were measured at field strengths of aPpx. 4000, 6000 and 8000 G and were found to be independent of field strength.

Acetone solution

NiL~CI 2

22,730

2k,390

17 ,Sh~

23,260 17,860

NIL~Br 2

22,730

NiL~I 2

23,700

15,620

NiLk(SCN) 2

21+,100

19,050

NILe(SeCN) 2

23,810

18,180

NiL~(N03) 2

2~,100

21% 500 16,670

15,620 I~,~90 -

27,030

2~,3ao 24,100

19,23o

1~,930

NiL~C1(ClO~)

mhos em2

Feff (e)

1.08

Ni~Br 2

Solid

,

(h)

1. NiL~CI 2

Table 3. Electronic spectra of Ni(II) complexesin cm '

NiL~(PF6) 2

J~M

2.

15. NIL2(N03) 2

Complex

A

23,260

25,000 17,860

NiL~Br(OI%.)

23,530

17,860

NtLkI(Cl%)

23,700 19,050 17,5t~0

NiLC12

17,860

NiL2Br2

19,230

1~,87o 1~,930

number through solvation. The orange-yellow, diamagnetic NiL4X2 (X = CI, Br, I, C104) and NiL4XC104 (X = CI, Br, I) complexes exhibit a broad and strong band in the region ca. 21,000-23,000 cm -1, characteristic of a nickel(II) complex in a strong tetragonal field[12]. The blue or green, paramagnetic NiL4(SCNh, NiL(SeCN)2 and NiL4(NO3)2 complexes exhibit bands in the region expected for octahedral complexes [13, 14], corresponding

to 3T,, *- 3A2g transition (v2) at 14,000-19,000 cm ' region and to 3Tig(P)~--3A2g transition (v3) at about 23,00024,000 c m ' region. NiL4(PF6)2 exhibits bands at 27,030, 25,320, 24,100, 19,230 and 14,930 cm -~ in acetone solution corresponding to the v3, v2 and v , transitions respectively. Magnetic susceptibility measurements. From the magnetic susceptibility data (Table 2) the reported complexes can be classified into three groups: orangeyellow, diamagnetic complexes with a singlet ground state ('E); blue or green, paramagnetic complexes exhibiting a moment of ca. 3.00 BM possess a triplet ground state (3A2) and yellow-green, paramagnetic complexes exhibiting a subnormal moment of ca. 2.2 B.M. possessing a single, ground state but having a thermally accessible triplet level. The NiL4X: (X = CI-, Br , I , C104 ) and NiL,XC10, (X = Cl-, Br, I ) complexes are orange-yellow and diamagnetic, with all the spins of the nickel(If) ion perfectly paired. The blue or green NiL4X2 (X = SCN , S e C N , NO3 ) complexes are paramagnetic, spin-free complexes with two unpaired spins on the central nickel(II) ion, with a magnetic moment of ca. 3.00 B.M., which is in excellent agreement with those expected for octahedral nickel(II) complexes[15]. The nickel(ll) complexes with coordination number six in almost all cases have a high-spin electronic configuration and possess regular or distorted octahedral stereochemistry. In such an environment the nickel(ll) ion has an orbitally non-degenerate ground state 'A2, and is not expected to have any orbital contribution, and the magnetic moment should be close to the spin-only value of 2.83 B.M. However, due to spin-orbit coupling between the first excited state 3T2g and the ground state 3A2g there is always a small orbital contribution to the magnetic moment increasing the spin-only value to 3.003.30 B.M. [16,17]. The thiocyanate and selenocyanate coordinate through the N-end to the nickel(II) ion (see

1894

PANCHANAN PUJARI and KAILASH C. DASH

below) and thus in these complexes an approximately NiN6 chromophore may be taken to define an almost octahedral configuration resulting in paramagnetism of the complexes. The nitrate ion occupies a position near fluoride in the spectro-chemical series and the complexes involving coordinated nitrate are expected to be highspin[18]. Imidazole lies above oxygen donors and just below NH3 and pyridine in the spectro-chemical series and well below chelating amines such as ethylenediamine or 2,2'-bipyridine. 2-Isopropylimidazole is expected to occupy a similar position in the spectrochemical series and due to its close proximity in position to that of NO3-, NCS- and NCSe- in the spectrochemical series, in these three cases high-spin, approximately octahedral or weak tetragonal paramagnetic complexes, with a triplet ground state (3A2)are formed. Since CI-, Br-, I- and CIO4are well separated in position from this ligand in the spectrochemical series, in these cases strongly tetragonal complexes are formed which are diamagnetic and possess a singlet ground state (~E). In the case of NiL(PF6)2 and NiL4(SCN)(CIO4) complexes magnetic moments of 2.2 and 2.3 B.M. are obtained respectively. It may be taken that in these complexes the spin-singlet state lies lower with a thermally accessible spin triplet level and the magnetic properties depend upon the Boltzman distribution between these two levels[19]. Thus, the magnetic moment varies with the nature of the axially coordinated univalent groups. IR spectra. The IR spectra (4000-250 cm-~) for both the pure ligand and the coordination complexes were obtained as Nujol mulls or as KBr discs and were

