Mayr, Herbert

Siehl, Hans Ullrich

English

Crossref

Stable vinyl cations. Direct spectroscopic observation of vinyl-substituted vinyl cations

Universität München: Elektronischen Publikationen

J.  Am. Chem. SOC. 1982, 104, 909-910 909 
appear to be significantly broader than the other resonances. Thus, 
the low-temperature spectra (Figure 1) give no evidence of freezing 
out of a "classical" norbornyl cation. This means only that if one 
views the spectra in terms of rapidly equilibrating classical ions, 
the barrier of that equilibration must be lower than about 3 kcal 
mol-I. Within this limitation, one can adopt a different view. The 
recent studies of the appropriately deuterium labeled norbornyl 
cation by Saunders show very small isotopic splittings owing to 
perturbation of ~ymmetry. '~ This result is stronger support, albeit 
less direct, for the "nonclassical" symmetrical norbornyl cation. 
We anticipate that we shall be able to conduct MAS 13C NMR 
spectroscopy in the 30 K range in the future and look forward 
to additional studies of this and other cations over this extended 
temperature range. 
Acknowledgment. We thank the National Science Foundation 
for financial support (Industry/University Cooperative Program, 
Grant CHE-8020743) and IBM Instruments, Inc., for postdoctoral 
support (V.K.M.). The skilled assistance of R. D. Kendrick is 
gratefully acknowledged. We thank Dr. J. R. Lyerla for his 
interest and help in this work and Professor Martin Saunders for 
stimulating discussions. 
Registry No. 2-Norbornyl cation, 24321-81-1. 
(15) Saunders, M. A.; Kates, M. R. J .  Am. Chem. SOC. 1980,102,6867. 
Stable Vinyl Cations. Direct Spectroscopic Observation 
of Vinyl-Substituted Vinyl Cations 
Hans-Ullrich Siehl* 
Institut fur  Organische Chemie 
der Uniuersitat Tubingen, D-  7400 Tubigen 
Federal Republic of Germany 
Herbert Mayr* 
Institut fur Organische Chemie 
der Universitat Erlangen-Niirnberg 
0-8520 Erlangen, Federal Republic of Germany 
Received September 8. 1981 
Vinyl cations, although well established as reactive interme- 
diatesIa and predicted to be rather stable thermodynamical1y,lb 
have been rather elusive toward direct spectroscopic observation. 
Attempts to generate vinyl cations via SbF5-assisted heterolysis 
of a-arylvinyl halides resulted in attack of SbF5 at  the alkene A 
system and formation of CT complexes.2 Protonation of alkynes 
under strongly acidic conditions also failed to give stable vinyl 
cation solutions;' rapid inter-4 and intramolecular5 sequential 
reactions took place instead. Hitherto the only spectroscopic 
evidence comes from treatment of a-alkynyl alcohols with sup- 
eracids, leading to alkynylcarbenium ions, which may be regarded 
as vinyl cations if the allenic resonance structure is important.6 
(1) (a) Stang, P. J.; Rappoport, Z.; Hanack, M.; Subrarnanian, L. R., 
'Vinyl Cations"; Academic Press: New York, 1979. (b) Mayr, H.; Schneider, 
R.; Wilhelrn, D.; Schleyer, P. v. R.  J .  Org. Chem., in press. 
(2) Siehl, H.-U.; Hanack, M. J .  Am. Chem. SOC. 1980, 102, 2686. 
(3)  Ferrocenyl-stabilized vinyl cations are accessible in this way and have 
been characterized by 'H NMR spectroscopy: Abram, T. S.; Watts, W. E. 
J .  Chem. Soc., Perkin Trans. 1 1977, 1522. 
(4) Olah, G.  A.; Staral, J .  S.; Spear, R.  J.; Liang, G. J .  Am. Chem. SOC. 
1975, 97, 5489. 
(5) Olah, G.  A.; Mayr, H.  J.  Am. Chem. SOC. 1976,98,7333; 1978,100, 
6544. 
