Bull. SOC. Chim. Bdg. vol. 98 I no 12 / 1989 0037-9646 I 89 / $ 2.00 + 0.00

Q 1989 Cornit6 van Beheer van het Bulletin v.z w

SYNTHESIS, PHYSICAL PROPERTIES AND SPECTROSCOPY OF Pd(ll) COMPLEXES WITH CH300CCSNH2

Patrick Jacobs + , Katerina Dimitrou*, Spiros P. Perlepes*, John Plakatouras* and Herman 0. Desseyn +

-+ Laboratorium Anorganische Scheikunde, Rijksuniversitair Centrum Antwerpen. Groenenborgerlaan 171, 2020 Antwerpen, Belgium.

*Laboratory of Inorganic Chemistry, Department of Chemistry, University of loannina, GR-451 10 loannina, Greece

Received : 13/11/1989 -Accepted : 22/11/1989

ABSTRACT

The new complexes frans-[Pd(HL)2Clg].O.5H20. frans-[Pd(HL)2Xp] (X = Br, I, SCN). [Pd(HL)(ON02)2].

cIs-[Pd(HL2)X21 (X = NCS. ONOZ) and [Pd(HL)4]X2 (X = CI, C104), where HL = H~NCSC'OOCHB, have been prepared. The

complex trans-[Pt(HL)pClq] was also isolated for comparison reasons. The complexes were characterized by elemental analyses,

conductivity measurements. X-ray powder patterns, TG/DTG studies and spectral ('H-NMR electronic, IR and far-IR, Raman)

studies. The vibrational analysis of the complexes is also given using deuterium isolopic substitutions. Monomeric square

planar structures are assigned for the 1 :2 and 1 :4 complexes in the solid state ; the 1 :1 nitrato complex is a polymer containing

both monodentate and bidentate bridging nitrato groups. The thioamide sulfur atom of HL is the donor atom to Pd(ll) and pt(ll).

The thiocyanate ions are coordinated through sulfur in the trans complex and through nitrogen in the cis product. This

differentiation in the mode of thiocyanate bonding is discussed.

INTRODUCTION

The discovery of dithiooxamide, HzNCSCSNH2, more than one and a half century ago [l] marked

the birth of a novel and exotic ligand family [2]. The complexes of dithiooxamide and substituted

dithiooxamides have been used in analytical chemistry for a long time, but their magnetic,

semiconducting and catalytical properties have only recently been accounted for and have induced new

searches on their structure [3-51. The surprising versatility of the dithiooxamide ligand family has caused

a considerable research effort to be directed toward the synthesis of new ligands containing the -CSNH2

group and study of their coordination chemistry [2].

For several years, our group has been engaged in the design of suitable thioamide ligands,

R-CSNHR (R = H, methyl-, ethyl-, cyclohexyl-, benzyl-, etc.) groups, and the study of the vibrational

-901 -

spectra of their complexes, mainly with d8, d9 and d1o transition metal ions. A first step was the study of

complexes with ligands in which R' contains no additional donor site (R' = -CH3, -C6H5) [6-81. In a

following step, we studied complexes with ligands in which R' is also a primary or secondary thioamide

group [4,9-201, i.8. ligands which belong to the dithiooxamide ligands family. We have recently initiated a

research study on the coordination chemistry of the ligands R'-CSNHR, with R' = -CONHCH3 [21],

-CONHp [22] and -COO- [23], Le. ligands in which R' contains an additional, non-thioamidic, potential

donor site. As a part of this study, we report here the synthesis and study of Pd(ll) complexes of

H2NCSC'OOCH3, abbreviated as HL, with a special attention to their vibrational spectra. From the

vibrational spectroscopy point of view, complexes of HL should present interest because of the presence

of two simple functional groups, each of which gives completely different spectral characteristics. From

the coordination chemistry point of view, HL presents interest because of the poor coordinating capability

and the electron withdrawing nature of the ester function. These properties are expected to alter the

coordinating behaviour of HL in comparison with the behaviour of the ligands with R' = -CONHCH3,

-CONH2 and -COO- ; hence marked differences in coordinating behaviour between HL and the previous

ligands (21-231 might be expected.

Finally i t is worth noting that the coordinating properties of the thioamide bond are also of

biological interest. Recent studies on the thioarnide analogues of natural peptides have shown that a

modification of a peptide bond, -CO-NH- to -CS-NH-, has a considerable effect on the biological activity

of biomolecules [24]. As in metallopeptide systems the amide linkage is usually a critical binding site of

many metal ions and it decides structural and thermodynamic features of the complexes formed, it is of

biological interest to follow the coordination ability of the thioamide linkage and to compare its

characteristic features with those of normal amide bonding.

EXPERIMENTAL

Elemental analyses, physicochemical measurements and spectroscopic techniques were carried out by published

methods [18.19,21,25]. HL was synthesized according to the reaction :

N =CC'OOCb +excess dry H2S (CH3)3 N H2 NCSCOOCb toluene, 0°C

The obtained precipitate was collected by filtration and purified by column Alp03 chromatography. The identity and purity of the

ligand was checked by C, H and N microanalyses, mass spectrum and IR and 'H-NMR spectroscopy. HL is readily soluble in

ether, acetic acid, alcohols and water. The detailed syntheses of the complexes are described below. Kz[Pd(SCN)4] was

prepared from Kz[PdC14] by a literature method [26].

trans-[Pd(HL)zCId 0.5Hfl

(a) A total of 0.29 g (1.63 mmol) of PdC12 was dissolved in 1 ml of concentrated (37 %) hydrochloric acid and 7 ml of

glacial acetic acid. To the brown solution obtained, a solution of HL (0.35 g, 2.94 mmol) in glacial acetic acid (3.5 ml) was added

- 902 -

dropwise. A brownish orange precipitate formed after 2-3 min. The reaction mixture was stirred at room temperature for

5 min, filtered and the solid Product washed with absolute ethanol and ether. It was dried in vacuo over P4O10.

(b) To a solution of lithium tetrachloropalladate(ll), prepared in situ from 0.21 g (1.17 mmol) of PdC12 and

0.99 g (2.34 mmol) of LiCl in 3.5 ml of refluxing methanol, a solution of HL (0.25 g, 2.10 mmol) in methanol (5 mi) was added

dropwise under vigorous stirring. A brownish orange powder was precipitated. The reaction mixture was stirred at room

temperature for 3 min and the solid was filtered off, washed with ethanol and ether and dried in vacuo over CaC12.

This compound was prepared from PdBr2 (0.31 g, 1.16 mmol), HL (0.25 g, 2.10 mmol), concentrated (48 %)

hydrobromic acid (1 ml) and glacial acetic acid (total volume : 10 ml) by following the procedure as described above for the

analogous chloro complex.