compared to gain information on the coordination around the metal ion and to detect possible impurities. The spectra indicated all the compounds to be pure. The bands arising due to the ligand, 2-isopropylimidazole, could be easily identified in the spectra of the complexes, since they were either split or shifted due to lattice effects or due to departures from idealised symmetry[20,21]. We consider here the bands arising due to vibrations in the polyatomic anions, NCS-, NCSe-, NO(, PFr- and ClOd (Table 4). (a) The pseudohalide ions. The linear triatomic pseudohalide anions NCS- and NCSe- can coordinate through both the N- and S or Se-end and thus function as ambidentate ligands. Mitchell and Williams [22] depicted that a change from M-NCS (or M-NCSe) to M-SCN (or M-SeCN) bonding closely parallels the Ahrland-ChattDavies [23] and Schwarzenbach[24] classification of metals. The Class " a " type metals or "hard acids" according to Pearson's concept [25] form N-bonded isothiocyanates (or isoselenocyanates) and class "b" metals or soft acids[25] form S-bonded thiocyanates (or selenocyanates). Since the Ni(II) ion belongs to the borderline case, it can coordinate either through the S (or Se) and/or the N-end of the pseudohalides. From the positions of the different anion vibrations it is possible to establish [26-29] if the complexes contain terminal N-bonded isothiocyanate and iso-selenocyanate, or S (or Se) bonded thiocyanates or selenocyanates. The NiL,(SCN)2 and NiL(SeCN)2 complexes exhibit C-N stretching frequencies around 2100 cm-', indicating the formation of terminal N-bonded pseudohalides [29].

Table 4. IR spectraof the polyatomicanions(cm-x) (a)

(b)

(e)

~(C ~ 7)

~(C - S)

NiL4(SCN) 2

2110 vs

8@0 s

482 vs

NIL4(SeCN) 2

2108 vs

NiL4(SCN)(CI04)

2110 vs

860 w

480 s

Pseudohalides

Nitrate

~1

~4

~2

~5

NiL4(N0~) 2

1412 m

13~@ m

1042 s

820 s

NiL2(N03) 2

1610 s

12@@ sh

1027 s

810 m

Perchlorate

~I

~2

~

620 s

440 s

1063 br 1115 m

620 s

950 s

440 s

1068 at 1120 at

620 s

953 s

440 s

106~ s 1113 br. st

620 s

NiL4(SCN)(C104)9~0 m

435 s

1068 s 1140 hr.

620 s

930 s

~iL4CI(CI04)

9@0 m

NIL4Br(CI04) NIL4I(CI04)

770 vs

~4

10@@ s 1110 br. st

NiL4(CI04) 2

~(N - C - S)

st

(d)

HexJ-eluoropbosphate 740 s

850 br 838 eh

568 sh 5@8 s

Abbreviations used : w-weuk,m-medlum, s-shurplve-verysharp , st-strong~br-broad~ sh-shoulder.

