Vinylmetallics as ligands

493

Journal

of Organometallic

Chemistry,

213 (1981)

493-502

ElsevierSequoia S.A., Lausanne- Printed in The Netherlands

VINYLMETALLICS

AS LIGANDS

VII *. ACETYLACETONATOBIS(q*-VINYLSILANE)RHODIUM(I) CQMPLEXES

JOHN W. FITCH and WILLIAM T. OSTERLOH Depnrtment of Chemistry, Southwest (iLS_A.)

Texas State University, San Marcos. Texas 78666

(Received October 9th, 1980)

summary Complexes of the type, [ (acac)Rh(ViSiR3),] and [ (acac)Rh(Vi2SiR2)], are readily prepared by displacement of ethylene from [(acac)Rh(C2H4),] _ The compounds,

which are iess reactive to air than similar carbon analogues, have

been characterized by ‘H and i3C NMR. The 13C NMR results suggest that the vinylsilane ligands function as better r-acceptors than do similar carbon analogues, and that dr-prr interactions between the silicon atom and the double bond are decreased upon coordination

of the vinylsilane.

Introduction Vinylsilanes readily form r-complexes with transition metals; however, the role played by the silicon atom in affecting the stability and reactivity of these complexes is not clear. For example, vinylmetalliccopper(1) complexes are surprisingly air stable and form more readily than similar all-carbon structural analogues 111. The platinum(O) complexes, [(Ph3P)2PtViSiR3], axe also more stable than their carbon analogues, increase in stability as R is changed from methyl to ethoxy and, in contrast to platinum(II) complexes [a], do not react with water 131. However, not all vinylsilane complexes are characterized by added stability (lack of reactivity) when compared to their carbon analogues. Palladium(II) reacts rapidly with vinyWanes causing silicon-carbon bond fission and yielding various products depending on reaction conditions [4]. Platinum(H) forms the ?r-complex, K[PtC13ViSiMe3], which undergoes slow hydrolysis in wet solvents *

ForpartVI see ref. 3.

0022-328X/81/0000-0000/$02.50,~

1981, ElsevierSequoia S.A.

494

forming K[PtClsCH2= CH,] and hexamethyldisiloxane 123. In order to gain additional information on the changes in physical and chemical properties of vinylsihmes upon coordination we have prepared a series of complexes of the types, [(acac)Rh(ViSiR&] and [(acac)Rh(Vi$SiR,)], and wish to report them at this time. Results and discussion Preparation of complexes

Vinylsilane-rhodium(I) complexes were prepared by the standard ligand exchange reaction shown below (eq. 1) [S]. [(acac)Rh(C,H,),]

+ 2 ViSiR3 -+ [ (acac)Rh(ViSiR&]

+ 2 C&H,.,

(1)

The new rhodium(I) compounds (listed in Table 1) derived from low-boiling ligands were isolated in pure form by removing the excess ligand in vacua (Procedure A). Complexes derived from high boiling ligands were isolated by low -temperature crystallization (Procedure B) or chromatographically (Procedure C). The vinyls&me complexes were observed to be stable in air for days although they were routinely stored under argon at 0°C. This stability is in contrast to the behaviour of the 3,3-dimethyl-1-butene complex, 5, which shows evidence of decomposition within hours upon exposure to air. This contrasting behaviour between vinylsilane complexes and their carbon analogues has been noted to be even more extreme in platinum(O) complexes 131. Cramer has noted that rhodium(1) complexes of ethylenes carrying electronegative substituents are more stable than those of comparable unsubstituted alkenes [6], and the observed stability of the vinyIsi.Ianecomplexes might have been anticipated since the -SiMe, group is a net weak electron-withdrawing group [ 7]_ The stability of the rhodium(I)-vinylsilane complexes to water is also notable. The ‘H NMR spectrum of a sample of [(acac)Rh(CH,=CHSiMe,),] in wet acetone-d6 showed no change in signal intensities over a period of two weeks at room temperature_ The complex can be refiuxed in wet acetone for two hours without evidencing any change. In this regard the complexes resemble the earlier reported Pt” complexes [3] more than Pt” complexes [2] which undergo hydrolysis in wet acetone. ‘H NMR spectra The most striking characteristic in the ‘H _NMR spectra of the new complexes is their sharpness at ambient temperature. Olefins in complexes of the type, [(acac)Rh(ol),] (01 = styrene, propene, vinylacetate), give rise to broad resonances unsuitable for complete analysis due to a large number of isomers [8] _

