Ni(II) and Zn(II) perthiocarboxylates

J.inorg. nucl.Chem.,1971, Vol. 33, pp. 18411o1849. PergamonPress. Prinledin Great Britain

Ni(ll)

AND

Zn(II)

PERTHIOCARBOXYLATES

A. F L A M I N I , C. F U R L A N I and O. P I O V E S A N A Institute of General Chemistry of the University of Perugia, and C N R Laboratory for Theory, Electronic Structure and Spectrochemical Behaviour of Coordination Compounds, Rome, Italy (First received 28July 1970: itt revisedJ~)rm 25 September 1970) A b s t r a c t - T h e synthesis by Brunt and Levi, modified by Fackler and Coucouvanis, from aldehydes and ammonium polysulfide has been extended to aliphatic aldehydes, and perthiopivalate ICH:0:~(CSSS has been prepared. This ligand forms 1:2 Znlll) and Nillll complexes, analogous to the already known complex perthiobenzoates and perthio-p-toluates. The properties and electronic spectra of all these complex perthiocarboxylates are reported; the 5-membered chelate ring in Ni perthiobenzoate and presumably also in the other perthiocarboxylates, is planar with SNiS angle of 95'. Although the coordination of the Ni(ll) complexes is quadratic, their spectra are strikingly different from those of most common [NiS4] chromophores in having high intensity visible bands and very tow ligand feld ~ 1 ( - - 12 kK); possible factors causing the low value of F~ are reviewed, and the importance of the bond angle at the coordinated sulfur atoms on the extent of in-plane rr :~:effects is stressed. INTRODUCTION

R~CENT developments of the chemistry of sulfur-containing complexes have revealed, besides "normal" coordination forms such as those of quadratic Nil Ill with bidentate anions like dtc-, xant *, etc. an increasing number of examples of unusual electronic and steric structures. Thus, dtc- in complexes can be oxidized

]

S--S to (R,,NCSS).2[1] o t t o

R2N~C /

\C=NR2

"+ [2] : complex dithiocarboxyl-

\s_s /

{e.g. N idtb2)[3], or binuclear structures with bridging ligands and metal-metal bonding ates exhibit either profound conjugation effects giving rise to peculiar spectra S

/ *dtc - dithiocarbamate R 2 N - - C

:

xant = xanthate R O - - C / /

%s /

S-

S-

5;

dtb = dithiobenzoate C~Ha--C

%s S dtpa = dithiophenylacetate C~H~--C H2--(7// SdttS = perthfl-p-toluatep-CH:~--CaH4--C

//

dtt = dithio-p-toluate p-H:~C--('~H4--( 'ffff \, S S dlbS

\

,

:

\

S

perthiobenzoate C~H~--C/ff

\

S--S //

S

dlpvS = perthiopivalate(CH:0:~C--C//

\ S--S-

I. H. C. Brinkhoff, J. A. Cras, J.J. Steggerda andJ. Willemse, Rec Tray. Chim. 88,633 11969). 2. ,I. Willemse and J. J. Steggerda, Chem. Commun. I 123 I 19691. 3. C. Furlani and M. k. kuciani, lnorg. Chem. 7, 1586 { 1968). 1841

S--S

1842

A. F L A M I N I , C. F U R L A N I and O. P I O V E S A N A

(e.g. Niedtpa4) [3, 4], or unusual octa-coordination [5] such as in Vdtb4, analogously to several octa-coordinated dithiocarbamates reported recently[6-8]. Besides, coordinated dithiocarboxylates R - C S S - may undergo easily oxidation to disulfides, or oxidative ring enlargement to perthiocarboxylates R - C S S S - [ 9 , 10], and in the present paper we report some new experimental results, namely the synthesis of complexes of perthiopivalic acid (CH3)3C-CSSSH, and a preliminary discussion of the electronic structure of Ni(l I) and Zn(l I) perthiocarboxylates, on the ground of the recently determined X-ray structure of Ni(dtbS)2. S /9 Perthiocarboxylates R - C / /

