Synthesis, structure and redox properties of isomeric [RuCl2(L)2] (L=N-aryl-1,2-arylenediimine) complexes formed by the oxidative dimerization of coordinated aromatic amines

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Synthesis, structure and redox properties of isomeric [RuCl

₂

(L)

₂

]

(L = N-aryl-1,2-arylenediimine) complexes formed by the oxidative

dimerization of coordinated aromatic amines

Amrita Saha

a

_{, Chayan Das}

a

_{, Kedar N. Mitra}

a

_{, Shie-Ming Peng}

b

_{, G.H. Lee}

b

_,

Sreebrata Goswami

a,

_*

a_{Department of Inorganic Chemistry, Indian Association for the Culti}_{6ation of Science, Calcutta}_{700 032}_{, India} b_{Department of Chemistry, National Taiwan Uni}_{6ersity, Taipei, Taiwan, ROC}

Received 11 July 2001; accepted 4 October 2001

Abstract

The di-arylamino complex [RuCl2(ArNH2)2L] (ArNH2= aromatic monoamine and L = N-aryl-1,2-diimino arene) reacts

spontaneously with aqueous H2O2 to produce two isomers of [RuCl2(L)2] via oxidative dimerization of the two coordinated

aromatic amines. The geometries of two isomers are cis with respect to two chlorides and are trans, cis(tc) and cis, cis(cc) in the following sequence: NH(imine), HN(imine) and ArN(imine), ArN(imine). Of the above two, the tc-[RuCl2(L)2] forms good X-ray

quality crystals. The structure of a representative has been solved by X-ray diffraction. The geometry of the cc-[RuCl2(L)2] is,

however, revealed by the spectral data. The tc-isomer possesses a C2-symmetry axis and the two-coordinated diimine ligands are

magnetically equivalent. As a result, a single NH resonance in tc-[RuCl2(L)2] appeared at approximatelyl 12.40. By contrast, the

cc-isomer is unsymmetrical and a complex pattern of1_{H NMR was observed in this case. Thus, there were two NH resonances}

observed in the range approximately l 14.11–11.88. A semi-empirical extended Hu¨ckel MO calculations on a representative example showed strong metal – ligand overlap. A highly intense electronic transition that appeared in the visible range spectra of the complexes was assigned to a transition involving two molecular orbitals with considerable metal and ligand contributions. Both the isomers of the complex showed a high potential anodic response (0.8 V) due to the RuIII_/RuII _{couple. The trivalent}

congeners [RuCl2(L)2]+ were generated in solution which showed characteristic rhombic EPR for the low spin d5 ions. The

distortion parameters have been computed using the observed g values. The axial distortion (D) in each case is found to be stronger than the rhombic (V) distortion. © 2002 Elsevier Science Ltd. All rights reserved.

Keywords:Ruthenium promoted reactions; Dimerization of aromatic amines; 1,2-Diimine; Structures; Redox

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1. Introduction

Metal complexes of redox non-innocent dioxolene ligand series together with their nitrogen analogues are of considerable interest [1 – 3] because of their excep-tionally rich redox and spectroscopic behavior, which originate from a strong degree of orbital mixing be-tween metal (dp) and ligand (pp) frontier orbitals. Since the first report [4] of the ruthenium complex of bqdi, [Ru(bqdi)3](PF6) (benzoquinonediimine) by Warren, the

ruthenium complexes of bqdi and its redox partners have been the focus of many recent studies [5,6].

As a part of our ongoing work [7,8] in the area of metal promoted assembly of diimine complexes based on oxidation of aromatic amines we report herein the synthesis [7c], characterization, crystal structure and redox properties of isomeric RuCl2(L)2(L =

N-aryl-1,2-arylenediimine). The diimine ligand L is generated in situ by the oxidative dimerization of two coordinated aromatic amines (Scheme 1). It is proposed that the metal ion mediates these transformations by holding the amine residues in close proximity and also by taking part in the redox processes. In the present study two di-arylamino complexes [7b] of ruthenium of

gen-* Corresponding author. Tel.: + 4971; fax: + 91-33-473-2805.

E-mail address:[email protected](S. Goswami).