(6) Richey, H. G., Jr.; Philips, J. C.; Rennick, L. E. J .  Am. Chem. SOC. 
1965,87, 1381. Richey, H.  G., Jr.; Rennick, L. E.; Kushner, A.  S.; Richey, 
J.  M.; Philips, J. C. Ibid. 1965, 87, 4017. Pittrnan, C. U. ,  Jr.; Olah, G. A. 
Ibid. 1965, 87, 5632. Olah, G. A,; Spear, R. J.; Westerman, P. W.; Denis, 
J.-M. Ibid. 1974, 96, 5855. 
0002-7863/82/1504-0909$01,25/0 
i H \ 1  2 3/H 
H h'C-CH3 I 
,c=c-c 4 6 
5 /  CH3 IC3 IC1 C 6 f 5  
lc4 cz 
A. ,..+&,..-+#-~~M-,. 1 *u'c- 
- 
250 200 150 100 50 PPm 
Figure 1. 13C NMR spectrum of cation 3a in S02C1F/S02F2 (2: l )  at 
-120 OC. Peaks marked X are due to CD3COC1 used as capillary lock. 
We now report the first generation and N M R  spectroscopic ob- 
servation of a-vinyl-substituted vinyl cations 3, which have pre- 
viously been encountered as short-lived intermediates. 
Grob and Spaar studied solvolysis reactions of 2-bromo-l,3- 
butadienes and found particularly large rate enhancements through 
methyl groups at  C4, in accord with intermediate vinyl cations 
3' - 3",' Reactions of a-allenyl alcohols 2 with aqueous acid 
Br 
1 
\ / 
\ /  
,c=c=c 
/ c ,  
HO 
2 - 
\ /  
\ + /  -L / ,c=c-c.+ c=c=c, + 
/c-  - ' /c-  
- 3' - 3"
to give a$-unsaturated ketones were also suggested to proceed 
via vinyl cations 3.* Since conjugated double bonds are more 
stable than cumulated double bonds, vinyl cations 3 can be ex- 
pected to form more readily from 2 than from 1. 
Clean solutions of 3a-c in S02C1F/S02F2 were obtained by 
reaction of alcohols 2a-c9 with SbF5 using Saunders' "molecular 
beam" method.I0 The 13C NMR spectra (Table I) were recorded 
at  -120 OC and assigned by off-resonance and proton-coupled 
spectra (Figure 1). Single-frequency proton-decoupled spectra 
were used to confirm the assignments. 
In all cations, C2 and C4 absorb at  very low field, indicating 
location of positive charge at these positions. Contribution of both 
resonance structures 3' and 3" is thus confirmed. The small 
chemical shift difference between C3 in 3 and 2 shows that this 
position does not carry significant charge. The C1 resonances are 
also quite similar in cations 3 and precursors 2, since the ClC2 
A system cannot interact with the electron-deficient orbital at C2. 
Hindered rotation around the C3-C4 bond is responsible for the 
nonequivalence of C5 and C6. 
Small, but significant, effects on chemical shifts are generated 
by methyl substitution at C,. The shift of C1 corresponds to the 
normal a'-CH3 effect in allenes ( E  10 ppm/CH3 group)" and 
is almost identical for alcohols 2a-c and cations 3a-c. While C3 
is practically unaffected by C1 substituents, the chemical shifts 
of C2 and C4 show opposite trends in series 2 and 3. In the alcohols 
2, C1-methyl groups exert the normal /3*-CH3 effect on C, (--3 
ppm/CH3 group) in allenes" and do not influence the remote C4. 
In cations 3, however, C2 is deshielded and C4 is shielded by methyl 
groups at C1. CC hyperconjugation, which is more effective than 
C H  hyperconjugation, can account for this effect since the C,-CH, 
bond is ideally oriented (dihedral angle of Oo) for interaction with 
(7 )  Grob, C. A.; Spaar, R. Helv. Chim. Acta 1970, 53, 2119. 