(a) This complex was isolated from Pdlz (0.38 g. 1.05 mmol), HL (0.25 g, 2.10 mmol), concentrated (57 %) hydroiodic

acid (1 0 ml) and glacial acetic acid (2 ml) by following the procedure as described above for the analogous chloro complex.

(b) A total of 0.38 g (1.05 mmol) of Pdl2 was dissolved in 7 ml of a nearly saturated aqueous KI solution under reflux.

A small quantity of undissolved Pdl2 was removed by fiitration. To the black filtrate obtained, a solution of HI (0.25 g, 2.10 mml)

in glacial acetic acid (2 ml) was added dropwise under vigorous stirring at room temperature. A yellowish brown precipitate

formed immediately, which was filtered. washed with ethanol and ether and dried in vacm over P4Olo.

(c) This produci can also be prepared by solid-state reaction under pressure 116,18,27]. using IPd(HL)4]Cl2 (for its

preparation see below) as starling material. When [Pd(HL)4]C12 was pressed in a KI matrix, the obseived IR spectrum was

completely different from the spectrum of [Pd(HL)&12 obtained in a KCI matrix. The spectrum in the KI matrix was more complex

and clearly showed the presence of the characteristic bands oftrans-[Pd(HL)plp] (preparations a and b) and the typical bands of

the free ligand, according the reaction :

excess KI pressure

[Pd(HL)4]C12 + 2KI trans- [Pd(HL)2 121 + 2HL + 2KCI

So, for the solid-state preparation of trans-IPd(HL)212], a total of 60 mg of JPd(HL)&12 was pressed with ca. 600 mg of KI for

5 min. The lhick pellet was ground to yield a yellow powder. The yellow powder was extracted with ether to be free of HL and the

remaining solid was treated with 10 ml of water for 1 min. to dissolve KCI and KI. The solid was filtered off. washed with water.

ethanol and ether and dried in vacuo over P4O10. The results of the chemical analyses, the X-ray powder pattern and IR. far-IR.

Raman and IH-NMR studies indicated the formation at the desired pure trans iodo complex.

(a) A filtered solution of HL (0.23 g, 1.92 mmol) in 6 ml of water, was added to a magnetically stirred daR red solution of

Kz[Pd(SCN)4] (0.40 g, 0.96 mmol) in 3 ml of water at 5°C. An orange precipitate formed immediately. but the reaction mixture

was stirred for 25 min at 54°C to promote homogeneity. The compound was collected on a sintered-glass filter funnel (No. 3).

-903-

washed with water, ethanol and ether and dried in vacuo over P4O10.

(b) This compound can also be prepared by using a 1:4.5 Kz[Pd(SCN)4]:HL molar ratio and following exactly the

procedure described just above.

OS-[Pd(HL)p(fiCS)d

A solution of 0.26 g (0.67 mmol) of Kz[Pd(SCN)41 in 3 rnl of water and 3 ml of methanol, was added to a solution of 0.40 g

(3.35 mmol) of HL in 6 ml of methanol at 20°C. The dark brown complex began to precipitate after 10 min and after a 1 h period of

stirring it was filtered, washed with methanol and ether and dried in vacuo over silica gel.

(a) A filtered solution of HL (0.15 g, 1.26 mmol) in 4 ml of water, was added to a stirred dark brown solution of Pd(N03)~

(0.30 g, 1.30 mmol) in 3 ml of water. A dark brown precipitate formed Immediately. After 5 min stirring at mom temperature, the

precipitate was filtered off (a colourless filtrate was obtainedl), washed with water, methanol and ether and dried in vacuo over

p4010.

(b) This compound can also be prepared by using methanol as a solvent and following exactly the above procedure a.

To a solution of HL (0.25 g, 2.19 mml ) in 4 ml of water and 1 ml of methanol, a solution of Pd(N03)~ (0.1 1 g. 0.48 mmol)

in 3 ml of water was added dropwise. under vigorous stirring, at room temperature. A dark orange precipitate formed

immediately. The reaction mixture was stirred for 30 min and the solid was filtered, washed with water, ethanol and ether and

dried in vacuo over P4O10.

(a) A total of 0.24 g (0.58 mrnol) of K2[PtC14] was dissolved in 3 ml of water. To the obtained dark red solution, a filtered

solution of HL (0.13 g, 1.06 mmol) in 4 ml of water was added at room temperature. A small quantity of a brown precipitate

formed. The reaction mixture was stirred for 3 h. The solid was filtered. washed with ethanol and ether and dried in vacuo over

p4010.

(b) This compound can also be prepared in a water-ethanol solvent mixture by using a 1:4.5 Kz[PtC14]:HL molar ratio.

(a) A solution of lithium tetrachloropalladate(II), prepared in sifufrom 0.11 g (0.65 m m l ) of PdC12 and 0.06 g (1.30 mml)

of LiCl in 3 ml of refluxing acetic acid, was added dropwise to a solution of HL (0.35 g, 2.94 mmol) in acetic acid (3 ml), under

stirring at room temperature. A yellow precipitate formed, which was fiilered. washed with ethanol and ether and dried in vacuo

over silicagel.

(b) This compound can also be prepared by using methanol as a solvent and following the above procedure a.

- 904 -

Pd(OH)2, precipitated by careful neutralization of a filtered solution 01 0.124 g (0.70 mmol) of PdC12 in 10 ml 01 2M HCI

with 20.5 ml ol an aqueous standard 1N KOH solution, was dissolved in 2 ml of concentrated (70 %) perchloric acid and 8 ml of

acetic acid. This brown solution was added dropwise to a solution of HL (0.35 g, 2.94 mmol) in 2 ml of acetic acid. The pale

yellow complex began to precipitate alter 2-3 rnin and after 15 rnin stirring it was filtered off. washed with ethanol and ether and

dried in vacuo over P4O10.

Attempts to prepare the complexes [Pd(HL)4]X2 (X = Br, I, NCS. NOS). using several preparative conditions, e.0. very

high HL : metal salt molar ratio, various reaction times and temperatures and a wide variety 01 solvent mixtures, were

unsuccessful. When X = I, NCS, NOS the 1:2 complexes described in Table 1 were again isolated. When X = Br, products with

poor analytical results were obtained ; the IR spectra of these complexes were complicated. The 1 :2 perchloro complex could

not be obtained. Eflorts to prepare the cis isomers of the 1 :2 chloro, bromo and iodo complexes also met with failure.

The deuterated ligand DL was obtained by dissolving HL in CD30D, stirring lor 35 min at room temperature, evaporating

the solution to dryness on a rotary evaporator and drying the remaining residue in vacuo over P4O10 lor several days. The

deuterated complexes were prepared exactly as described for the normal complexes using DL and deuterated solvents

(CD3OD, C H Q O D , DCL, DBr, DI. DC104, D20).

RESULTS AND DISCUSSION

Physical properties of the the complexes, electronic and 1H-NMR spectra.