\

can be prepared by a modification of the

S--Sdithiocarboxylate synthesis by Bruni and Levi[11] from aldehydes and ammonium polysulfide employing excess polysuifide and basic medium. A mixed nickel(l I) complex Ni(dtb)(dtbS) was obtained in 1952 by Hieber and Briick[9], but was misformulated as a Ni(IV) derivative (dtb)2NiS2Ni(dtb)~. It was left to Fackler and Coucouvanis [10] in 1967 to clarify the structure of the latter compound and to prepare other transition metal complexes of perthiobenzoic and perthiotoluic acids such as Ni(dttS)~, and Fe(dtt)2(dttS)[12]. We extended the preparative methods of Bruni and Levi[11] and of Fackler and Coucouvanis[10] to the aliphatic perthioligand (CH3)3C-CSSS-, and started a systematic investigation of the electronic structure and spectra of metal perthiocarboxylates M(II)L2 with M = Ni, Zn, and L = dtbS, dttS and dtpvS. PREPARATION AND PROPERTIES Perthiocarboxylato complexes can be obtained (i) by oxidative sulfuration of dithiocarboxylato complexes (e.g. (CsHsCSS)2Ni ° " s ' - ~ (CnHsCSS)(CrHsCSSS)Ni[9, 10]), or (ii) as Zn(ll) complexes (from which other metal complexes can be obtained by double exchange) by sulfurating oxidation of aldehydes with ammonium polysulfide and reaction with Zn salts[10, 11]: 2 R - C H O + 4(NH4)~S,

) 2R-CSSSNH4 + 2NH4OH + 2(NH4)2S2,-3

(1)

Following the original preparations by Fackler and Coucouvanis[10], we reprepared by the latter method Ni(dtbS)2, Ni(dttS)2 and Zn(dttS)2[10] and prepared Zn(dtbS)z. We tried also to extend reaction (1), which was originally applied to aromatic aldehydes, to aliphatic aldehydes; while the reaction failed with acetic and other simple aldehydes, it worked, although with low yield, with trimethylacetic (pivalic) aldehyde. Zn(dtpvS)2.3 g sulfur are added to 20 ml of 20% (NH4)zS and 100 ml tetrahydrofurane, and heated for a short time; then 0-1 moles (8-6 g) of pivalic aldehyde are added, the mixture is refluxed for about 4. 5. 6. 7. 8. 9. 10. I I. 12.

M. Bonamico, G. Dessy and V. Fares, C h e m . C o m m . 1106 (1969). O. Piovesana, Proc. X I I I I.C.C.C. (Kracow and Zakopane, 1970). D . C . Bradley, R. H. Moss and K. D. Sales, C h e m . C o m m . 1255 (1969). D. Brown, D. G. Holah and C. E. F. Rickard, J. chem. Soc. A423 (1970). M. Colapietro, A. Vaciago, D. C. Bradley, M. B. H. Hursthouse and I. F. Rendall, C h e m . C o m m . 743 (1970). W. Hieber and R. Briick, Z. anorg, allg. Chem. 269, 13 (1952). J. P. Fackler, Jr., D. Coucouvanis, J. A. Fetchin and W. C. Seidel, J. A m . chem. Soc. 90, 2784 (1967). G. Bruni and T. G. Levi,A ttiA ccad. N a z . Lincei 32, 5 (1923). D. Coucouvanis and S. J. Lippard, J. A m . chem. Soc. 90, 3281 (1968).