(8) Gelin, R.; Gelin, S.; Albrand, M. Bull. SOC. Chim. Fr. 1972, 720. 
Olsson, L.-I.; Claesson, A.; Bogentoft, C. Acra Chem. Scand. 1973, 27, 1629. 
(9) Alcohols 2a-c were prepared by a standard procedure: Cowie, J.  S.; 
Landor, P. D.; Landor, S. R. J .  Chem. SOC., Perkin Trans. 1 ,  1973, 720. 
(IO) Saunders, M.; Cox, D.; Lloyd, J .  R. J .  Am. Chem. SOC. 1979, 101, 
6656. 
(1 1 )  Stothers, J. B. " C ~ ~ ~ O ~ - ' ~ N M R  Spectroscopy"; Academic Press: 
New York, 1972; p 76. 
0 1982 American Chemical Society 
910 
Table I. ''C NMR Chemical Shifts of Vinyl Cations 3a-c and Their Precursors 2a-d 
J.  Am. Chem. SOC. 1982,104, 910-913 
2 2 
compd R 1  R1 c-1 c-2 c- 3 c-4 c-5 '2-6 C,-CH3 
3a H H 79.01 241.88 11 3.68 261.58 32.81 36.91 
3b CH3 H 90.34 243.97 114.43 259.16 32.64 36.63 9.46 
101.55 245.39 113.97 257.64 32.43 36.44 16.29 
2a H H 
2b CH3 H 88.55 201.28 100.39 69.36 29.52b 29.59b 13.99 
77.28 205.24 99.90 68.89 29.33 
3c CH3 CH, 
2c CH, CH3 97.86 197.92 99.01 69.54 29.5 1 20.10 
Specific assignments of C-5 and C-6 in the ions 3a-c tentatively analogous to  allyl cations. Assignment tentative. 
Table 11. Calculated Bond Lengths (A) and ?r Densities p of 
Vinyl Cations 3a 
compd C1-Ca Ca-C3 C 3 C 4  p2 Q 3  p4 
3a 1.285 1.352 1.410 0.543 1.162 0.458 
3b 1.287 1.358 1.403 0.518 1.167 0.481 
3c 1.289 1.363 1.398 0.496 1.171 0.500 
All calculations were carried out with the Gaussian 76 series of 
p r o g ~ a m s ' ~  at the STG3G basis set level.15 C-C bond lengths were 
optimized; standard values were selected for angles and C-H bond 
lengths. 
the electron-deficient orbital a t  C2.I2 If corrections for the 
precursors are considered, 10.8-ppm deshielding of C2 and 4.6-ppm 
shielding of C4 can be attributed to the hyperconjugative electron 
donation of two methyl groups at C1. As in ordinary allyl cations, 
electron donation at  one end increases the positive charge at  this 
terminus (C,) and decreases the charge at the opposite end (C4).I3 
This interpretation of spectroscopic data is supported by ab initio 
M O   calculation^^^ (Table 11). The optimized bond lengths show 
that Cl-C2 is identical with that in allene (1.288 A),'6 whereas 
C2<3 and C3-C4 are similar as in allyl cations,13 in accord with 
the delocalized structure 3. The positive charge is located at C2 
and C4, the termini of the allylic cation. 
Hyperconjugative electron donation by C1 methyl groups is also 
confirmed by the calculations. As derived from the 13C N M R  
spectra, electron density increases at C4 and decreases a t  C2 when 
methyl groups are added at  Cl. Simultaneously elongation of 
C2-C3 and shortening of the C3-C4 bond takes place. In valence 
bond terminology the contribution of resonance structure 3' in- 
creases from 3a to 3c. Calculated A densities indicate similar 
charge distributions in cation 3a and allyl cation 4.13 This is 
verified by the similarity of the 13C chemical shifts of the tertiary 
carbons in both species. The other carbons cannot directly be 
compared, since they are differently hybridized. 