Table 1 summarizes the colours, analytical data, yields and A M values of the complexes. The

complexes are microcrystalline or powder-like, stable in atmospheric conditions and soluble only in DMF

and DMSO. The 1 :4 complexes have a moderate solubility in water ; however, their aqueous solutions

are not stable having a tendency for hydrolysis. The insolubility of [Pd(HL)(ON02)2] is a strong evidence

of its polymeric character. Because of the insolubility of the prepared complexes in suitable solvents, we

could not grow crystals for single-crystal X-ray structural analysis. The X-ray powder diffraction patterns

indicate that each product represents a definite compound, which is not contaminated with starting

materials or species with other stoichiometries. The data indicate that the anhydrous form of

trans-[Pd(HL)2CI2].0.5H20, trans-[Pd(HL)~Br2] and trans-[Pd(HL)pI2] are isostructural. The patterns of

trans-[Pd(HL)2(SCN)2] and cis-[Pd(HL)2(NCS)2] are different. The small number of diffraction lines

observed for the 1 :1 nitrato complex suggests a polymeric structure [28].

The AM values of the complexes [Pd(HL)4]X2 (X = CI, ClO4) in DMSO are in accord with those

being formulated as 1 :2 electrolytes [29]. The value for trans-[Pt(HL)2C12] indicates a non-electrolyte [29].

The AM values of the other complexes indicate a partial ionization in DMSO [29]. However, the facts that

(i) the conductivities of the solutions change with time, (ii) the solution and solid state d-d spectra of these

complexes differ, and (iii) the 1H-NMR spectra are complex and change with time, can be attributed to the

- 905 -

strong donor capacity of DMSO, which frequently leads to displacement of anionic ligands and change of

electrolyte type [29].

TABLE 1

Colours, yields. analytical results and molar mnductivity values for the campiexes prepared

Cwnplex Colaur Yield Analytical resultsC AM' W)

M X C H N ( S d m d " )

frens-[Pd(HL)2Cl2]-O.SH2Oe brownish orange

frans-[Pd(HL)zBrz] yellowish orange

frme[Pd(HL)zl2] yellowish brown

frane[Pd(HL)p(SCN)2] orange

ds-(Pd(HL)2(NCS)2] dark brown

[Pd(HL)(ONQ)zl dark brown

ciePd[HL)2(0NQ)d dark orange

hen+[R(HLkCld brown

[Pd(HL)&Iz yellow

[Pd(HL)41(C10412 pale yellow

64a

8 2a

3 7a

59a

2gd

96b

27a

65a

8 78

4Ia

24.99(25.06) 16.32(16.70) 17.40(16.97)

20.eo(21.09) 31.3q31.68) 14.61(14.29)

18.03(17.78) 42.41 (42.41 ) 13.00( 12.04)

23.30(23.09) d 20.33(20.85)

22.80(23.09) d 20.67(20.85)

30.06(30.44) d 10.26(10.31)

23.26(22.70) d 15.20(15.37)

39.50(38.66) 14.01(14.06) d

15.93(16.27) d 22.t6(22.04)

14.80(13.61) d 18.56(18.43)

2.25(2.62)

1.90(2.00)

1.76(1.69)

2.22(2.19)

2.40(2.19)

d

d

d 3.1 O(3.09)

2.39(2.58)

6.41( 6.60)

5.43( 5.55)

4.53( 4.68)

11.89(12.16)

12.02(12.16)

11.60(12.02)

lt.91(11.%)

d 8.40( 8.57)

7.10( 7.17)

378

239

118

148

109

I

409

2

72

81

~ ~~~~ ~

a Based on the metal. bBased on the ligand. CTheorellcal values in parentheses. dNo data available. eThe water percentage was lound by TG

analysis. fValues of molar mnduclivity for ca. 103 M solutlons in DMSO at 25°C ;the mnduclivities of the solutions were measured 10 min alter

dissolution. DThe condunlvities of the solutions drange with time; fw example me AM value of frms{Pd(HL)~Cl&O.5Hfl after 19 h ia 54Smi??n0l-~.

hThrougout lhis paper Pd(SCN) denotes an S-bonded thiocyanate ion and Pd(NCS) an N-bonded (Isothiocyanato) ion. i =insoluble : M = metal : X = CI, B, I, SCN, NCS, ON@, Clod.

The thermal decomposition of the prepared complexes was studied using TG/DTG techniques.

The TG curve of trans-[Pd(HL)2C12].0.5H20 shows a first mass loss between 80 and 105"C, which

corresponds exactly to the release of the water content ;the low temperature of water loss shows that this

is lattice held. The complexes decompose above 11 0-1 60°C with rather simple (trans-[Pd(HL)nBrp],

trans-[Pd(HL2)12, [Pd(HL)4]C12 or complex (all the other complexes) degradation mechanisms as

revealed from the number of DTG peaks. The absence of TG plateaus during decomposition indicates

that stable intermediates cannot be formed. So, complexes of other stoichiometries, i.e. [Pd(HL)2X2],

[PdLz] and [PdLX], cannot be prepared by the thermal decomposition of the complexes obtained by

normal synthesis. Some complexes, however, decompose with the formation of stoichiometric non-

stable intermediates, as stoichiometric compounds can be assigned to the curves' infleiions. Mass loss

calculations show that trans-[Pd(HL)2C12].0.5H~O decomposes with loss of two hydrogen chloride

molecules, trans-[ Pd(H L)2Br21, cis-[ Pd(HL)2(NCS)2] and cis-[Pd(HL)2(0N02)2] with loss of one ligand

molecule and one HX molecule, trans-[Pd(HL)2(SCN)2] with loss of two ligand molecules and

[Pd(HL)&12 and [Pd(HL)4](C104)2 decompose with loss of four ligand molecules. The final residue in

most cases was metallic Pd.

All the prepared complexes are diamagnetic.

Table 2 gives details of the solid state electronic spectra of the Pd(ll) and Pt(ll) complexes of HL.

Square planar structures are assigned for the complexes on the basis of their d-d frequenties [30-321.

The LMCT and MLCT transitions were assigned according to the work of Clark and Turtle [31].

TABLE 2

Solid state electronic spectral data (29400-1 1200 cm-1) forthe Pd(ll) and Pt(ll) complexesb 01 HL

Complex

~

Solid state electronic (diffuse reflectance) bandsa ( lo3 cml)

d-d

17.70C, 20.41d 20.83 shd 17.39shc. 20.666 20.53d 14.66C. 18.97d 18.46C 14.998, 18.32C

19.42c n.0.