Ni(II) and Zn(II) perthiocarboxylates

1843

8 min, then cooled, diluted with 200 ml water and extracted with 300 ml ether. To the aqueous layer, 10 g hydrated ZnCI2 in 100 ml water are added, whereby Zn(dtpvS) 2 immediately precipitates; it is freed from Zn(OH)z by dissolving in tetrahydrofurane, filtering and evaporating to dryness: recrystallization from absolute ethanol. Yield 0.5 g 1 - 3 percent), m,p. 136-138°C. Ni(dtpvS).,. A concentrated solution of Zn(dtpvS)., in CSz (1.3 m-moles or 0.5 g in 30 ml) is mixed at room temperature with a concentrated solution containing an excess of NiCIz.6H~O (2.1 m-moles or 0.5 g) in 10 ml abs. ethanol; after completion of the color change from yellow to deep red, the solution is evaporated to dryness, Ni(dtpvS)2 is extracted with CS., and recrystallized from the same solvent. Yield 0-4 g (85 per cent), m.p. 118-120°C. Analytical and other data of the investigated substances are reported in Table 1. It is to be noted that addition of sulfur in the preparative Reaction ( I ) is usually not complete, ,~o (N H02Sn has to be used in large excess, and in any case a mixture of bis-perthiocarboxylatozinc and bis-dithiocarboxylatozinc is obtained. The separation can be easily achieved by crystallization from CS~:ethanol I : I for Zn(dtbSh which is much less soluble than Zn(dtbh, less easily for Zn(dttSh which is instead more soluble than the corresponding bis-dithiocomplex: with Zn(dtpvSh the formation reaction is more shifted towards nearly pure bisperthiocomplex, although the overall yield is lower. All complexes listed in the table are strongly colored, moderately soluble in all organic solvents, insoluble in water by which however they are not decomposed. The solubility in organic solvents is larger for dtpvS than for the aromatic perthiocomplexes; nickel is more soluble than zinc with the former, and less soluble than zinc with the latter ligands. In solution they are all nonconductors, and monomeric; this was proved by m.w. measurements in CHCI.~ for Ni(dtbS)2 and Ni(dttS)2 by Fackler and Coucouvanis[10], and we completed measurements for the newer compounds; e.g. for Ni(dtpvS)z 0.5 per cent in C6H6. m.w. found (osmometrically) 370 T- 15 (calc. 384). The solid zinc complexes are diamagnetic, while the nickel complexes exhibit a small positive susceptibility (of the order of Xmot + 500 × 10 6) practically independent of temperature and grossly inversely proportional to the applied magnetic field. Indeed, data for Ni(dtbS)z taken with the Faraday balance extrapolate to g e , - 0 at I/H ~ 0. The residual paramagnetism can slightly decrease after repeated crystallizations, but does not disappear completely; it is due to traces of ferromagnetic impurities, and we assume that the electronic structure of the Ni{ 1I) chromophores is indeed spin-paired as with all other [NiS4] diamagnetic quadratic complexes. The results of an infrared investigation on dithio- and perthio-carboxylato complexes will be discussed in a subsequent paper. However some remarks are pertinent here: Ni( ! !) and Zn(ll) complexes with the same ligands show only small differences in their IR spectra. More relevant, the enlargement of the chelated ring from four to five members seems to have two effects: (1) the presence for the perthiocomplexes of an S - S stretching band (sometimes a doublet) in the 500-600 cm -1 region which is completely lacking from the spectra of the dithiocomplexes; (2) to shift the ffs and ~as CSS stretching frequencies to higher energies. E.G. the most reasonable assignments for ~s and Vas are 945 and 980 cm -1 for Ni(dtb)2 and 970 and 1030 cm -1 for Ni(dtbS),.,. In Zn(dtpvS)2 and Ni(dtpvS)~, vs-s is clearly located at 525-568 cm ~ and 520 cm -1 respectively. The CSS vibrations are assigned: ~a~ = 1060 and 1080 cm-L ~ = 982 and 1000 cm ~ for Zn and Ni compounds. Further work is in progress to corroborate the above assignments of the CSS modes. Electronic spectra have been recorded both in solution and in the solid state. Reflectance and solution spectra are completely similar, except for shifts of some bands of the nickel complexes, a,s will be discussed below; also, no difference was observed for the same substance in different solvents. and the solutions were stable and followed Beer's law quite accurately. The spectra of zinc perthiocarboxylates are characterized by an intense visible band (Pmax between 22 and 25 kK, ema~ 12--15 × I0 '~) which cannot certainly be an L ~ M charge transfer in view of the presumably very low optical electronegativity of zinc[13], but might be either a M ~ L charge transfer (and indeed a similar transition could not occur with Ni(ll), whose highest d orbital is empty), or alternatively the n-~ transition of the ligand (22-23 kK in dithiocarboxylates [3]), shifted and intensified by mixing with the metal orbitals. Then, in the u.v. region (30-38 kK) there are further intense bands which by analogy with the 13. C. K. Jergensen, Chemical Applications of Spectroscopy (Edited by B. C. Wybourne), Interscience, N e w York (1962).

1844

A. F L A M I N I ,

C. F U R L A N I

g

and O. P I O V E S A N A

,4

o ,g

e-

ooz ¢.q

ca

.g

¢,a

z~ r,

e~'g:

~z ~

mg~m~m

~0

= ~~ E

<

d

~z

~

°°

~o~ e>

e-

0

gg

pe e~ g

0

~

eeg~

.o

~ ~J2

0 e~ 0

¢0

~.

¢-i

Z

2

~

N

N

1845

Ni(ll) and Z n ( l l ) perthiocarboxylates

-

,. 3-

1

/

,

.~ .i

,,.,,

I

f12

i i

/ !