0 5 5 4  7 0.L57 
CH3 \ /A\ /CH3 P 
L 
As part of an allene unit, C3 and C2 absorb at higher or lower 
field, respectively, than the corresponding allyl carbons of 4. C2 
- 
~~ 
(12) Apcloig, Y.; Schleyer, P. v. R.; Pople, J. A. J .  Am. Chem. Soc. 1977, 
99, 5901. 
(13) Mayr, H.; Forner, W.; Schleyer, P. v. R. J .  Am. Chem. SOC. 1979, 
101,6032; 1980,102, 3663. 
(14) Binkley, J. S.; Whiteside, R. A,; Hariharan, P. C.; Seeger, R.; Pople, 
J.; Hehre, W. J.; Newton, M. P. QCPE, Program No. 368, Indiana University. 
(15) Hehre, W. J.; Stewart, R. F.; Pople, J. A. J. Chem. Phys. 1969, 51, 
2657. 
(16) Radom, L.; Lathan, W. A.; Hehre, W. J.; Pople, J. A. J. Am. Chem. 
SOC. 1971.93, 5339. 
(1 7) For MO calculations of the unsubstituted and monomethyl-substituted 
cations, see: Mirejovsky, D.; Drenth, W.; Duijneveldt, F. B. v. Red.  Trau. 
Chim. Pays-Bas. 1979, 98, 388. 
in 3a-c is, to our knowledge, the most deshielded sp hybridized 
carbon reported so far for carbocations. Experiment and theory 
are thus in good agreement concerning the delocalized structure 
of vinyl cations 3a-c. 
Acknowledgment. Support of the Deutsche Forschungsge- 
meinschaft to H.U.S. and the Fonds der Chemischen Industrie 
to H.M. is gratefully acknowledged. 
Registry No. 2a, 34761-53-0; 2b, 56037-78-6; 2c, 2424-45-5; 3a, 
80375-40-2; 3b, 80375-41-3; 312, 80375-42-4. 
Preparation, Structural Characterization, and Reactivity 
of (PEt3)2(CO)Rh-Co(C0)4. A Quantitative Study of 
the Reversible Heterolytic Cleavage of the Polar Rh-Co 
Bond 
David A. Roberts,la William C. Mercer,Ia 
Susan M. Zahurak,la Gregory L. Geoffroy,*la 
C. W. DeBrosse,lB Marion E. Cass,lb and 
Cortlandt G. Pierpont*lb 
Department of Chemistry, The Pennsylvania State University 
University Park, Pennsylvania 16802 
and the University of Colorado 
Boulder, Colorado 80309 
Received June 15. 1981 
A large number of heteronuclear metal-metal bonded com- 
pounds have been prepared and characterized, but relatively few 
have had their chemistry explored and even less have been shown 
to express catalytic activity.24 This is surprising in view of the 
unique reactivity features which should accrue as a result of 
adjacent metals with differing sets of chemical properties. The 
lack of catalytic activity appears to be due in part to  the absence 
of open coordination sites or easily dissociable ligands in the 
complexes examined and the fact that relatively few contain one 
of the more active second-row metals such as rhodium. Another 
problem inherent with low-valent metal-metal bonded complexes 
is their susceptibility to cleavage of the metal-metal bond during 
reaction.2s5j6 A number of examples of such cleavage reactions 
have been reported but little quantitative data are a~ailable.~-lO 
We describe herein the preparation and characterization of 
(PEt3)2(CO)Rh-Co(C0)4 (l), a coordinatively unsaturated polar 
(1) (a) Pennsylvania State University. (b) University of Colorado. 
(2) Roberts, D. A.; Geoffroy, G. L. In "Comprehensive Organometallic 
Chemistry"; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon 
Press: Oxford, in press; Chapter 40. 
(3) Gladfelter, W. L.; Geoffroy, G. L.,  Adu. Orgammer. Chem. 1980, 18, 
207. _
(4) Vyazankin, N. S.; Razuvaev, G. A.; Kruglaya, 0. A. Organomet. 
(5) Meyer, T. J. Prog. Inorg. Chem. 1975, 19, 1. 
(6) Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 379. 