Rotational line broadening has also been observed in [(acac)Rh(CH,=CH&] at ambient temperature 151. These complications are apparently prevented in vinyMane complexes by the bulkiness of the ligands. Upon coordination to rhodium(I), the ABC pattern observed in free vinylsikmes is replaced by an ABX pattern as modified by io3Rh-iH coupling (chemical shifts and coupling constants are listed in Table 2 for those compounds analyzed). Complete spectral analyses for compounds 2,3,4 and 6 were not possible because of overlapping signals, but qualitative assignments are con-

495

3

(

c

a Protons numbered

1’

2\

C=C

‘Si

B

[(acac)Rh(CH2=CHSiMo&] CH2=CHCMc3 [(acnc)Rh(CH2=CHCMc3)2] (CH2=CH)2SiMe2 ’ [(ocnc)Rh(CH2=CH)2SiMe2] (cH2=cH)2sl(c~H5)2 c [(acnc)Rh(CH2=CH)2Si(CgZIS)2] (CH2=CHSLMe2)20 [(ecnc)Rh(CH2=CHSfMc~)20]

CH2=CWSiMc3

Compound

L_------

6.63 4.01 4.90 3.52 6.76 2.78 G.66 2,77 ii.71 3.04

1

R (mm)

--....--em

5.87 3.36 4.80 2.84 6.18 3.12 6.93 3.b3 G-38 4.G3

2

----.-_.---_

14.60 11,37 10.71 8.G7 14.60 11.80 14.70 11.76 14.84 12.10

23

Hz. C Ref. 26.

20,40 lb46 1?,4b 13,38 20,20 lb42 20,40 l&86 20.64 15,69

13

J (Hz)

ppm; J error 0.08-0.10

6.11 1.19 6.81 1.83 6.44 3.38 6.11 3.29 6.10 3.61

3

COMPLEXES a* I’

: Nucleus No. 4 is lo3Rh. b 6 error 0.004-0.009

--_._-

NO.

100 MHz 1H NMR ANALYSES OF OLEPINS AND RHODIUM(I)-OLEFIN

TABLE 2

1.79 1.43 -0.03 3.70 1.13 3.60 O.GO 3,82 1x53

3.80

12

0.07 1.00 0.80 067 0.92

1.84 1.68 1.66 2.02

1.41 1.81 1.84 1.61

34

2.11

24

1.73

14

497

sistent with the results obtained from the completely analyzed spectra. Structural assignments for the compounds studied followed from the analysis of the *H NMR spectrum of 7 since’the orientation of the divinylsilane in 7 must be as shown.

Following Cramer’s descriptions for “inner” and “outer” hydrogen [ 51, hydrogens 1 must be “inner” hydrogens because they are shifted to highest field (6 2.78 ppm). They are attached to the same carbon as hydrogens 2 (6 3.72 ppm) since Jl,* = 1.1 Hz, wh ich is typical of geminal protons. The other hydrogen, 3 (6 3.39 ppm), is thus the “outer” hydrogen attached to the siliconbearing carbon. This assignment is supported by Jls3 and J2,3 which are 15.4 and 11.8 Hz, respectively, for the bans and cis coupling constants. Thus “inner” hydrogens in this complex appear at higher field than “outer” hydrogens, in agreement with earlier work [ 51. The spectra of 8 and 9 can be analyzed in the same way. The spectral correlations are not so straightforward in 1 (or other complexes containing monovinyl ligands) since the vinylsilane could adopt several conformations, three of which are schematically shown. However, unsymmetrical

x)-Rh-(x li (A)

X’

x

-Rh-

Rh-

(B)

permutations of A-C are ruled out by the very symmetrical spectra. The key to the structural assignment is the observation that in 1 both protons 2 and 3 (see Table 2 for proton numbering scheme) resonate at a higher field (3.36 and 1.19 ppm, respectively) than proton 1 (4.01 ppm). Thus, these protons are likely “inner” protons and compound 1 has been tentatively assigned either the structure A or B. Structure B is favored on steric groubds_ Similar structures are proposed for the other monovinylsilane complexes and for 5, the ah-carbon analogue of 1. However, as noted in Table 4, compounds 2 and 3,may contain a small amount of a second isomer_ J?inal.ly,it will be noted that small ro3Rh-lH couplings (9-2 Hz) are observed for all compounds studied, and these values