22.07 22.22 22.12 24.69 20.41 21.87 19.93 22.73 22.99

24.39 26.80 24.75 26.32 22.73 22.99 22.04 26.52 28.17

aAssignments have been given by studying literature for Pd(ll) and Pt(ll) complexes with sulfur ligands [30-321. bA satisfactory diffuse reflectance spectrum of cis-[Pd(HL)n(NCS)2] could not be recorded because of its dark colour. C M ~ ~ t probably due to the 'A lg + lAzg transition under D4h symmetry. d M ~ ~ t probably due to the l A l g + lEg transition (D4h). eDue to 'AIg + 3Eg, 3A2g. 'Due to 'Alg --t ~ B z u . lBgU transition. DDue to l A l g + lBzU transition. LMCT = ligand- to-metal charge transfer ; MLCT = metal-lo-ligand charge transfer : n.0. = not observed.

The 1H-NMR spectrum of HL in DMs0-d~ exhibits two sharp singlets at 6 (downfield from TMS)

3.45 and 3.76 ppm assigned to the methyl protons and two broad singlet signals at 6 10.02 and

10.40 ppm due to the thioamide protons. The appearance for two resonances for the methyl protons

indicates that HL is a mixture of two isomeric forms having the methyl group in syn and anti positions with

respect to the sulfur atom of the thiocarbonyl group (33,341. The doublet character of the -CSNH2 signal

shows that there is slow (on the NMR time scale) rotation about the thioamide bond due to the partial

double C===N bond character [35]. The occurrence of the -CSNH2 signals at almost the same 6 values

in the spectra of (Pd(HL)4]X2 (X = CI, ClO4) and trans-(Pt(HL)2Cl2], relative to the free ligand, indicates

the non-involvment of the thioamide nitrogen in coordination. The spectra of these three complexes

exhibit only a sharp singlet at 6 3.76 - 3.78 ppm assigned to the methyl protons ;the single resonance

pattern observed is a strong evidence to suggest that the coordinated ligand HL takes up one of the two

(syn, anti) conformations in the complexes. In the spectrum recorded for the Pt(ll) complex, no coupling

of the methyl protons with 195Pt appears to occur; this rules out coordination of the ester group. It is worth

noting that in the same spectrum the -CSNH2 resonance are ca. 0.8 ppm apart, as against a 0.4 ppm

separation in free HL and Pd(ll) complexes. The HL coordination to Pt(ll) through sulfur enhances

largely the double bond character of the CN bond, due to a strong Pt-S interaction ; consequently,

coordinated HL shows a NH2 proton signal separation larger than that for free HL (341.

The 1H-NMR spectra of the other Pd(ll) complexes in D M s 0 - d ~ change with time ; so, it is difficult

to interpret the data. For example, a 'H-NMR spectrum acquired on a fresh solution (the spectrum was

recorded 2 min after dissolution) of trans-[Pd(HL)2C12]~0.5H20 displays the signals typical for the above

-907-

described complexes. Over a period of 5-6 min, signals due to this complex diminish and signal due to

another species appear and grow. After only 10-12 min, the peaks of the second component become as

large as those for trans-[Pd(HL)2C12]~0.5H20 and additional minor peaks appear. With time, the

spectrum becomes even more complicated. Possible other Pd(ll) species in solution are

[Pd(HL)2(DMSO-d6)CI]+, [Pd(HL)2(DMSO-d6)2]2+, [Pd(DMSO-d6)2C12], [Pd(HL)(DMSO-d6)2CI]+, etc.

(34,361. The 1H-NMR results are consistent with the conductivity data, which indicate the presence of

ionic species in the DMSO solutions of the 1 :2 Pd(ll) complexes.

The vibrational spectra of the complexes

The IR assignments for trans-[Pd(HL)2C12]~0.5H20 and for its -NH2/ND2 substituted analogue are

gathered in Table 3. Table 4 summarizes the most characteristic fundamentals of the complexes

trans-[Pd(HL)2X2] (X = CI, Br, I) and trans-[Pt(HL)2Cl2]. The IR vibrational analysis of the complexes

trans-[Pd(HL)2(SCN)2] and cis-[Pd(HL)2(NCS)2] is given in Table 5. Table 6 gives the vibrational

analysis of the 1.4 complexes. The assignments of the most intense Raman bands for some

representative complexes are given in Table 7. Fig. 1 shows the IR spectra of the complexes

trans-[Pd(HL)2CI2], trans-[Pd(HL)2(SCN)2] and [Pd(HL)d]C12. The far-IR of the complexes

trans-[Pd(HL)2Xp] (X = CI, Br, I) are shown in Fig. 2.

TABLE 3

The infrared vibrational analysis (cm-') of frans-[Pd(HL)zClz]~O.5H~O

3470 vw 3299 m 3232 rn 3147 rn 2965 w 2912 vw

1738 vs 1599 vsb

1453 s 1438 rn 1431 rn 1300 s 1222 mb 1175 rn

974 sh 966 m

915 m

3462 vw

2492 m 2417 w 2333 s 1736 vs

1508 s 1489 s 1456 w 1434 m 1282 s

1176 w 1154 m 1124 vw

967 w 954 m 913 w

- 908 -

TABLE 3 (Contd.)

frans-[Pd(H2NCSC'OOCH~)2CI2].0.5H20 tfans-[Pd(D2NCSC'OOCH~)~Cl~]~O.5D20 Assignments=

834 rn 803 m 778 rn 752 s

803 vs 718 mb 655 sb 580 m

463 w 450 w 424 w 400 rn 364 rnb 325 s 288 rnb 250 w 186 rn 155 m 130 m 1 1 1 w 104 w 98 w 71 m 62 m

579 w 566 rn 533 mb 486 w 474 w 463 w 450 w 426 w 376 w 353 rnb 324 s 284 rnb 250 w 184 rn 153 rn 132 rn 113 w 104 w 93 w 78 m 68m

6(C'02) v(CS) + v(CC') V(CS) + v(CC') v(CS) + v(CC') v(CS) + v(CC) + 6 ( C 0 2 ) W H z ) W H z ) G(NCS) 6(NCS) + o(ND2) G(NCS) + w(ND2) 7 W 2 )

ring vibrations and lattice modes

aThe weak bands due to water modes are not presented in the table. b = broad : m = medium ; s = strong : sh = shoulder : v = very ; w = weak.

TABLE 4

Most characteristic and diagnostic infrared fundamentals 01 the trans-(Pd(HL)eXz) complexes.

Mode trans-IPd(HL)2C12].0.5H20 frans-[Pd(HL)zBr2] frans-[Pd(HL)~lz) frans-[Pt(HL)~Clz]

VadNH2) 3299 rn 3309 rn 3316 m 3257 rn VS(NHZ) 3147 rn 3150 rn 3154 s 3165 rn v(C'=O) 1738 vs 1735 vs 1740 vs 1736 vs

S(NH2) 1599 vsb 1591 vs 1584 vs 1613 sb

v ( W 1453 S 1447 s 1450 s 1435 sa

p(NH2) 1222 mb 1221 m 1218 m 1255 sb v(CC') + v(CS) 915 m 913 m 912 rn 905 m v(CS) + v(CC) + 6(C02) 803 vs 803 s, 786 m 802 s. 777 m 801 rn, 761 m o ( N W 718 mb 702 mb 673 rnb 680 wbb

W-b) 655 sb 646 sb 643 sb 680 wbb

4MS) 288 mb 283 rn 278 rn 277 rnb

av(CN) + 6(CH3). b0verlapping vibrations. X = CI. Br, I ; M = Pd, Pt.