(7) Ellis, J. E. J. Organomet. Chem. 1975, 86, 1. 
(8) Pearson, R. G.; Dehand, J. J.  Organomet. Chem. 1969, 16, 485. 
(9) Davison, A.; Ellis, J. E. J .  Organomet. Chem. 1972, 36, 113. 
(10) (a) Burlitch, J. M. J .  Am. Chem. Soc. 1%9, 91,4562. (b) Ibid. 1969, 
Chem. Rev. 1968, 3A, 323. 
91, 4563. 
0002-7863/82/lS04-0910.$01.25/0 0 1982 American Chemical Society 


Open Access LMU ( Ludwig-Maximilians-Univ. München)

J.  Am. Chem. SOC. 1982, 104, 909-910 909 appear to be significantly broader than the other resonances. Thus, the low-temperature spectra (Figure 1) give no evidence of freezing out of a "classical" norbornyl cation. This means only that if one views the spectra in terms of rapidly equilibrating classical ions, the barrier of that equilibration must be lower than about 3 kcal mol-I. Within this limitation, one can adopt a different view. The recent studies of the appropriately deuterium labeled norbornyl cation by Saunders show very small isotopic splittings owing to perturbation of ~ymmetry. '~ This result is stronger support, albeit less direct, for the "nonclassical" symmetrical norbornyl cation. We anticipate that we shall be able to conduct MAS 13C NMR spectroscopy in the 30 K range in the future and look forward to additional studies of this and other cations over this extended temperature range. Acknowledgment. We thank the National Science Foundation for financial support (Industry/University Cooperative Program, Grant CHE-8020743) and IBM Instruments, Inc., for postdoctoral support (V.K.M.). The skilled assistance of R. D. Kendrick is gratefully acknowledged. We thank Dr. J. R. Lyerla for his interest and help in this work and Professor Martin Saunders for stimulating discussions. Registry No. 2-Norbornyl cation, 24321-81-1. (15) Saunders, M. A.; Kates, M. R. J .  Am. Chem. SOC. 1980,102,6867. Stable Vinyl Cations. Direct Spectroscopic Observation of Vinyl-Substituted Vinyl Cations Hans-Ullrich Siehl* Institut fur  Organische Chemie der Uniuersitat Tubingen, D-  7400 Tubigen Federal Republic of Germany Herbert Mayr* Institut fur Organische Chemie der Universitat Erlangen-Niirnberg 0-8520 Erlangen, Federal Republic of Germany Received September 8. 1981 Vinyl cations, although well established as reactive interme- diatesIa and predicted to be rather stable thermodynamical1y,lb have been rather elusive toward direct spectroscopic observation. Attempts to generate vinyl cations via SbF5-assisted heterolysis of a-arylvinyl halides resulted in attack of SbF5 at  the alkene A system and formation of CT complexes.2 Protonation of alkynes under strongly acidic conditions also failed to give stable vinyl cation solutions;' rapid inter-4 and intramolecular5 sequential reactions took place instead. Hitherto the only spectroscopic evidence comes from treatment of a-alkynyl alcohols with sup- eracids, leading to alkynylcarbenium ions, which may be regarded as vinyl cations if the allenic resonance structure is important.6 (1) (a) Stang, P. J.; Rappoport, Z.; Hanack, M.; Subrarnanian, L. R., 'Vinyl Cations"; Academic Press: New York, 1979. (b) Mayr, H.; Schneider, R.; Wilhelrn, D.; Schleyer, P. v. R.  J .  Org. Chem., in press. (2) Siehl, H.-U.; Hanack, M. J .  Am. Chem. SOC. 1980, 102, 2686. (3)  Ferrocenyl-stabilized vinyl cations are accessible in this way and have been characterized by 'H NMR spectroscopy: Abram, T. S.; Watts, W. E. J .  