498

are typical of those reported earlier for other rhodium(I) complexes [5,9]. The large upfield shift of “inner” protons in rhodium(I) complexes has been attributed to long range shielding resulting from the anisotropic magnetic fields of the two coordinated olefins [ 5]_ Certainly this effect appears to dominate any effect which might arise from electronic perturbations in the ligands due to silicon’s presence since compound 5, the carbon analogue of 1, also has a very highly shielded proton attached to the carbon atom bearing the t-butyl group. 13c”NMR spectra Table 3 contains 13C NMR data for several of the new complexes Gong with similar data for the uncoordinated ligands. Several effects appear to be significant. Upon coordination to rhodium(I) ah vinylsilanes studied experienced large upfield shifts in their 13C NMR spectra. The chelated complex, 7, showed an especially large coordination shift. These upfield coordination shifts (A613C) are listed in Table 4 and are observed to be 10 to 15 ppm larger than the shifts observed in 5, 6 or the propene complex. The silicon atom seems to have considerable influence on the magnitude of the coordination shift when it is bound directly to the coordinating double bond, but apparently does not appreciably affect it when it is located in the allylic position. (see compound 6, Table 4) Although the origin of A613C in coordinated alkenes is still the subject of some debate [13-181, the magnitude of the coordination shift has been shown to be related to the degree of metal-to-olefin backbonding [19-211. The larger coordination shifts experienced by the vinyls&me ligands when compared to allyisiiane or carbon analogues are therefore likely related to increased backbonding from rhodium to the vinylsilane. Thus the data indicate that the vinylsilanes studied are somewhat better rr-acceptors than either allyltrimethylsilane or simple aikenes such as propene or 3,3-dimethyl-1-butene. In addition to the bulk upfield shift effect observed for all vinylsilanes upon coordination, the individual C(1) and C(2) resonances (C(1) is the carbon atom attached to silicon) in 1 to 4 respond differently as ethoxy groups replace methyl groups in the hgands (Table 3). As the number of ethoxy groups in the ligand is increased, C(1) becomes progressively more shielded (613C changes from 64.1 ppm in 1 to 54-6 ppm in 4), but A613C remains essentially constant at ca. 76 ppm. On the other hand 613C for C(2) remains essentially constant at about 69 ppm but A613C increases from 62.1 ppm in 1 to 66.7 ppm in 4. The value of J(103Rh-13 C) remains virtually unchanged for C(1) and C(2) (12.5 to 14.3 Hz) as ethoxy groups replace methyl groups. These observations are likely related to changes in chr-pn bonding when a vinylsilane is coordinated to a transition metal. The lSC NMR spectra of the free ligands seem to be fairly well understood in terms of models involving &r-pabonding. The chemical shifts of C(1) and C(2) in vinyltrimethplilane (140.2 CH2=CH 2

(X = CH3 or O&H,)

I\

Six3 and 131.1 ppm, respectively) are reversed in vinyhriethoxysilane (129.3 and 136.6 ppm, respectively) [lo], and this reversal has been analyzed in terms of

-_-

RESULTS

CHz=CHCH3 [(acnc)Rlr(CH2-CHCH3)2]

[(acac)Rh(CH2=CH)$il(CeH&]

CHz=CHCMq [(acoc)Rh(CH2=CHCMe3)2]

CH2=CHSi(OEt)3 [(ncac)Rh(CH~=CHSi(OEt)~)2]

CHZ=CHSiMqOEt [(acac)Rh(CH2=CHSlMeZOEt)2]

Compound

NMR SPECTRAL

76.5 (15.G)

(988) 3.36.8

(13.1) 132.3 66.7 (14si) 68.4 (14.8) 118.6 66.0 (13.7)

136.6 69.9 (12.6) 109.0 67.2 (1l.G) 112.7 69.8

134.6 699 (13.0)

134.8 89.1 (13.2)

129.3 64.6 (13.8) 149.0 90.4 (14.8) 138.1 76.9 (14.1) 138.2 60.6 (10.2) 48.4

131.1 69.0 (12.7) 132.6 69.4 (12.8)