,

,

L ,

40

30

20

(./..~,

'" ',,"

/

~t I il ! ~ 10

-9 ( k K } Fig. I. Absorption spectra o f metal ( M = Ni, Zn) bis-perthiocarboxylato complexes in chloroform. - . . . . M(dtpvS)~: . . . . . M(dtbS)2; - M(dttS)=.

known spectra of dithiocarboxylates and related s u b s t a n c e s [3, 14] can be assigned as 7r-~'* transitions of the thiocarbonyl group. T h e spectral patterns are essentially the same for all three ligands, apart from smaller shifts and from the obvious lack of the ~--conjugation band[3, 14] in the 3 0 - 3 2 kK region from the s p e c t r u m of the perthiopivalate. T h e spectra of the Ni complexes are more rich, beginning on the low-frequency side with a d - d band of low intensity (e - 30-50) around 12 kK, followed possibly by a second d - d band evident only as a shoulder, then by an intense band at 18-20 kK having an unusually high intensity (~ - 2.5 x 10.~) for a visible band, and cannot therefore be a pure d - d band; it might be strongly intermixed with a neighbouring charge transfer band, or, in other words, in the corresponding excited level the delocalization between ligand and metal is more pronounced than u~ual for ligand-field levels. In the chargetransfer region there are two b a n d s at 24 and 29, or 26 and 31 k K (log E 3.5-4, respectively 4,3-4.6): the 2 4 - 2 6 k K band is likely to be a c h a r g e - t r a n s f e r band S --~ Ni, more in line with other [NiS4] c h r o m o p h o r e s [ 17]. T h e band at 2 9 - 3 2 k K could be tentatively identified with a 7r-Tr* transition of the thiocarbonyl group of the ligands, possibly shifted on complexation: this assignment finds indirect support in the similar position of the C - - S band in the related metal dithiocarboxylates [3, 14], although a larger uncretainty persists here since the s p e c t r u m of the free ligands is not known owing to the 14. 15. 16. 17.

M. Bossa. J. chem. Soc. B. 1182 (1969). D. C o u c o u v a n i s and J. P. Fackler, J r., J. A m. chem. Sot'. 89, 1346 (1967). J. P. Fackler, Jr., and D. C o u c o u v a n i s , J. Am. chem. Soc. 89, 1745 (1967). C. K. J e r g e n g e n , J . inorg, nucl. Chem. 24, 1571 (1962).

1846

A. F L A M I N I , C. F U R L A N I and O. P I O V E S A N A

impossibility of isolating the free acids or the sodium salts in pure form. In farther u.v. there are absorptions at 35-39 kK which again repeat the spectral patterns of the ligands as they appear in the spectra of the zinc complexes. The compounds are remarkably stable and require no special care to avoid decomposition. Among their reactions, a very characteristic one is the desulfuration by triphenylphosphinell0, 15, 16]; which occurs easily and sharply with the newly prepared perthiopivalates as well as with aromatic perthiocarboxylates[10, 15, 16], and leads from zinc bis-perthiocarboxylates directly to zinc bis-dithiocarboxylates, while nickel bis-perthiocarboxylates are desulfurated stepwise, first to dithioperthiocomplexes, and then to bis-dithiocomplexes. The larger stability of the mixed dithioperthio-complexes of Ni(ll) compared to those of Zn(ll) is evident also in their easy formation on mixing equimolar solutions of bis-dithio and bis-perthio-nickel complexes. The violet mixed complex is formed almost completely, and the spectrum evolves towards the typical patterns of the dithio-perthionickelcomplexes with a band (for the aromatic ligands) of very strong intensity (E - 1 × 104) at 19 kK; the spectrum is therefore distinctly different from those of the greenish bis-perthio whose intense visible band falls at 18 kK with e - 3 × 103, and of the blue bis-dithio-nickel complexes with intense visible band at 17 kK with ~ - 1 x 104 and characteristic profile with shoulders at 15.5 and 18.5 kK. On the other hand, mixing bis-dithio- and bis-perthiocarboxylates of zinc produces a spectrum which is just the sum of the spectra of both species, characterised by shoulders only in the visible (first maximum at 27-29 kK) for the dithio, and a maximum ~ 22.5 kK (log ~ - 4.2) for the perthiocomplexes.