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Scheme 1.

solution resulted. The crude mass was extracted with dichloromethane and was washed thoroughly with 1 M hydrochloric acid and finally washed with water. The acid free solution was dried over anhydrous sodium sulfate. Hexane was then layered over the red solution and after 2 days shiny brown crystals were obtained. Yield 60%. Calc. for C24H24Cl2N4Ru (540): C, 53.33; H,

4.44; N, 10.37. Anal. Found: C, 53.46; H, 4.50; N, 10.52%.

A similar reaction using p-toluidine instead of aniline afforded (1b) in 65% yield. Calc. for C28H32Cl2N4Ru

(596): C, 56.37; H, 5.37; N, 9.39. Anal. Found: C, 56.48; H, 5.27; N, 9.50%.

2.1.2. Synthesis of dichloro

bis(N-phenyl-1,2-benzoquinone diimine)ruthenium(II), tc-[RuCl₂(La₎

2] and cc-[RuCl2(L

a₎

2] (2a)

To a dichloromethane solution of [RuCl₂(PhNH₂)₂ -(La_)]·CH

2Cl2 (0.108 g, 0.2 mmol), 0.5 ml 30% weight/

volume hydrogen peroxide solution was added and the mixture was stirred at room temperature for 30 min. An intense blue color resulted, which was evaporated at room temperature. The crude product, thus obtained, was purified on a TLC plate. Two intense blue bands moved with a 25% chloroform – acetonitrile mixture. The two geometrical isomers were collected by evapora-tion of the solvent at room temperature. The first blue isomer (tc-) was crystallized by slow diffusion of dichloromethane tetrahydrofuran solution of the com-plex into hexane. Analysis and yields of the two isomers are given below:

tc-[RuCl₂(La₎

2] (first blue), yield (35%). Calc. for

C₂₄H₂₀N₄Cl₂Ru (537): C, 53.63; H, 3.72; N, 10.43. Anal. Found: C, 53.65; H, 3.43; N, 10.52%.

cc-[RuCl2(La)2] (second blue), yield (35%). Calc. for

C24H20N4Cl2Ru (537): C, 53.63; H, 3.72; N, 10.43.

Anal. Found: C, 54.02; H, 3.33; N, 10.62%.

The tc and cc isomers of [RuCl2(Lb)2] (2b) were

similarly prepared following the above procedure. tc-[RuCl2(Lb)2], yield 32%. Calc. for C28H28N4Cl2Ru

cc-[RuCl2(Lb)2], yield 32%. Calc. for C28H28N4Cl2Ru

2.2. Crystal structure determination

The crystallographic data for the compound tc-[RuCl2(La)2] (2a) are collected in Table 1. Slow

diffu-sion of a dichloromethane – THF solution of tc-[RuCl2(La)2] in hexane yielded thin needle like

crys-tals (0.50 × 0.05 × 0.04 mm3_{). Intensity data were}

col-lected on a SMART CCD diffractometer with

graphite-monochromated Mo Ka radiation (u=0.7107 eral formula [RuCl2(ArNH2)2(L)] were chosen for the

study of oxidation reaction. Oxidative deprotonation [9] of coordinated aromatic amines leading to the for-mation of metal – imido complexes was known in the literature. However, oxidative dimerization of coordi-nated amines of above type has been noted by us only in the recent past [7]. Furthermore, the transformation described in this work represents an example of an oxidation reaction, which led to the oxidation of inter-nal substrate. Such reactions are important but are scanty [10].

2. Experimental

The solvents and chemicals used for synthesis were of analytical grade. The supporting electrolyte tetraethy-lammonium perchlorate (TEAP) and solvents for elec-trochemical work were obtained as before [11]. 2.1. Physical measurements

A Shimadzu UV 2100 UV – Vis spectrophotometer was used to record electronic spectra. The IR spectra recorded with a Perkin – Elmer 783 spectrophotometer.

1_{H NMR spectra in CDCl}

3 were recorded with a

Bruker Advance DPX300 spectrophotometer. SiMe4

was used as internal standard. A Perkin – Elmer 240C elemental analyzer was used to collect microanalytical data (C,H,N). Electrochemical measurements were per-formed under dry nitrogen with an EG&G PARC model 273A potentiostat/galvanostat based electro-chemistry system. All potentials, reported in this work, are referenced to the saturated calomel electrode (SCE) and are uncorrected for junction contributions. EPR measurements were made with a Varian 109C E-line X-band spectrometer fitted with a flat cell. Spectra were calibrated with the help of DPPH (g = 2.0037). Elec-tronic spectra were recorded on a JASCO V-570 spectrophotometer.