V(M-X)t 325 s 260 vs 170 s 357 vs

- 909 -

+ I r 1 4 I I

4000 3000 2000 1500 1000 S00 vlcrn-1

I 1 1-

4000 31300 2000 Is00 1 ~ 0 0 500 v/crn-1

crl %T

4 In I II If\ I llllr v I ' I1

-1 1 I 1 I 1 I I 4000 3000 2000 IS00 I000 500 vlcrn-1

Fig. 1 . The IR spectra of trans- [Pd(HL)2C12] (a), ffans[Pd(HL)2(SCN)p] (b)

and [Pd(HL)4]C12 (c).

- 910.

' t %T

400 3 0 0 200 1 00 vlcm-1

%T

I

5 0 0 400 300 200 1 0 0

5a0 400 300 200 180

Fig. 2. The far-IR spectra of trans-[Pd(HL)2Clp] (a), trans- [Pd(HL)ZBrz(b)

and trans- [Pd(HL)212] (c).

-911 -

The IR spectrum of the free ligand HL exhibits the vas(NH2) mode at 3348(s), v,(NH2) at 3164(m),

v(CH3) at 2949(m), v(C'=O) at 1721(vs), 6(NH2) at 1632(s), v(CN)+S(CH3) at 1436(m) and 1327(m),

v(C-0) at 1308(m), p(NH2) at 1235(m), p(CH3) at 1182(m), v(O-CH3) at 998(s), v(CC)+v(CS) at 930(m)

and 788(m), G(C'O2)+v(CS) at 81O(s), w(NH2) at 719(w), z(NH2) at 662(sb) and S(NCS) at 570(rn) cm-1.

The spectra of all the complexes clearly show the typical bands of the primary thioamide group,

coordinated via the sulfur atom [6,7,10,12,14,18,19], and the characteristic bands for the uncoordinated

methyl ester group [37]. Only few ester fundamentals are coupled to thioamide modes in the

4000-500 cm-l region.

The absence of large systematic shifts of the vas(NHz), vS(NH2), 6(NH2), p(NH2), w(NH2), r(NH2),

v(C'=O), v(C'-O) and v(O-CH3) bands in the spectra of the complexes, compared with the frequencies of

these modes in the spectrum of the free ligand, implies that there is no interaction between the thioamide

nitrogen and the ester oxygens and the metal ions.

The bands with v(CS) character show a frequency decrease, whereas the bands with v(CN)

character present a frequency increase in the spectra of the complexes. These shifts are consistent with

S-coordination [6,7,18,19]. On coordination via sulfur, the positively charged metal ion stabilizes the

negative charge on the sulfur atom. The thioamide function occurs in its polar resonance form and, thus,

the double bond character of the CN bond increases while the double bond character of the CS bond

decreases. The v(CS) frequency decrease is larger for frans-[Pt(HL)2Cl2], as expected from the stronger

Pt(ll)-S bonds. The frequency of the v(CN) vibration lowers in going from [Pd(HL)4]C12 to

trans-[Pd(HL)2C12].0.5H20 and also from trans-[Pd(HL)2(SCN)2] to cis-[Pd(HL)2(NCS)2], implying a

stronger Pd-S(HL) bond in the complexes in which the Pd(ll) atom is bonded to four sulfur atoms ; this is

also confirmed by the higher v(PdS) ligand frequency observed for these complexes. As the Pd(ll)-S

bond weakens when going from the PdllS4 coordination spheres to PdllS2C12 or PdllS2N2 ones the CS

bond becomes stronger, but the v(CS) shift is not apparent as the v(CS) vibration is not pure [la].

The low frequencies and the rather broad character of the v(NH2) bands in the spectra of all the

prepared complexes indicate the presence of strong hydrogen bonds between the thioamide hydrogens

and the coordinated (X = CI, Br, I, SCN, NCS, ONO2) or uncoordinated (X = CI, C104) X- anions

[ 10,14,16,18].

TABLE 5

The inlrared vibrational analysis (cm-1) of the complexes ?rans-[Pd(HL)z(SCN)2] and cis-[Pd(HL)2(!!lCS)2]

rrans-[Pd(HzNCsC'COCH3)&CN)d @ans-[Pd(D2NCSC'COCH3)2(SCN)i us. (Pd(HzNCSCoOCH3)~~CS)~ Assignmentsa

3473 vw

3266 m 3057 mb 2955 w

2158 vw 2119vs

1740 vs 1629 sb

1477 s

1436 s 1300 vs

1214 rn

1174 m

975 m

921 w 891 w

803 s

708 w

681 mb

580 rn

466 w 429 m

445 w

3464 vw

2955 w

2466 m 2291 s

2119 vs

1737 vs

1544 m 1520 s 1502 m

1436 s 1285 vs

1198 w 1161 m 1116 m

957 m

891 vw 855 w 834 m

801 m 776 m 754 m

686 m

598 m 580 m 555 rn 498 w

466 w 428 m

447 w

3345 m 3248 mn 3152 mb 2956 w 2924 w

2098 sb 1735 vsb 1593 sb

1444 s 1439 S 1402 w

1301 m 1257 m 1215 mb

1167 m 1026 w 979 m

912 w 889 w

805 m

772 m n.0.

648 wb

578 w

477 m 467 w

452 w 443 w 429 w 429 w 401 w

- 913 -

TABLE 5 (Conld.)

fraans-[Pd(H2NCSC'~CH3)2(SCN)~ trans-[Pd(O~NCSC'oOCH~)2(sCN)~] us-[Pd(H2NCSc'OOCH~)z~CS)~] Assignmentsa

377 w 360 sb

295 s 284 s h

194 s

166 rn 138 m 125 w

103 w 95 rn 88 w 74 w

399 w 387 w 357 sb

295 s 284 m

193 s

165 m 138 w 122 w

103 w 98 m 87 w 76 w

261 m v(PdN)isothiocyanate 182 w 172 w 165 w 150 w ring vibrations 126 w and 120 w lattice modes 105 w

aAssignments for the cis complex are supported by deuterium isotopic substitution. n.0. = not-observed

In the 1.4 complexes the lower vas(NH2) and vs(NH2), corresponding with higher 6(NH2), p(NH2),

o(NH2) and .r(NH2) modes, are observed for the chloride complex, due to the fact that the strength of the

intrermolecular NH-.X- hydrogen bond increases from c104- to CI-. The v(CN) mode should therefore be

found at higher wavenumber according to the series clod- < CI- [19] ; this trend was observed (Table 6) .