DISCUSSION

Analytical formulas, m.w. measurements and the substantial diamagnetism suggest that the Ni(ll) complexes described here are quadratic monomeric complexes of spin-paired nickel, containing [NiS4] chromophores with chelate rings S spanned by the - - C ~ \

group, like the one ascertained by X-ray structure

S---Sdetermination in the case of Fe(dtt)2(dttS) [12]. Indeed such 5-membered rings have actually been found by Bonamico[19] who completed recently the X-ray structure of Ni(dtbS)2: Fig. 2 reports the relevant structural parameters, including almost perfect planarity within the chelate ring (the plane of the phenyl group is instead rotated about 34 ° from the former ring, thus reducing the importance of possible electronic conjugations between both ~'-systems), and bond angles significantly larger than in similar four-membered sulfur-containing chelate rings: SNiS is 95 ° (against 78 ° in Nidtb2 [20], or 88 ° in bis(diphenyldithiophosphinatonickel( l l) [18])), NiSS is 106 °, and NiSC 110 ° against NiSC 86 ° in Nidtb2[20] or NiSP 85 ° in the compound of Ref. [18]. Also in some aspects of chemical and of spectroscopic behaviour do Ni(II) bis-perthiocarboxylates differ markedly from other [NiS4] complexes, e.g. they hardly form adducts with pyridine and other bases, and particularly their electronic spectra exhibit strong differences from those of most other [NiS4] complexes like Nidtc~, Nixant2, Nidtp2, Nidtpi2, etc. While namely the latter complexes exhibit well established typical patterns allowing rather clear classification into d - d bands (log E of the order of 2; ~1 between 14 and 16 kK, ~2 between 19 and 22 kK)[16, 17], c.t. bands (mostly around 26 kK) and ligands internal transitions (above 20-34 kK), nickel perthiocarboxylates 18. P. Porta, A. Sgamellotti and N. Vinciguerra, inorg. Chem. 7, 2625 (1968). 19. M. Bonamico, private communication; submitted to Chem. Comm. (1970). 20. M. Bonamico, G. Dessy and V. Fares, Chem. Comm. 324 (1969).

N i( l 1) and Zn( 11) perthiocarboxylate s

%y

107 °

1847

$

,, 95.

C

1.41

C

Fig. 2. Molecular structure of Ni(dtbS)~ (after Bonamico [ 19]).

have their d-d band (the only one which by its intensity can be confidently assigned as such) rather low ( - 12 kK) and outside the usual range. We shall here attempt a discussion of the possible factors affecting such particularly low frequency of the first d-d band. Firstly, we rule out a possible assignment as a spin-forbidden d-d or a spin-forbidden 7r-~* transition (actually never observed with certainty in similar cases); further effects which could conceivably play a role, but are also to be discarded in the present case include (i) a possible $1 very low spectrochemical position of $3 i n - - C " f

\

owing to its high negative

S2---S3charge (-1 in the free ligand, practically localized on $3 alone): however, by the same token S~, being closer to neutral, should have a higher spectrochemical position (comparable, e.g. to thioureas) and the average should not change significantly from any other chelating dithioanion (see Fig. 3); or (ii) distorted coordination geometry. In fact, diamagnetic [NiS4] chromophores are actually more or less rectantular (D2h) rather than square (O4h): however, point charge model calculations show that increasing rectangular distortion should decrease t,l (Fig. 3), contrarily to what happens with perthiocarboxylato complexes where coordination is closer to square owing to the large S-S bite (3.1 ,~[19]). The fact that solid Ni(dtbSh is miscible to a limited extent into Zn(dtbS)2 crystals had led us originally to consider the possibility of an out-of-plane helicoidal distortion (D2a if distorting a square, or D2 of distorting a rectangle), which would be really effective in reducing in-plane ligand field strength, hence Vl (see Fig. 3); since however Ni lies on an inversion center in its crystal site [ 19], and its coordination must be therefore strictly planar, also this effect has to be ruled out as the main reason for the low value of ~1, and could possibly only be responsible for the smaller spectral differences observed between pure and mixed crystals (see footnote, Table 2). Distorted bond geometry is more likely to be the actual determining effect. Although more types of relative orderings have been proposed and discussed in literature for quadratic d s complexes, larger agreement exists on the assignment of d ~ as the highest occupied orbital of the partly filled shell, mainly because of its strong in-plane ¢r*-antibonding effect. However, in "normal" [NiS4] chromo-

1848

A. F L A M I N I , C. F U R L A N I and O. P I O V E S A N A

15

.

.

.

.

.

.

x_2-__y~ .

.

.

.