2.1.1. Synthesis of dianiline dichloro

N-phenyl-1,2-benzoquinone diimine ruthenium(II) [RuCl2(La)(PhNH2)2] (1a)

A sample of RuCl3·3H2O (0.25 g, 0.95 mmol) was

added to 0.5 ml of aniline, and the mixture was heated on an oil bath at 140 °C for 30 min. The brown color changed gradually to violet and then an intense red

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Table 1

Crystallographic data for [RuCl2(L a₎

2]·THF

Empirical formula C28H28Cl2N4ORu

608.51 Mr monoclinic Crystal system P21/c Space group

Unit cell dimensions

a (A, ) 11.5638(7) 20.5507(11) b (A, ) 11.8331(7) c (A, ) 96.062(2) i (°) 2796.3(3) U (A,3₎ Z 4 1.445 Dcalc(Mg m−3) 0.0698 R1 [I\2|(I)] 0.1520 wR2 [I\2|(I)] 1.033 Goodness-of-fit

The complex 1 is poorly soluble in common organic solvents. A suspension of 1 in CH2Cl2, however, reacts

instantaneously [7c] with 5% aqueous H₂O₂ at room temperature (300 K) to afford [RuCl₂L₂] (2) (Eq. (2)): [RuCl₂(ArNH₂)₂L]H2O2[RuCl₂L₂] (2) Preliminary work up followed by chromatographic purification yielded the two geometrical isomers of [RuCl2L2] (vide infra). In this reaction, the two

coordi-nated amine residues have undergone oxidative cou-pling to give a 1,2-diamino ligand. This transformation,

12 is a combination of many operations: (i)

isomer-ization of the starting complex 1 in order to bring two ArNH2 in close proximity (transcis); (ii) oxidative

coupling of two ArNH2 residues to ortho-semidine

(C – N bond making); and (iii) further oxidative dehy-drogenation, ortho-semidine1,2-diimine (diamine diimine).

3.2. Formulation and spectral characterization of isomeric [RuCl2(L)2]

The crude product, obtained from the reaction (1), contained two major blue fractions, which were sepa-rated on a preparative TLC plate. These were obtained in almost identical yields (ca. 35% each). FAB mass spectra of these two compounds are nearly identical which indeed confirm that these two are isomers. Due to the unsymmetric nature of L, there exist [13] five geometrical possibilities for [RuCl2L2]. The present

complexes are characterized by two sharp and medium Ru – Cl stretches in the region 310 – 350 cm− 1_{(Table 2).}

Thus, the RuCl₂ fragments in these complexes are cis and three geometries A – C are possible (Scheme 2). The relative orientations (trans/cis) of the coordinated imine nitrogen atoms [N(H), N(H)] and aryl substituted imine nitrogen atoms [N(Ar), N(Ar)] in the isomeric [RuCl2(L)2] are trans, cis and cis, cis respectively (vide

infra). We shall refer these two isomers as tc- (first blue) and cc- (second blue) isomers. Of the above two com-A, ). A total of 13 424 unique reflections were collected

and 5719 with I]2|(I) were used in the refinement [12]. Refinement of positional and anisotropic thermal parameters for all non-hydrogen atoms converged to R = 0.0698. The final Fourier difference maps showed residual extrema at 0.591, − 0.570 e A, − 3_.

3. Results and discussion

3.1. The starting compounds and synthesis

Hydrated ruthenium(III) chloride reacts [7b] with neat ArNH₂ at 130 °C to yield the brown complex, [RuCl₂(ArNH₂)₂L] [ArNH₂= aniline, L = La _and

ArNH₂= p-toluidine, L = Lb_{, Eq. (1)], which was used}

as the starting complex for carrying out the synthesis. RuCl3·nH2O ArNH2 130 − 135 °C [RuCl2(ArNH2)2L] 1 (1) ArNH2 L 1 aniline La _1a p-toluidine Lb _1b Table 2 IR and NMR data IRa_(cm−1₎ Compound 1_{H NMR}b_(ppm) w(CN) w(Ru–Cl) lMe w(N–H) lN–H(imine) tc-[RuCl2(La)2] 3160 1540 360, 350 12.47 360, 355 1545 3150 cc-[RuCl2(La)2] 12.46, 12.00 tc-[RuCl2(Lb)2] 3160 1550 360, 345 2.33, 2.023 12.33 3165 1560 cc-[RuCl2(Lb)2] 360, 340 2.38, 2.32, 2.03, 1.94 14.11, 11.88 360, 340 [RuCl2(ArNH2)2(La)]c 3300, 3100 1580 14.66 3310, 3140 [RuCl2(ArNH2)2(Lb)]c 1580 350, 340 2.17, 2.08, 1.97, 1.96 14.51 a_{In KBr.} b_{In CDCl} 3. c_{lN–H(–NH2}_{) appeared at 5.09, 4.84; 5.07, 4.80 ppm.}

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Scheme 2.