CZ

24.7 26.5

CJ

-3.3 1.8 2.4

-1.4

-44 -3,l

-1.9 -0.2 -008

0.1

-1.G

SI-CH:,

3

--

-.--

-

of

= 1.0 Hz)]

ref. 12 ref. 12

appear in parentheses

The methyl groups urc non-cquivnlent chelntcd silanc

ref. 11 CM03 134.5, J(103Rh-13C C(CH3) (30.3) ref. 10

(Hz);

In tlm

;

OCCH3 (d; 68.8, G8.1); OCH2C (18.8) Note C(1) rcsonancc overlaps OCHzCH3 resonance at 68.8 Contaminated with smnll amount of another isomer ref. 10 OCCH3 (68.7); OCH2C (18.6)

OCCH3 (68,fi); OCH2C (19.0) Contaminated with a small amount nnother isomer

Comments

peaks); the values of J( 10%-13C)

--..----_- ._-_

---_

26,9

186.6

(1.9)

27.0

(2.3) 99.4

27.3

27.1

26.8

26.8

26.9

26.9

CH3 -

99.3

(1.8)

9809

(1.8)

99.0

(2.0)

99.0

(2.1)

99.1

(2.1)

99.2

(2.3)

99.2

CH

a, b

186.4

186.8

18G.9

18b.9

18b.9

186.8

186.7

co

acac

COMPLEXES

for 103Rh.coupled

OLEFINS AND THEIR (acac)RhI ...__-_-_I__--_-_

140.2 64.1 (14.3) 138.3 62.3 (14.3)

Cl

FOR SEVERAL

’ Chemical shifts are rcportcd in ppm downfield from TMS (menn value reported spectra observed in CsDg. b Cnrbons numbered C=C 2 llsi; ;=%C_s,

No,

%

TABLE 3

5Gi.l TABLE

4

UPFIELD [(acac)Rhk]

COORDINATION

SHIFTS

(L = ALKENE) _-_.-__

OF VINYLIC

______.__-_._

..__

.

AhI3C

L in [(acac)RhL;?]

Compound

CARBONS

= IN COMPOUNDS

OF FORMULA

.~--- --- ------------

__.-- ._._-_._--..-.. (ppm)

No. cm -._-.-

_ ____-.-.

.-....._-..

CHZ=CHSiMq CHpCHSiMe2(0Et) CHz=CHSiMe(OEt)2 CHz=CHSi(OEt)3 CHz=CHCMq CHZ=CHCHZSiMq
1 2 3 4 6 6 7 -

__ __.-. H = Carbons

numbered

\

H’2

C=C

/

cm __.______.-.

___ ~~_____

.-_-

.

_.--..-._.-.~-~

--__

62.1 63.2 65.0 66.7 51.8 52.9 65.6 53.6

76.1 76.0 75.7 74.7 58.6 59.2 87.6 61.3

-_._.--

.._.-_.

_. - .._.-.

. . . _..~..

H _ b Data taken from ref. 12.

l’x

the relative importance of canonical forms I and II below [ 221. CH,= CH-Six3 ++ CH,-CH= %X3 (1) (II) Our results suggest that the vinyl group to silicon dr-pr bonding decreases upon formation of t.he 7r-complex since the C(2) resonances are essentially identical from 1 to 4. Of particular interest is the behaviour of the -Si-CH3 resonances in 1 to 3. In each case the methyl carbon becomes slightly less shielded upon complex formation, again in keeping with the suggestion of decreased dzr-p~~ bonding in the coordinated ligands. However, field anisotropy in the complexes could be responsible for these small changes in the methyl resonances_ Experimental General

Pentane and hexane were first shaken with sulfuric acid several times to remove olefinic impurities. They were than washed with dilute sodium bicarbonate, dried by refluxing over calcium hydride, and distilled. Benzene was also dried and distilled from calcium hydride prior to use. Tetrahydrofuran was purified by distillation from sodium benzophenone ketyl under argon. Alcoa F-20 alumina was neutralized by stirring with 0.5 M nitric acid, washed with water, and dried-at 120°C for 8 hours. Melting points (uncorrected) were determined on a Thomas Hoover Uni-melt apparatus. Carbon and hydrogen analyses were performed locally on a Coleman iMode 33 Analyzer or were obtained from Galbraith Laboratories, Knoxville, Tennessee_ Gas chromatograms were obtained on a Gow-Mac Gas Chromatograph with a Series 550 thermal conductivity detector. A column (4’ X l/4”) containing a stationary phase of carbowax was used. Sixty MHZ ‘H NMR spectra were determined with a Perkin Elmer Model R-12A Spectrometer while 100 MHz spectra were obtained on a Varian Model HA-100 Spectrometer. Samples were analyzed in benzene-d,, chloroform-d or carbon tetrachloride; benzene (6 7.24 ppm) or tetramethylsilane served as internal