Chem. Soc., Perkin Trans. 1 1977, 1522. (4) Olah, G.  A.; Staral, J .  S.; Spear, R.  J.; Liang, G. J .  Am. Chem. SOC. 1975, 97, 5489. (5) Olah, G.  A.; Mayr, H.  J.  Am. Chem. SOC. 1976,98,7333; 1978,100, 6544. (6) Richey, H. G., Jr.; Philips, J. C.; Rennick, L. E. J .  Am. Chem. SOC. 1965,87, 1381. Richey, H.  G., Jr.; Rennick, L. E.; Kushner, A.  S.; Richey, J.  M.; Philips, J. C. Ibid. 1965, 87, 4017. Pittrnan, C. U. ,  Jr.; Olah, G. A. Ibid. 1965, 87, 5632. Olah, G. A,; Spear, R. J.; Westerman, P. W.; Denis, J.-M. Ibid. 1974, 96, 5855. 0002-7863/82/1504-0909$01,25/0 i H \ 1  2 3/H H h'C-CH3 I ,c=c-c 4 6 5 /  CH3 IC3 IC1 C 6 f 5  lc4 cz A. ,..+&,..-+#-~~M-,. 1 *u'c- - 250 200 150 100 50 PPm Figure 1. 13C NMR spectrum of cation 3a in S02C1F/S02F2 (2: l )  at -120 OC. Peaks marked X are due to CD3COC1 used as capillary lock. We now report the first generation and N M R  spectroscopic ob- servation of a-vinyl-substituted vinyl cations 3, which have pre- viously been encountered as short-lived intermediates. Grob and Spaar studied solvolysis reactions of 2-bromo-l,3- butadienes and found particularly large rate enhancements through methyl groups at  C4, in accord with intermediate vinyl cations 3' - 3",' Reactions of a-allenyl alcohols 2 with aqueous acid Br 1 \ / \ /  ,c=c=c / c ,  HO 2 - \ /  \ + /  -L / ,c=c-c.+ c=c=c, + /c-  - ' /c-  - 3' - 3"to give a$-unsaturated ketones were also suggested to proceed via vinyl cations 3.* Since conjugated double bonds are more stable than cumulated double bonds, vinyl cations 3 can be ex- pected to form more readily from 2 than from 1. Clean solutions of 3a-c in S02C1F/S02F2 were obtained by reaction of alcohols 2a-c9 with SbF5 using Saunders' "molecular beam" method.I0 The 13C NMR spectra (Table I) were recorded at  -120 OC and assigned by off-resonance and proton-coupled spectra (Figure 1). Single-frequency proton-decoupled spectra were used to confirm the assignments. In all cations, C2 and C4 absorb at  very low field, indicating location of positive charge at these positions. Contribution of both resonance structures 3' and 3" is thus confirmed. The small chemical shift difference between C3 in 3 and 2 shows that this position does not carry significant charge. The C1 resonances are also quite similar in cations 3 and precursors 2, since the ClC2 A system cannot interact with the electron-deficient orbital at C2. Hindered rotation around the C3-C4 bond is responsible for the nonequivalence of C5 and C6. Small, but significant, effects on chemical shifts are generated by methyl substitution at C,. The shift of C1 corresponds to the normal a'-CH3 effect in allenes ( E  10 ppm/CH3 group)" and is almost identical for alcohols 2a-c and cations 3a-c. While C3 is practically unaffected by C1 substituents, the chemical shifts of C2 and C4 show opposite trends in series 2 and 3. In the alcohols 2, C1-methyl groups exert the normal /3*-CH3 effect on C, (--3 ppm/CH3 group) in allenes" and do not influence the remote C4. In cations 3, however, C2 is deshielded and C4 is shielded by methyl groups at C1. CC hyperconjugation, which is more effective than C H  hyperconjugation, can account for this effect since the C,-CH, bond is ideally oriented (dihedral angle of Oo) for interaction with (7 )  Grob, C. A.; Spaar, R. Helv. Chim. Acta 1970, 53, 2119. (8) Gelin, R.; Gelin, S.; Albrand, M. Bull. SOC. Chim. Fr. 1972, 720. Olsson, L.