Fig. 2. Molecular structure and atom numbering scheme for tc-[RuCl2(La)2].

pounds, the tc-[RuCl2(La)2] formed suitable crystals for

X-ray structure determination. The geometry of the cc-isomer was however, revealed by the spectral data. The complexes showed well resolved1_{H NMR}

spec-tra in CDCl3at 300 MHz, chemical shift data are listed

in (Table 2) and the spectra of two representatives are shown in Fig. 1. The spectrum of the tc-isomer is simpler than that of the cc-isomer. Each kind of proton in the tc-isomer gave rise to only one signal (singlet or multiplet), which indicates that the two L ligands in this isomer are magnetically equivalent. By contrast, the corresponding cc-isomer showed two signals for each kind of protons. In this geometry the two ligands are magnetically inequivalent and their spectral pattern is as expected. For comparison, while the tc-[RuCl2L2]

showed only one NH resonance [7d] at l 12.40, the

corresponding cc-[RuCl2L2] displayed two such

reso-nances at approximately l 14.11 and 11.88. The two isomers of the complex with methyl substituted ligand, [RuCl₂(Lb₎

2] showed two (tc-) and four (cc-) methyl

resonances, respectively, in the range l 2.38–1.94. The

1_{H NMR together with the IR spectral data, thus}

confirms the geometries of the two isomers of [RuCl2L2].

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3.3. Crystal structure, tc-[RuCl2(L

a₎

2]

The first blue fraction of RuCl₂(La₎

2 formed suitable

X-ray quality crystals for structure determination. A view of the molecule is shown in Fig. 2, which shows that one of the N-phenyl rings is disorder. Selected bond parameters are collected in (Table 3). The coordi-nation sphere involves RuCl2N4. Two chlorides are cis

to each other and the aryl substituted imine nitrogens are also in relative cis positions. The imine C – N bond lengths, average 1.316 A, are within the range of values expected for the diimine oxidation state of L [5e,7]. These bonds are much shorter than the two C – N single bonds, viz. C(7) – N(2) and C(19) – N(4), average 1.434 A, . Moreover, the C–C lengths (average 1.348 A,) at positions that would have localized double bond for the diimine form of the ligands are significantly shorter than the other C – C lengths (average 1.432 A, ) in the same ring. The Ru – Cl lengths in this complex compare well with the corresponding lengths in the related com-plexes. The overall gross structural features of this conforms well with those of a related osmium(II) com-plex [7a], tc-[OsBr2(La)2]. Moreover, the average of

Ru – N lengths in the present complex (average 1.995 A, ) significantly smaller than RuII_{– N single bonds. For}

comparison, the Ru – N bond distances [10,11] in RuII_–

NH3 and RuII– bpy (bpy = 2,2%-bipyridine) lie in the

range 2.14 – 2.06 A, ; which are appreciably longer than [9,12] Ru(II) – N(azo) lengths (ca. 1.98 A, ) in [RuCl2(pap)2] complex where very strong metal (dp)

and ligand (pp) interactions were noted. Thus the short Ru – N distances in the tc-[RuCl₂(La₎

2] complex

confi-rms strong dp–pp interactions. This effect has also been reflected in the high metal based anodic potentials (vide infra).

3.4. Electronic spectra and redox properties

From the crystallographic data it is obvious that in the ruthenium complexes, under consideration, there exist strong metal – ligand dp–pp interactions. In order

to have some insight into the nature of redox and spectroscopically relevant orbitals, a standard extended Hu¨ckel MO calculation using the crystallographic parameters was performed on tc-[RuCl2(La)2] using the

CACAO programme [14] by Mealli and Proserpio. A

partial energy level scheme is shown in Fig. 3. In this complex, the molecular orbitals, LUMO and HOMO-1 are heavily mixed orbitals where as HOMO is almost a pure ruthenium orbital. Pictorial representations of these three molecular orbitals are deposited as supple-mentary information.