10

I

0.8

1

1.2

90 °

R4 R~

80 °

70 °

SNiS

I

I

I

I

90 °

80 °

70 °

60 °

~'L or (1T-~LI

Fig. 3. Point-charge ca|culation o f d-energy levels in distorted quadratic c h r o m o p h o r e s o f [ N i S d type (left); square geometry (SNiS' = 90 °, Os = Os, = 90 °) but t w o different kinds o f sulfur ligands (S and S', D2~ point group); f o r R4/R'4 < 1,/~4 = 9, R2 = 13.5 k K ; f o r R4/R'4 > ], R4 = 9 and R= = 13,5 (constant). (centre) Rectangular distortion ( - f o u r equal ligands, D2,; --- S ~ S', C2h: R J R 4 ' = 1.28); R4 = 9, R 2 = 1 3 , 5 k K . (right) H e l i c o i d a l distortion (D2a) on a square c o o r d i n a t i o n (four equal ]igands, $]Ni$ 90 ° in the undistorted square); R4 = 9, R= = 13.5 k K .

R , =- Rn(r) = f [

[R3,(r)], rr"( - ~ r 2 dr.

phores the PSNi or CSNi angle is close to 90 ° or even less (e.g. 85 ° in Nidtpi2 [18]), and this leaves little or no availability of rr lone pairs of sulfur for in-plane dative L---> M ~--bonding (in VB language, both bonds C-S (P-S) and S-Ni would be close to pure p-o- bonds; even allowing for curved bonds formed by trigonal hybrids, dative ~--bonds in-plane would occur with little efficiency); thus, assuming that the orbital energy sequence dxz, d~,z < dz2 < dxu < d~2_,2 still holds, the ¢r* antibonding effects raising d ~ should not be very conspicuous, and ~,, i.e. the frequency of the d~, ~ d~_~ transition, should be relatively large. Instead, in five-membered rings such as those perthiocarboxylato complexes [19], there is less strain, and the SSNi and CSNi angles come actually closer to 120° (Fig. 2), leaving a lone pair of trigonai sulfur available for stronger exploitation of ¢r*-antibonding effects on d ~ , hence lower p~; the energy of dz2-,~ should not change drastically from a p-o- to a trigonal hybrid o--bond between S and Ni, since in a second order perturbation treatment[21] the lower Hs values of a trigonal S hybrid, or higher ]Hs-HM] would be compensated by better S-M overlap in (Hs + HM)2S~s/(Hs--HM)[21]. Indeed the lowest ~ values are observed in [NiS4] chromophores with 5-membered rings such as mnt[22] or perthiocarboxylato complexes, Vl of 4-membered rings lying all higher[17, 18]. With thioureas (tu) and other similar neutral conjugated monodentate iigands, the neutrality of sulfur 21. C. K. Jergensen, Orbitals in A toms and Molecules Chap. 7. Academic Press, New York (1962). 22. C. K. Jergensen, lnorg. Chim. Acta Rev. 2, 65 ( 1968); and references therein.

N i( 1l) and Zn(l I) perthiocarboxylates

1849

and possible 7r-backdonation effects should make u~ very much higher, and the fact that actually it turns to be only slightly higher than with anionic sulfur ligands (e.g. 16.1-16.7 kK in [Nitu4]'-'+ [23] is possibly explained again by good availability of lone pairs for in-plane dative S ---, M bonds raising the d~,~ orbital more than in dtc, dtp and similar Ni(ll) complexes. This explanation contrasts with a suggestion recently put forward in the literature[24] that the spectrochemical series of sulfur ligands should not depend on the 4 - o r 5-membered character of the chelate rings they form, but in view of both experimental evidence and theoretical considerations we are of the opinion that not only the bond angles subtended at the metal, but also those subtended at the sulfur donor atoms are of importance, and need in principle to be considered in determining covalent bond effects, hence quantitative values of p.f. shell orbital energies, and ultimately spectrochemical sequences, although a quantitative assessment of the relative importance of both steric factors is of course more difficult and hardly feasible at the present status of bond theories for complexes. In addition, we have to consider that a ligand-field treatment such that discussed above, is of limited validity in cases like the present one, where mixing of electronic states occurs especially in the higher excited states destroying the sharply defined character of the excited states as ligand field, charge transfer or ligand 7r* states. Thus, discerning the first excited state or a pure l.f. state is just an approximation and a rather poor one: further clarifications will come only from complete M.O. calculations; a W.H. treatment of these complexes is in progress at our Laboratory, and its results will be reported subsequently. 23. T. Tarantelli, P. Riccieri and C. Furlani, ./. inor~, nucl. Chem. 31, 3585 ~1969): and references therein. 24. O. Siiman and.I. F r e s c o . J . Am. chem. Soc. 92, 265211970).