The presence of strong hydrogen bonds in the case of the chloride complex is also reflected by the

appearance of a low-frequency ammonium-type v(NH) band at ca. 2960 cm-1.

Weaker hydrogen bonds are present in the trans 1:2 halo complexes as deduced from the

frequencies of the v(NH2) bands. The weaker hydrogen bonds are in accordance with the lower basicity

of the coordinated halides. According to the positions of the v(NH2), 6(NH2), p(NH2), w(NH2) and r (NH2)

bands, the strength of the hydrogen bonds increases according to the series I < Br < CI, as expected.

TABLE 6

The infrared vibrational analysis (cm-1) of the complexes [Pd(HL)2]CIz and [Pd(HL)4(C104)2]

[Pd(H2NCSC'OOCH3)4]Cl2 [Pd(D2NCSC'OOCH3)4]Cl2 [Pd(H2NCSC'OOCH3)4](ClO4)2 Assignmentsa

3469 vw 3289 rn 3142 mb

3467 vw

2960 m 2958 sb

2460 rn 2392 m 2349 w 2192 sb

1741 vs 1638 mb

1741 vs

1527 s

- 914 -

TABLE 6 (Contd.)

1477 rn

1432 rn 1304 s 1229 rn

1220 rn

1176 rn

985 rn

926 w

814 rn 801 s

763 w 680 rnb

580 rn 566 w

464 w 453 w 431 w 405 rn 390 rn 366 s 293 rn 262 rn 203 s 182 rn 161 rn 138 s

108 w 103 w 95 w

1502 rn

1432 rn 1289 sb

1193 w 1175 w 1160 w 1145 vw

1109 rn

985 rn 961 s 921 w

836 rn

812 w 800 rn 786 w 775 rn 753 s

643 w

605 rn

578 w 558 w 543 rn 502 rn

465 w

405 w 386 w 364 s 280 rn 262 rn 202 s 180 rn 162 rn 137 s

113 w 104 w 81 w

1449 rn 1437 rn

1300 s

1220 s

1180 rn

1143 rn 1122

1026 s 975 rn

914 s

801 s

699 rnb

624 rn

605 w 579 rn

478 w 472 w 457 w 439 w 399 rn 393 rn 360 vs 281 rn

202 s

169 rn 138 w 130 w 113 w 100 w 80 w

v(PdS) v(PdS)

ring vibrations and lattice modes

aAssignments for [Pd(HL),$](CIO& were assigned by isotopic substitution. bModes of the hydrogen-bonded C ~ V perchorate ion

915 -

The hydrogen bonds appear to be stronger in the trans thiocyanato complex than in the

cis isothiocyanato complex. This is due to the fact that the NH,-NCS hydrogen bonds are, in general,

stronger compared with the NH.-SCN bonds which are expected to be present in the isothiocyanato

complex.

The splitting of some ligand fundamentals (i.e. vCN, vCS) in the spectra of cis-[Pd(HL)z(NCS)z]

and cis-[Pd(HL)2(ON02)2] confirms the proposed cis structures for these complexes [19].

The IR spectrum of cis-[Pd(HL)2(NCS)2] exhibits the v(CsN), v(CS) and G(NCS) modes in the

regions characteristic of terminal N-bonded isothiocyanate groups [38]. The compound

trans-(Pd(HL)2(SCN)2] has the v(C=N) band at 2119 cm-1, the v(CS)+z(NHz) at 681 cm-1 and the

G(NCS) band at 429 cm-1 ; these frequencies are characteristic for terminal S-bonded thiocyanato

ligands [38]. The v(C=N) band in the trans complex is very sharp, whereas in the cis complex it is broad.

These features are in accordance with literature [39,40] ; in general, the C=N stretching band is sharp in

the S-bonded complexes and broad in the N-bonded complexes.

The 1:4 stoichiometry found for [Pd(HL)4](C104)2 and the facts that this complex is an 1:2

electrolyte in DMSO and has a d-d spectrum typical for a square planar structure are consistent with the

presence of ionic Td perchlorate groups. However, three bands are observed in the range of the

stretching vibrations of the perchlorate group and three in the range of the bending modes in the

spectrum of [Pd(HL)4](ClO4)2 recorded in KCI, KBr, Nujol and perfluorocarbon. This is consistent with

the symmetry reduced from Td for the unperturbated clod- ions to CsV for the monodentate perchlorato

ligands [38,41]. This discrepancy has also been recently [19] observed in the IR spectra of

[Cu(LH2)2](C104)2 (LH2 = dithiooxamide, N-methyldithiooxamide, N,N'-dimethyldithiooxamide), which

have a Culls4 coordination sphere, and it has been explained assuming an interaction of each C104- only through one oxygen to form a hydrogen bond ; thus, the ion should have a C3v symmetry behaving

as a "pseudomonodentate" group. This explanation was confirmed by the crystal structure of

[Cu(H2dbzdto)2](C102)2 (Hndbzdto = N,N'-dibenzyldithiooxamide)] [4] ; in this square planar complex

each clod- ion forms only one strong hydrogen bond with one of the thioamide groups. In the spectrum

ofcis-[Pd(HL)2(0N02)2] in Nujol and hexachlorobutadiene the vibrational fundamentals of the nitrato

groups are strongly indicative of the presence of monodentate nitrates, because the separation of the two

highest frequency modes v4(B2) and vr(A1) is lower than 160 cm-1 [38]. The absence of a single strong

band in the 1400-1350 cm-l confirms that ionic D3h nitrates are absent [38]. It is worth noting that the IR

spectrum of this complex in Nujol or hexachlorobutadiene and of its powdered pressed KCI or KBr pellet

differ in the regions of the nitrato absorptions ; the obtained spectrum in KCI or KBr is indicative of the

simultaneous presence of ionic D3h nitrates and coordinated Cpv nitrato groups. The IR spectrum of the

1 :1 nitrato complex is very complex in the regions of the nitrato absorptions and supports a structure with

possible bidentate bridging (the presence of bidentate bridging nitrato groups is revealed by the

appearance of nitrate bands above 1530 cm-1 [42]) and monodentate nitrato ligands being

- 916 -

simultaneously involved.