The electronic spectral data are collected in (Table 4). Two representative spectra of isomeric (tc- and cc-) [RuCl2(La)2] are shown in Fig. 4. There are two major

transitions in the range 1600 – 250 nm. The lowest en-ergy transitions, occurring in the near IR region, is weak (u, ca. 950 nm; m, ca. 950 M− 1_cm− 1_{) and broad}

compared to the next highly intense transition (u, ca. 600 nm; m, ca. 23 100 M− 1_cm− 1_{) in the visible region.}

However, the intensity of NIR transition for the present complex is too intense for a d – d transition. This transition may be ascribed to HOMO Ru(dp) LUMO transition. The next highly intense transition at approximately 590 nm is due to a transition between two heavily mixed metal – ligand orbitals, LUMO and HOMO-1. This type of transitions are known to exist in other related complexes [7d,15] and their intensities are

Fig. 3. Partial energy level diagram of tc-[RuCl2(La)2]. Table 3

Selected bond lengths (A, ) for tc-[RuCl2(La₎ 2] 2.402(2) Ru–Cl(1) C(1)–C(2) 1.429(9) 2.388(2) C(2)–C(3) 1.360(10) Ru–Cl(2) 1.996(5) Ru–N(1) C(3)–C(4) 1.422(11) 2.005(5) Ru–N(2) C(4)–C(5) 1.337(11) 1.982(6) Ru–N(3) C(5)–C(6) 1.413(9) Ru–N(4) 1.999(6) C(6)–C(1) 1.453(9) 1.314(8) C(1)–N(1) C(13)–C(14) 1.429(12) 1.333(8) C(6)–N(2) C(14)–C(15) 1.332(14) 1.302(10) C(13)–N(3) C(15)–C(16) 1.420(2) 1.314(9) C(18)–N(4) C(16)–C(17) 1.362(14) 1.436(9) C(7)–N(2) C(17)–C(18) 1.445(11) 1.432(8) C(19)–N(4) C(18)–C(13) 1.447(10)

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Table 4

Electronic spectral data

Compound Absorptiona_umax_{[nm (}_{m (M}−1_cm−1_))] 948b_{(950), 590 (22682), 486}b_(6993), tc-[RuCl2(La)2] 432b₍₅₇₄₈₎ cc-[RuCl2(La)2] 970b(942), 598 (23124), 482b(6350), 432b₍₅₆₄₂₎ 930b_{(850), 592 (19500), 475}b_(9580), tc-[RuCl2(Lb)2] 406b₍₉₉₃₃₎ cc-[RuCl2(Lb)2] 930b(840), 598 (20100), 476b(9240), 419b₍₉₈₇₀₎ [RuCl2(ArNH2)2(La)] 585b(2070), 492 (6840), 365 (2650) [RuCl2(ArNH2)2(Lb)] 590b(6750), 495 (15350), 380 (5390)

a_{Data obtained from absorption spectra.} b_Shoulder.

except the minor differences in band positions and intensities.

The redox properties of the ruthenium complexes have been studied by cyclic voltammetry with a plat-inum-working electrode in the range 1.8 to − 1.5 V. Voltammetric data are collected in (Table 5) and repre-sentative voltammograms are displayed in Fig. 5. All the potentials are referenced to the saturated calomel electrode (SCE). Both the isomers of [RuCl2(L)2] show

almost identical voltammograms. The difference in ge-ometry seems to have negligible effects on the redox potentials. The complexes exhibited a reversible anodic response near 0.80 V and two irreversible cathodic responses in the potential range − 0.60 to − 1.60 V. The oxidation process in these complexes occurs at HOMO, which is primarily a metal orbital. One-elec-tron coulometric oxidation of both the isomers of [RuCl2(La)2] afforded reddish – brown solutions. The

voltamograms of the electrogenerated trivalent com-plexes are superposable (initial scan cathodic) on those of the corresponding isomers of [RuCl2(La)2] (initial

scan anodic) showing that the metal redox processes are reversible. The EPR of the Ru(III) complexes were studied in acetonitrile – toluene glass at 77 K, which display rhombic EPR spectra with three distinct reso-nances (Table 6). The spectrum of a representative example cc-[RuCl2(La)2]+ is shown in Fig. 6. The

triva-lent ruthenium complexes may also be generated by the chemical oxidation of the bivalent complexes by Br₂. Unfortunately, the oxidized complexes are not stable

Fig. 4. Solution spectra of tc-[RuCl2(La)2] ( — ) and cc-[RuCl2(La)2] (---) in dichloromethane.