501

references and locking signals. The chemical shifts and coupling constants reported were obtained using the programs NMRIT and NMRENl [23]. Carbon-13 NMR spectra were determined on a Varian FT SO-A Spectrometer operating at 20 MHz or on a Bruker HX-90 Spectrometer operating at 22.62 MHz. Benzene-d, was used as solvent, for deuterium lock, and as internal reference (center C6D6 resonance at 6 128.00 ppm). The 13C spectra were obtained in proton-decoupled, off-resonance decoupled, and occasionally in coupled modes to aid in spectral assignments. Starting materials

Commercially available silanes (PCR, Inc. Gainesville, Fla.; Research Organic/ Inorganic Chem. Corp., Sun Valley, Calif.; Petrarch Systems, Levittown, Pa.) were distilled prior to use. The olefin, 3,3dimethyl-1-butene was used as purchased from Aldrich Chemical Co. Dimethyldivinylsilane was prepared by the Grignard method 1241, and the fraction boiling at 78-82°C was collected and used. Di-p-chlorotetrakis(ethylene)dirhodium(I) [25], and acetylacetonatobis(ethylene)rhodium(I) [5] were both prepared according to the literature and showed acceptable analyses and IR and NMR spectra. Preparation of rhodium(l)

complexes

Illustrative examples of the basic procedure used are given. Procedures used in the synthesis of compounds not given here as examples are found in Table 1 as are yield and analytical data for all compounds prepared.

Procedure A: Preparation of acetyLacetonatobis(vinyltrimethyisilane)rhodium(I) (I). [(acac)Rh(CH,=CH,),] (0.50 g, 1.9 mmol) was placed in a 50 ml Schlenk tube which was sealed with a serum cap, evacuated, and filled with argon. Vinyltrimc&ylsilane (3 ml) was syringed into the flask which had been cooled to -78°C. Brisk ethylene evolution occurred as the reaction mixture warmed to room temperature. The excess ligand was removed in vacua to yield initially a viscous liquid which crystallized to give a free-flowing yellow-orange solid. Procedure B: Preparation of acetylacetonato(diphenyldivinylsilane)rhodium(I) (8)_ Compound 8 was prepared as described above using [(acac)Rh(CH2=

CH,),] (0.50 g, 1.9 mmol) as starting material. After evolution of ethylene was complete (about 5 min) absolute ethanol (20 ml) was added, and the solution was cooled to -78°C. The yellow product which precipitated was isolated by filtration, washed with cold absolute ethanol (5 ml) three times, and dried in vacua. The analogous vinyltriethoxysilane complex was similarly prepared using per&me as the recrystallization solvent.

Procedure C: Preparation of acetylacetonatobis(uinylmethyldiethoxysilane)rhodium(I) (3). Compound 3 was prepared as described above- After most of

the excess ligand was removed in vacua, the reaction mixture was dissolved in pentane (3 ml) and placed on a short alumina column. The column was eluted with pentane until the complex had moved three fourths of the way down the column at which time it was stripped from the column with ether/methanol (l/l ~01%). An orange, slightly viscous liquid was obtained upon removal of solvents in vacua.

502

Preparation of acetylacetonatobi‘s(3,3-dimethyi-I-butene)rhodium(I)

(5).