Assignments of the far-IR metal-ligand stretching bands have been carefully made by noting (i) the

frequencies of the internal modes of the free ligand HL, (ii) bands principally dependent on X (X = CI, Br,

I, NCS, SCN), (iii) the variation in band position with deuteration, and (iv) extensive literature reports

[4,6-14,16-22,38,43]. The presence of one v(M-X)t (X = CI, Br, I, SCN) vibration (BgU under D2h

symmetry) and one v(MS) vibration (Bzu under D2h symmetry) in the f a r m spectra of trans-[Pd(HL)2X2]

(X = CI. Br, I), trans-[Pt(HL)2CI2] and trans-[Pd(HL)2(SCN)2] confirms their trans geometry [38]. In the

isothiocyanato complex three Pd(ll)-ligand vibrations are present, instead of the expected four for a

cis geometry. The fourth vibration is probably overlapped with one of the bands present at 278 and

261 cm-1. The far-IR spectrum of [Pd(HL)4](C104)2 shows a band of medium intensity at 281 cm-1,

assigned to the Pd-S Eu stretching vibration of the square planar D4h [Pd(HL)4]2+ ion [6]. Two v(PdS)

bands are observed in the far-IR spectrum of [Pd(HL)4]C12 ; this splitting must be due to a deviation of the

ideal square planar structure or to a crystal packing effect. The two expected Raman v(PdS) vibrations

(Alg, Blg under D4h symmetry) are observed at 288 and 270 cm-1.

TABLE 7

Most intensea Raman bands (cm-1) for some representative complexes.

Assignments trans-[Pd(HL)pBrz] trans-[Pd(HL)glp] frans-[Pd(HL)2(8CNjz] [Pd(HL)&I2]

1095 801 579

269

2120 1100 1092 801 810,803 580 572 313 274 288,270

aln general, satisfactory Raman spectra could not be obtained because the complexes are highly fluorescent in the laser beam.

CONCLUSIONS

(1) From the overall study presented it is concluded that in all the complexes prepared the

thioamide sulfur atom of HL is the donor atom to Pd(ll) and Pt(ll).

(2) Monomeric square planar structures are assigned for the 1 :2 and 1 :4 complexes in the solid

state. The proposed geometries are shown below:

-917 -

trans-[Pd(HL)2Xz] (X = CI, Br, I) and

trans-[Pd(HL)2(SCN)~]

trans-[ Pt( H L)2C12]

cis-[Pd(HL)n(NCS)a] [Pd(HL)4]X2 (X = CI, clod)

For the 1 :2 nitrato complex, a cis structure with two monodentate nitrate groups is proposed. For the

nitrato compound, a polymeric planar structure with both monodentate and bidentate bridging nitr

groups can be tentatively assigned.

:1

to

(3) The trans 1 :2 complexes can be synthesized directly from the [PdX4]2- (X = CI, Br, I, SCN) and

[PtCl4l2- ions by reaction with the ligand. The isolation of the trans complexes can be explained by the

fact that HL has a greater trans effect compared to X- [44]. The cis structures of the 1 :2 isothiocyanato

complexes can be explained by the fact that these complexes could be prepared only by using a high

HL:Pd(ll) molar ratio (see Experimental). Assuming firstly the formation of the ion [Pd(HL)4]2+ in solution,

the nitrato and isothiocyanato ligands occupy cis positions in a next step because they are lower than HL

in the trans effect (trans-directing) series. We could not isolate the [Pd(HL)4](N03)2 and [Pd(HL)4](SCN)2

complexes ; this is possibly due to solubility reasons. An ideal route for the preparation of

cis-[Pd(HL)2Xz] (X = CI, Br, I) complexes would be the reaction of [Pd(HL)4]Cl2 or [Pd(HL)4](C104)2 with

an excess of X- ; unfortunately the attempted reactions failed to give pure products. The failure to isolate

the [Pd(HL)4]X2 (X = Br, I, SCN) complexes can be explained by the HSAB theory (451. The Br , I- and

SCN- ions are softer bases than CI- and c104- and, hence, they have a stronger tendency to coordinate

to Pd(ll) which is a soft acid. Another reason for the isolation of [Pd(HL)4]C12 and the non-isolation of

[Pd(HL)4]X2 (X = CI, Br) is the strength of the hydrogen bonds. When X = CI, the hydrogen bonds are

stronger ; this feature favours the formation of an ionic compound when excess of ligand is present. The

fealure to isolate the 1 :2 perchlorato complex is also due to the poor coordinating capability of this ion.

-918-

(4) According to the HSAB theory [45] the Pd-X bond strength increases on going from

CI + Br + I. Therefore the Pd-S bond strength and also the frequency of the v(PdS) vibration

decreases according to the same series (see Table 4).

(5) As it has been stated in this paper, the Pd-S(HL) bond strength increases on going from

frans-[Pd(HL)~C12]~0.5H20 to [Pd(HL)41C12 and also from cis-[Pd(HL)z(NCS)2] to frans-[Pd(HL),(SCN)2].

This is due to the symbiotic effect [46], according to which soft ligands flock together in a complex with a

soft central ion. So, the tendency towards symbiosis of soft HL and thiocyanato ligands makes the mixed

complexes of definitely soft and hard ligands, i.e. the complexes frans-[Pd(HL)2C12].0.5H20 and

cis-[Pd(HL)g(UCS)2], less stable.

(6) The V(PdS)ligand frequencies in the present complexes are lower than in the complexes

[Pd(DT0)2]X2 (101 and cis-(Pd(DTO)2]X2 (141 ; this Is attributed to the absence of extra stabilization, due

to the chelate effect, in the complexes of HL. The v(PdS) bands of the HL complexes are found at

approximately the same frequencies as in the complexes of TBA [7], where TBA = thiobenzamide. The

low v(PdS) frequencies observed for the present complexes are due to the withdrawing properties of the

ester function.

(7) A final interesting point to be discussed is the differentiation in the mode of SCN- bonding in

the trans thiocyanato and cis isothiocyanato Pd(ll) complexes. The N-bonding in cis-[Pd(HL)p(NCS)p] is

clearly due to the antisymbiotic frans influence [45,47,48], according to which two soft ligands in mutual

trans positions have a destabilizing effect on each other when attached to soft metal ions. The concept of

the antisymbiotic trans influence predicts that the site frans to a strong trans director will become harder

in complexes of soft metal ions. So, soft ligands can make a soft metal ion to invert normal preference for

S over N in thiocyanate complexes ; the inversion results when the SCN- ligand binds trans, i.e. the

complex has a cis structure, to a soft ligand. This behaviour has been rationalized over the years in

terms of electronic and steric effects ; there has been considerable discussion concerning the relative

importance of these two effects [49]. In our case because of the fact that HL is a strong trans director, i.e.

it has a great frans effect, the other trans site becomes hard and consequently the SCN- ion binds with

nitrogen at this site. This results in a cis isothiocyanato complex.

Acknowledgements

The authors would like to thank the N.F.W.O. for their financial support.

Patrick Jacobs thanks the I.W.O.N.L. for their financial support.

The following text presents research results of the Belgian program on interuniversity attraction poles

initiated by the Belgian State - Prime Minister’s Office - Science Programming. The scientific

responsability is assumed by its authors.

REFERENCES?

1.

2.

3.

4.