-I.; Claesson, A.; Bogentoft, C. Acra Chem. Scand. 1973, 27, 1629. (9) Alcohols 2a-c were prepared by a standard procedure: Cowie, J.  S.; Landor, P. D.; Landor, S. R. J .  Chem. SOC., Perkin Trans. 1 ,  1973, 720. (IO) Saunders, M.; Cox, D.; Lloyd, J .  R. J .  Am. Chem. SOC. 1979, 101, 6656. (1 1 )  Stothers, J. B. " C ~ ~ ~ O ~ - ' ~ N M R  Spectroscopy"; Academic Press: New York, 1972; p 76. 0 1982 American Chemical Society 910 Table I. ''C NMR Chemical Shifts of Vinyl Cations 3a-c and Their Precursors 2a-d J.  Am. Chem. SOC. 1982,104, 910-913 2 2 compd R 1  R1 c-1 c-2 c- 3 c-4 c-5 '2-6 C,-CH3 3a H H 79.01 241.88 11 3.68 261.58 32.81 36.91 3b CH3 H 90.34 243.97 114.43 259.16 32.64 36.63 9.46 101.55 245.39 113.97 257.64 32.43 36.44 16.29 2a H H 2b CH3 H 88.55 201.28 100.39 69.36 29.52b 29.59b 13.99 77.28 205.24 99.90 68.89 29.33 3c CH3 CH, 2c CH, CH3 97.86 197.92 99.01 69.54 29.5 1 20.10 Specific assignments of C-5 and C-6 in the ions 3a-c tentatively analogous to  allyl cations. Assignment tentative. Table 11. Calculated Bond Lengths (A) and ?r Densities p of Vinyl Cations 3a compd C1-Ca Ca-C3 C 3 C 4  p2 Q 3  p4 3a 1.285 1.352 1.410 0.543 1.162 0.458 3b 1.287 1.358 1.403 0.518 1.167 0.481 3c 1.289 1.363 1.398 0.496 1.171 0.500 All calculations were carried out with the Gaussian 76 series of p r o g ~ a m s ' ~  at the STG3G basis set level.15 C-C bond lengths were optimized; standard values were selected for angles and C-H bond lengths. the electron-deficient orbital a t  C2.I2 If corrections for the precursors are considered, 10.8-ppm deshielding of C2 and 4.6-ppm shielding of C4 can be attributed to the hyperconjugative electron donation of two methyl groups at C1. As in ordinary allyl cations, electron donation at  one end increases the positive charge at  this terminus (C,) and decreases the charge at the opposite end (C4).I3 This interpretation of spectroscopic data is supported by ab initio M O   calculation^^^ (Table 11). The optimized bond lengths show that Cl-C2 is identical with that in allene (1.288 A),'6 whereas C2<3 and C3-C4 are similar as in allyl cations,13 in accord with the delocalized structure 3. The positive charge is located at C2 and C4, the termini of the allylic cation. Hyperconjugative electron donation by C1 methyl groups is also confirmed by the calculations. As derived from the 13C N M R  spectra, electron density increases at C4 and decreases a t  C2 when methyl groups are added at  Cl. Simultaneously elongation of C2-C3 and shortening of the C3-C4 bond takes place. In valence bond terminology the contribution of resonance structure 3' in- creases from 3a to 3c. Calculated A densities indicate similar charge distributions in cation 3a and allyl cation 4.13 This is verified by the similarity of the 13C chemical shifts of the tertiary carbons in both species. The other carbons cannot directly be compared, since they are differently hybridized. 0 5 5 4  7 0.L57 CH3 \ /A\ /CH3 P L As part of an allene unit, C3 and C2 absorb at higher or lower field, respectively, than the corresponding allyl carbons of 4. C2 - ~~ (12) Apcloig, Y.; Schleyer, P. v. R.; Pople, J. A. J .  Am. Chem. Soc. 1977, 99, 5901. (13) Mayr, H.; Forner, W.; Schleyer, P. v. R. J .  