Fig. 5. Cyclic voltammograms (scan rate 50 mV s− 1_{, solvent CH} 3CN, 0.1 M TEAP) of tc-[RuCl2(La_{)2] ( — ) and cc-[RuCl2(L}a_{)2] (---).} Table 5

Cyclic voltammetric data

Compound Oxidation E1/2 Reduction E1/2(V)a (V)a 0.81 −0.66b_{, −1.41}b tc-[RuCl2(La)2] cc–[RuCl2(La)2] 0.80 −0.65b, −1.41b 0.70 tc-[RuCl2(Lb)2] −0.75b, −1.48b cc-[RuCl2(Lb)2] 0.70 −0.78b, −1.55b 0.60 −0.73b [RuCl2(ArNH2)2(La)] 0.52 −0.82b [RuCl2(ArNH2)2(Lb)]

a_{Experiments were carried out in CH}

3CN at 298 K using TEAP as supporting electrolyte. The reported data correspond to a scan rate of 50 mV s−1_.

b_{Irreversible cathodic response, the potential corresponds to Epc.}

Table 6

EPR g-valuesa_{and derived parameters of the complex [RuCl} 2(La)2]+ Compound Derived energy parametersb_(cm−1₎

g1 g2 g3 D V 2.256 2.404 1.882 tc-[RuCl2(La)2]+ 5529 2014 2.469 2.272 cc-[RuCl2(La)2]+ 1.909 6676 3136 a_{In 1:1 dichloromethane–toluene solution at 77 K.}

b_{Taking the value of the spin–orbit coupling constant (}_{u) for} Ru(III) as equal to 1000 cm−1_.

generally very high. For example, a similar transition at approximately 620 nm was noted [7a] for a related osmium complex, [OsBr2(La)2]. The spectral patterns of

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Fig. 6. EPR spectrum of cc-[RuCl2(La)2]+ in frozen 1:1 dichloromethane – toluene solution at 77 K, showing computed split-tings of t2orbitals. DPPH = diphenylpicrylhydrazyl.

Acknowledgements

Financial support received from the Council of Sci-entific and Industrial Research, New Delhi is gratefully acknowledged.

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enough for their isolation. They revert to the parent divalent complexes rapidly. The observed rhombicity of the EPR spectra in the present cases is understandable in terms of the gross molecular symmetry of these complexes, containing three non-equivalent axes. The distortion parameters have been computed [16] using the observed g values. The axial distortion (D, which split t2 into a and e) is indeed much stronger than the

rhombic distortion (V, which splits e). Thus the EPR spectral data of the complexes indicate significant dis-tortion from ideal octahedral arrangements which in-deed in line with the observed structure of the parent bivalent complexes.

In conclusion it may be stated that the present work further demonstrates that aromatic primary amines are susceptible to metal mediated oxidative dimerization processes via the formation of a C – N bond. The iso-meric complexes, RuCl₂L₂, thus formed, represent an interesting family of ruthenium – diimines that show strong metal d(p)–ligand p(p) interactions. Chemical reactions including the chloride substitution at [RuCl2(L)2] are now under scrutiny.

4. Supplementary material

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 160040. Copies of this infor-mation may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1233-336-033; e-mail: deposit@ ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

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[14] C. Mealli, D.M. Proserpio, J. Chem. Educ. 67 (1990) 399.

[15] (a) A.B.P. Lever, H. Masui, R.A. Metcalfe, D.J. Stufkens, E.S. Dodsworth, P.R. Auburn, Coord. Chem. Rev. 125 (1993) 317; (b) G.A. Mines, J.A. Roberts, J.T. Hoop, Inorg. Chem. 31 (1992) 125.

[16] (a) B. Bleany, M.C.M. O’Brien, Proc. Phys. Soc. Lond., Sect. B 69 (1956) 1216;

(b) J.S. Griffith, The Theory of Transition Metal Ions, Cam-bridge University Press, London, 1961, p. 364;

(c) S. Bhattacharyya, A. Chakravorty, Proc. Indian Acad. Sci. 95 (1985) 159;