A solution of 3,3-dimethyl-1-butene (10 ml) in benzene (10 ml) was added to [(acac)Rh(CH,=CH,),] (0.50 g, 1.9 mmol) under an argon blanket. The mixture was heated for 1 hour at 75°C after which solvent and excess ligand were removed in vacua to yield the product as a yellow solid. ‘NN.R

results for compounds

with overlapping peaks

Spectra were recorded inbenzene-& and referenced to the proton impurity at 6 7.24 ppm. 6(ppmj, Multiplicity: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet. Chemical shifts were measured from the center of multiplets. Compound 2: 6 0.28s, 0.55s(SiCI?s), 1_22t(-OCH,CH,), l-l-l.5m(CH2=CH-), l.TOs(acac-CIYs), 3.4-4_Om(CH,=CH-), 3.83q(-OCH,CH& 5_12s(acac-H). Compound 3: 6 0.40s(SiCN3), 1.2t, 1.3t(-OCH,CIYs), l-l-1.5m(CH,=CH-), 1_75s(acac-CH3), 3.4-4.2m(CN,=CH-), 3_7q, 3_8q(-OCH&H,), 5_22s(acac-H). Compound 4: 6 1.25t(-0CH,CH3), 1.2-l.Gm(CH,=CH-), 1_85s(acac_CH,), 3.3-4.lm(CH,=CH-), 4.05q(-OCHzCHs), 5.20s(acac-&I). Compound 6: 6 O.l2s(SiCIY& 1.1-1.7m(CH,=CHC~z-), 1_80s(acac-CIYs), 2.4-3.5m(CH2=CHCH,-), 5_16s(acac-H). Acknowledgements The authors are pleased to acknowledge the support of this work by the Robert A. Welch Foundation (Grant No. AI-306) and the assistance of Dr. B.A. Shoulders of the University of Texas at Austin in obtaining some of the NMR results. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 i9 20 2l 22 23 24 25 36

J-W- Fitch, D-P. FIores and J.E. George, J. Organometal. Chem. 29 (1971) 263. EM_ Hasebke and J-W_ Fitch, J. Organometal. Chem.. 57 (1973) C93. J.W. Fitch. K-C- Ghan and J-A. FroeIich, J. Organometal. Chem.. 160 (1978) 477. W.P. Weber. R.A. Felix. AX. Willard and K-E. Koenig. Tetrahedron Letters (1971) 4701; K. Yamamoto. K. Shinohara. T. Ohuchi and M. Kwnada. Tetrahedron Letters (1974) 1153. R Cramer, J- Amer. Chem. Sot.. 86 (1964) 217. R. Cramer. J. Amer. Chem. Sot.. 89 (1967) 4621. H. Bock and K SeidI. J. Amer. Chem_ Sot.. 90 (1968) 5694. A-C. Jesse. H-P. Gyben, D.J. Stufkins and K. Vrieze. Inorg. Chim. Acta.. 31 (1978) 203. R. Cramer and G-S. Reddy. Inorg. Chem_. 12 (1973) 346. P_E Rakita and L-S. Worsbam. J. Organometal. Cbem.. 139 (1977) 135. G. Mirjajama, K_ Takahasi and K. Niskimoto. Org. Msg. Reson.. 6. (1974) 413. K.R AI%. V. Aris and J.&L Brown. J. Organometal. Chea. 42 (1972) C67. M.H. Cbishohn. H-C. Clark. L-E Manzer and J.B. Stothers. J. Amer. Chem. Sot.. 94 (1972) 5087. D-G. Cooper._R.P. Hughes and J. Powell, J. Amer. Chem. Sot.. 94 (1972) 9244. D-J. Thoennes and C-L. Wilkis, J. Magn. Reson.. 13 (1974) 18. J. Evans and J-R Norton, Inorg. Chem.. 13 (1974) 3042. M-H. ChishOh. EC. Clark. L.E. KbbEr. J-B_ Stothe+sand J-E-H_ Ward. J_ Amer. Chem. Sot.. 97 (1975) 721. D-G- Cooper and J. PowelI. Inorg. Chem., 15 (1976) 1959. R.G. Solomon and JX. Kochi. J. OrganometaL Chem_. 64 (1974) 135. C.A. ToIman, kD. E&Ii& and LX. Manzer. horg, Chem_. 14 (1975) 2353. P-W_ CIark. P. Hanisch and kJ_ Jones. Inorg. C&em., 18 (1979) 2067. R-K. Harris, J. Jones and S. Ng. J. Msg. Reson. 30 (1978) 521. J-D. Smalen and C-k Reilly. J. Chem. Phys., 37 (1962) 21. S-D- Rosenberg. J-J. Walburn. T-D_ Stankovich. A.E. Balint and H.E. Ramsden, J. Org. Chem., 22 (1957) 1200. R Cramer, Inorg. Che+. 1 (1962) 722. T. Hobgood and J.H. Goldstein. Spectrochem. Acta. 19 (1963) 321.