Am. Chem. SOC. 1979, 101,6032; 1980,102, 3663. (14) Binkley, J. S.; Whiteside, R. A,; Hariharan, P. C.; Seeger, R.; Pople, J.; Hehre, W. J.; Newton, M. P. QCPE, Program No. 368, Indiana University. (15) Hehre, W. J.; Stewart, R. F.; Pople, J. A. J. Chem. Phys. 1969, 51, 2657. (16) Radom, L.; Lathan, W. A.; Hehre, W. J.; Pople, J. A. J. Am. Chem. SOC. 1971.93, 5339. (1 7) For MO calculations of the unsubstituted and monomethyl-substituted cations, see: Mirejovsky, D.; Drenth, W.; Duijneveldt, F. B. v. Red.  Trau. Chim. Pays-Bas. 1979, 98, 388. in 3a-c is, to our knowledge, the most deshielded sp hybridized carbon reported so far for carbocations. Experiment and theory are thus in good agreement concerning the delocalized structure of vinyl cations 3a-c. Acknowledgment. Support of the Deutsche Forschungsge- meinschaft to H.U.S. and the Fonds der Chemischen Industrie to H.M. is gratefully acknowledged. Registry No. 2a, 34761-53-0; 2b, 56037-78-6; 2c, 2424-45-5; 3a, 80375-40-2; 3b, 80375-41-3; 312, 80375-42-4. Preparation, Structural Characterization, and Reactivity of (PEt3)2(CO)Rh-Co(C0)4. A Quantitative Study of the Reversible Heterolytic Cleavage of the Polar Rh-Co Bond David A. Roberts,la William C. Mercer,Ia Susan M. Zahurak,la Gregory L. Geoffroy,*la C. W. DeBrosse,lB Marion E. Cass,lb and Cortlandt G. Pierpont*lb Department of Chemistry, The Pennsylvania State University University Park, Pennsylvania 16802 and the University of Colorado Boulder, Colorado 80309 Received June 15. 1981 A large number of heteronuclear metal-metal bonded com- pounds have been prepared and characterized, but relatively few have had their chemistry explored and even less have been shown to express catalytic activity.24 This is surprising in view of the unique reactivity features which should accrue as a result of adjacent metals with differing sets of chemical properties. The lack of catalytic activity appears to be due in part to  the absence of open coordination sites or easily dissociable ligands in the complexes examined and the fact that relatively few contain one of the more active second-row metals such as rhodium. Another problem inherent with low-valent metal-metal bonded complexes is their susceptibility to cleavage of the metal-metal bond during reaction.2s5j6 A number of examples of such cleavage reactions have been reported but little quantitative data are a~ailable.~-lO We describe herein the preparation and characterization of (PEt3)2(CO)Rh-Co(C0)4 (l), a coordinatively unsaturated polar (1) (a) Pennsylvania State University. (b) University of Colorado. (2) Roberts, D. A.; Geoffroy, G. L. In "Comprehensive Organometallic Chemistry"; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, in press; Chapter 40. (3) Gladfelter, W. L.; Geoffroy, G. L.,  Adu. Orgammer. Chem. 1980, 18, 207. _(4) Vyazankin, N. S.; Razuvaev, G. A.; Kruglaya, 0. A. Organomet. (5) Meyer, T. J. Prog. Inorg. Chem. 1975, 19, 1. (6) Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 379. (7) Ellis, J. E. J. Organomet. Chem. 1975, 86, 1. (8) Pearson, R. G.; Dehand, J. J.  Organomet. Chem. 1969, 16, 485. (9) Davison, A.; Ellis, J. E. J .  Organomet. Chem. 1972, 36, 113. (10) (a) Burlitch, J. M. J .  Am. Chem. Soc. 1%9, 91,4562. (b) Ibid. 1969, Chem. Rev. 1968, 3A, 323. 91, 4563. 0002-7863/82/lS04-0910.$01.25/0 0 1982 American Chemical Society 

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Stable vinyl cations. Direct spectroscopic observation of vinyl-substituted vinyl cations

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