5.

6.

7.

0.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

M. Gay-Lussac, Ann. Chim. (Paris), $&, 136-232 (1815).

M.R. Green, N. Jubran, B.E. Bursten and D.H. Busch, Inorg. Chem., z, 2326-2332 (1987) and

refs. therein.

M. Abboudi, A. Mosset and J. Galy, Inorg. Chem., a, 2091-2094 (1985) and refs. therein.

L. Antolini, A.C. Fabretti, G. Franchini, L. Menabue, G.G. Pellacani, H.O. Desseyn, R. Dommisse

and H.C. Hofmans, J. Chem. SOC., Dalton Trans., 1921-1928 (1987).

R. Veit, J.-J. Girerd, 0. Kahn, F. Robert and Y. Jeannin, Inorg. Chern., a, 4175-4180 (1986) and

refs. therein.

A.J. Aarts, ii.0. Desseyn and M.A. Herman, Transition Met. Chern., 2, 144-146 (1978).

A.J. Aarts, :?.O. Desseyn and M.A. Herman, Transition Met. Chem., 4, 46-48 (1979).

A.J. Aarts, H.O. Desseyn and M.A. Herman, Bull. SOC. Chim. Belg., 88, 25-29 (1979).

H. Hofmans, H.O. Desseyn, A.J. Aarts and M.A. Herman, Bull. SOC. Chim. Belg.. a, 19-31 (1982).

H. Hofmans, H.O. Desseyn, R. Dommisse and M.A. Herman, Bull. SOC. Chim. Belg., 91, 175-108

(1 982).

H. Hofmans, H.O. Desseyn and M.A. Herman, Spectrochim. Acta, Part A, a, 1213-1220 (1982).

H. Hofmans, H.O. Desseyn and M.A. Herman, Spectrochim. Acta, Part A, 3, 1307-1318 (1982).

B Slootmaekers, S.P. Perlepes and H.O. Desseyn, Spectros. Int. J., 4, 265-276 (1985).

H. Hofmans, P. Geboes and H.O. Desseyn, Spectrochim. Acta, Part A, 42,969-975 (1986).

P. Geboes, E.L. Esmans and H.O. Desseyn, Thermochim. Acta, m, 17-24 (1988).

P. Geboes and H.O. Desseyn, Spectrochim. Acta, Part A, 44, 963-968 (1988).

P. Geboes, B. Slootmaekers and H.O. Desseyn, Spectrochim. Int. J., 6, 227-248 (1988).

P. Geboes and H.O. Desseyn, Inorg. Chim. Acta, m, 169-177 (1989).

8. Slootmaekers, S.P. Perlepes and H.O. Desseyn, Spectrochim. Ada, Part A, in press (1989).

H. Hofmans and H.O. Desseyn, Bull. SOC. Chim. Belg., s, 83-88 (1986).

S.P. Perlepes, P. Jacobs, H.O. Desseyn and J.M. Tsangaris, Spectrochim. Acta, Part A, G, 771-780 (1987).

S.P. Perlepes, P. Jacobs, H.O. Desseyn and J.M. Tsangaris, Spectrochirn. Acta, Part A, Q,

1007-1 01 6 (1986).

S.P. Perlepes, P. Jacobs, J. Plakatouras and H.O. Desseyn, manuscript in preparation.

T. Kowalik, H. Kozlowski, I. Sovago, K. Varnagy, G. Kupryszewski and K. Rolka, J. Chem. SOC.,

Dalton Trans., 1-4 (1 987).

V. Hondrellis, S.P. Perlepes, Th. A. Kabanos and J.M. Tsangaris, Synth. React. Inorg. Met.-Org.

Chem., a, 83-112 (1988).

N.J. DeStefano and J.L. Burmeister, Synth. React. Inorg. Met.-Org. Chern., 2, 313-315 (1973).

H. Hofrnans, H.O. Desseyn, J. Sharnir and R. Dommisse, Inorg. Chim. Acta, M. L227-L229 (1981).

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

44.

45.

46.

47.

48.

49.

S.P. Perlepes, Th. Kabanos, V. Lazaridou and J.M. Tsangaris, Inorg. Chim. Acta, m, 27-37

(1 986).

W.J. Geary, Coord. Chem. Rev.. z, 81- (1971).

S.I. Shupack, E. Billig, R.J.H. Clark, R. Williams and H.B. Gray, J. Am. Chem. SOC., m, 4594-4602

(1 964).

R.J.H. Clark and P.C. Turtle, J. Chem. SOC., Dalton Trans., 2142-2148 (1977).

A.B.P. Lever, "Inorganic Electronic Spectroscopy", 2nd ed., Elsevier, Amsterdam, 1984,

pp. 341-353, 544-552.

J.A. Lepoivre, H.O. Desseyn and F.C. Alderweireldt. Org. Magn. Res., 6. 279-283 (1974).

G. Faraglia and F. Barbaro, J. Coord. Chem., B, 349-357 (1989).

R.J. Abraham and P. Loftus, "Proton and Carbon-13 NMR Spectroscopy", Heyden, London, 1978,

pp. 165-174.

R.E. Gramer, R.E. Kirkup and M.J.J. Carrie, Inorg. Chem., Z, 123-130 (1988).

H.O. Desseyn, B.J. van der Veken and M.A. Herman, Bull. SOC. Chim. Belg., &, 115-127 (1976).

K. Nakamoto, "Infrared and Rarnan Spectra", 4th ed., Wiley, New York, 1986, pp. 123. 124, 198,

199, 251, 254-257, 283-287,324-328.

A. Sabatini and 1. Bertini, Inorg. Chem., 9, 1665-1667 (1965).

G. Beran, A.J. Carty, P.C. Chieh and H.A. Patel, J. Chem. SOC., Dalton Trans., 488-495 (1973).

JL. Pascal, J. Potier, D.J. Jones, J. Roziere and A. Michalowicz, Inorg. Chem., 24, 238-241 (1985).

S.P. Perlepes, D. Nicholls and M.R. Harrison, Inorg. Chim. Acta, m, 61-68 (1985) and refs.

therein.

A. Sabatini and I. Bertini, Inorg. Chem., 4 959-961, (1965).

F.A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry", 5th ed., Wiley, New York, 1988,

pp. 1299-1 301.

"Hard and Soft Acids and Bases", edited by R.G. Pearson, Dowden, Hutchinson and floss, Inc.,

Pennsylvania, 1973.

C.K. Jergensen, Inorg. Chem., 3, 1201 (1964).

R.G. Pearson, Inorg. Chem., 2, 712-713, (1973).

J.B. Melpolder and J.L. Burmeister, Inorg. Chim. Acta, 49, 115-120, (1981).

J.H. Nelson, J.J. McDougall, N.W. Alcock and F. Mathey, Inorg. Chem., 21, 1200-1204 (1982).