Tetragonalnickel(II)complexeswith singletand triplet groundstates

1895

(b) The nitrate ion. The nitrate ion, inspite of its low isopropylimidazole only diamagnetic complexes, Ni(2position in the spectrochemical series near the fluoride isopropylimidazole)4X: (X= C I , B r , I , CIOn ), are ion[18] has been found to coordinate in a variety of obtained may be rationalised assuming greater steric environments. Gatehouse et a/.[30, 31] have shown that hindrance in the latter case, which prevents the close the D3h symmetry of the ionic nitrate is lowered to C2~ for approach of the axial halides to the central nickel(II) ion. linear monodentate or bidentate (or bridging) coordination However, the small strongly coordinating anions NCS , and to C, for non-linear coordination. The NiL4(NO3)2 NCSe- and NO3 seem to offset these steric factors and complex exhibit bands at 1412 (v~), 1335 (v4), 1042 (v2), form paramagnetic, 6-coordinated, nearly octahedral 820 (v3) and 772 (vs) cm-' region indicating unidentate complexes. Except in these three complexes, the obcoordination of the nitrate group and the NiL2(NO3)2 served magnetism and spectra may be explained by complex exhibits bands at 1610 (v,), 1255 (v4), 1027 (v2) assuming a diamagnetic, 6-coordinated structure. and 810 (v3) cm ' which is characteristic of the bidentate Maki[40] has discussed the conditions for diamagnetism nitrate group. The difference between the vt and v~ in d 8 compounds having a tetragonal component in an frequencies has been used[32,33] as a guide to the octahedral ligand field. The octahedral or weakly distorted covalent nature of the nitrate group; the greater the tetragonal nickel(II) complexes are paramagnetic with difference, the more covalent is the bonding. The two unpaired spins, since the triplet 3A2~ state difference is observed to be far less in the tetrakis-(2- (paramagnetic) always lies below the singlet tE state isopropylimidazole) complex as compared to the bis- (which imposes diamagnetism) for all values of the complex. This shows a greater covalency in the bis- crystal-field splitting, Ao. The situation is changed due to complex. Thus, the coordination number in both cases is axial distortion which lowers the symmetry from O,, to found to be six. D4h and is accompanied by a further loss of degeneracy of (c) The perchlorate ion. Even though C104- ion is the d-orbitals. With large axial distortion, the energy usually believed to be a noncoordinating anion, it has been separation between the dx2 ? and dz2 orbitals may exceed recently [34] demonstrated that when complexes involving the electron-pairing energy and there will be a change in this anion are prepared in nonaqueous medium in the magnetic moment from 3.0B.M. to zero. Thus, if the presence of dehydrating agents, such as triethylorthofor- separation of the in-plane ligand and the axial out-of-plane mate or 2,2'-dimethoxypropane, it may be involved in ligands in the spectrochemical series is relatively small. coordination. Vibrational spectroscopy has been exten- then paramagnetic, high-spin, tetragonal complexes with a sively used to demonstrate perchlorate coordination[35]. 3A2 ground state will be formed. However, if the The Ta symmetry of the free CIO4- ion is lowered[36] to separation in the spectrochemical series is large or if the C3~ and C2,, symmetry upon monodentate and bidentate binding-strengths of the in-plane and the out-of-plane coordination, respectively. This lowering of symmetry on ligands do not compare closely, the singlet state drops in coordination results in the splitting of the originally energy relative to the triplet, resulting in diamagnetic, degenerate modes and in the previously forbidden IR 6-coordinated, strongly tetragonal complexes with a 'E modes becoming allowed. Thus, the originally triply- ground state. During this change from the triplet to the degenerate modes, v3 and v4 of the free ion, are each split singlet ground state it is believed that the in-plane ligand into a non-degenerate and a degenerate mode and the field remains essentially constant, which in the present previously IR inactive modes of the free ion, v~ and u2, case is indicated by the position and intensity of the become IR active. The IR spectra of the perchlorato infrared bands due to the imidazole ligand being the same complexes is reported in Table 4. In these perchlorato for all the complexes, except for very negligible changes. In the present series of complexes, Ni(2complexes the IR spectra of the perchlorate anion seems to be intermediate in type between those of the free and isopropylimidazole)aX2, those complexes with anions of coordinated anions. In each case the v3 mode shows negligible or relatively low-coordinating ability, e.g. definite evidence of weak splitting. In all cases, IR bands X = CI , Br , I , C 1 0 4 , are diamagnetic, while those with were obtained at 930, 1060 and 1120 and 620 cm -~ region, anions of good-coordinating power, e.g. X= S C N , indicating that the perchlorate anions are slightly distorted SeCN, NO3, are paramagnetic. In the case of the on account of weak coordination. The perchlorate ion in diamagnetic nickel(If) complexes, the axial perturbation, these complexes may be visualised as being placed in long due to X, of the planar ligand field generated by four tetragonal positions resulting in an octahedral structure 2-isopropylimidazoleligands is sufficiently weak to permit with strong tetragonal distortion similar to that observed a singlet ground state ('E) for the Ni(lI) ion. In other for Cu(en)2(CIO4)2137] and Cu(en)2(BF4)2138] complexes. cases, i.e. when X = SCN , SeCN , NO~ , the complexes (d) The hexafluorophosphate ion. The highly symmetri- are paramagnetic with a triplet ground state (3A2). The cal PF6 ion with an Oh symmetry has been shown to be NiL4(PF6)2 and NiL(SCN)(CIO4) complexes having interinvolved in a type of very weak bonding, known as mediate magnetic moments are believed to possess a "semicoordination", in the complex Cu(py)4(PF6)2139]. spin-singlet state as the ground term but with a spin-triplet On coordination the IR inactive bands of the PF6 ion e.g. which is thermally accessible. All these complexes are, v~, v2, v~ and v6 are expected to be activated and the IR therefore, 6-coordinated and tetragonal in structure, active bands v3 and v4 to split. In the NiL4(PF6)2 reported having a trans-NiL4X2 configuration, since the cis here the v3 band of PF# is split and bands at 850 and complexes would not be expected to show such a large 838 cm '~ are obtained, whereas v4 shows an indication of splitting and exhibit this phenomenon [ 13]. The diamagnesplitting with a shoulder at 568 cm ' and a band at 555 tic NiLC12 and paramagnetic NiL_,Br2 and NiL2(NO3)2 are cm '. The PF~ ions are thus believed to be in long axial also believed to be octahedrally coordinated where the positions. 6-coordination is achieved through bridging of the anions, similar to the aniline complexes of Co(II) and Ni(II)[41]. GENERAL CONCLUSION The fact that nickel(II) halides form paramagnetic, Acknowledgement--The authors thank M/s BASF, West GerNi(2-methylimidazole)4X2 complexes, while with 2- many for a sampleof 2-isopropylimidazole,and Mr. J. G. Mohanty

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of I.I.T., Kanpur for C, H, N analyses, Electronic mull spectra and IR spectra in KBr phase. REFERENCES

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