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Implications of the photoelectron spectrum of a methylene-bridged, dinuclear transition-metal complex
Journal of Electron Spectroscopy and Related Phenomena, 24 (1981) 215-219 Elsevler Sclentlfic Publlshmg Company, Amsterdam - Prmted in The Netherlands
Short commumcatlon
IMPLICATIONS OF THE PHOTOELECTRON SPECTRUM OF A METHYLENE-BRIDGED, DINUCLEAR TRANSITION-METAL COMPLEX
J VITES and T P FEHLNER Department of Chemrstry, Unzversrty of Notre Dame, Notre Dame, IN 46556 (Fwst received 1 Apr111981,
(US A )
in final form 10 June 1981)
Bndged methylene transltlon-metal complexes of the type I have generated considerable chemical [l] and theoretical interest [ 2,3] as they model bonding features thought to play a malor role m the catalytic hydrogenation of carbon monoxide [4] These compounds have been termed &metallacyclopropanes, and the qualitative features of the bondmg have been analyzed from this point of mew [Z] Recently, CH, Fe, (CO)*, an example of I I
that 1s particularly simple from the pomt of view of photoelectron (PE) spectroscopy, has been prepared and characterized [ 51 As the PE spectra of the other members of the cyclopropane series including this compound have been measured (C3H6 [6], CzHqFe(C0)4 [7], and Fe3(C0)12 IS]), the PE spectrum of CH2 Fez(CO)s allows an emplrlcal correlation of the radical cation states correspondmg to the Walsh orbltals of the C,Fes_, rmg In addition it provides an example of the PE spectrum of the CH2 fragment bound to a dmuclear metal moiety As surface-bound CH2 fragments appear to be active mtermedlates m the Fischer-Tropsch reaction [ 91, the electronic charactemstlcs of this species are of great interest The gas-phase He(I) PE spectrum of CH2 Fez(CO)s 1s displayed in Fig l(a) Band assignment has been cmed out by usmg relative band mtensltles, relative band mtensltles as a function of photon energy, the spectra of model compounds, and the calculated properties of CH2, CZH4 and C3Hs The analysis suggests that band 1 contams eight lomzatlon potentials (IP’s), mcludmg the SK Fe 3d “lone pars”, band 2 contams one IP correlating with
0368-2048/81/0000-0000/$02
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lonlzatlon potentlol (EV) Fig 1 (a) The He(I) PE spectrum of CH2Fe,(CO)s at 30°C with Xe,Ar cahbratlon Vaporlzatlon of molecular CHz Fez (CO) s was confirmed mass-spectrometmcally (b) He(I)--Ne(I) difference spectrum with mtensltles adlusted such that the band 2 difference intensity 1s zero (c) CHZ Fez (CO)s-Fe3 (CO) 12 He(I) difference spectrum mth mtenslttes adlusted such that the CO band (4) difference mtenslty IS zero
the 3a, orbital of CH, (see IIb), band 3 contains one IP correlatmg wth the lb2 orbital of CH2 (see IIa), and band 4 contams twenty-four IP’s correlatmg mth the 50 and ?r orbltals of the CO hgands In terms of the cyclopropane analogy, band 2 corresponds to the lonlzatlon of the Walsh orbital IIIa [ 101.
a
b
Ionlzatlons correspondrng to the other two Walsh orbltals of CH2 Fez (CO)s are expected to have high Fe 3d character and to he m band 1 IIIb will have high Fe 3d character as it correlates with the double bond of the Fe, (CO), fragment, and Ilk because It correlates with the high-lymg empty 1 b l orbltal of CH, (11~) Partial resolution of band 1 1s achieved by correlatmg two observations First, the He(I)-Ne(1) difference spectrum (Fig l(b)) demonstrates that the two low-IP shoulders on band 1 have higher carbon 2~ character than the rest
217
of the band Second, we have noted that the PE spectra of metal carbonyl clusters m general exhibit d-band IP’s that are amazmgly msenskve to cluster size, structure or even metal Such bemg the case, the difference spectrum
a
b
(Fig l(c)) between CH2Fez(C0)s and Fe3(C0)12 normahzed to match CO band mtensltles demonstrates “extra” mtenslty m band 1 over that expected for a metal carbonyl cluster alone Hence, although the latter difference spectrum does not provide the precise location of the two IP’s, It does suggest that the higher carbon 2~ character observed for the shoulders of band 1 m the He(I)-Ne(1) difference spectrum may be associated with the two other Walsh IP’s This assignment permits lme correlation of the PE spectra of C, H2,,Fe3_,, (CO) 12-4n molecules and this 1s gwen m Fzg 2 The observed correlation demonstrates the appropnateness of the cyclopropane analogy, at least m terms of a molecular-orbltal descnptlon of the bondmg of the tnangular fragment The usefulness of the metallacyclopropane analogy for M,(CO) 12
Fig 2 Line correlation diagram of the Walsh orbltals for C,HznFe3_n(CO)1z-sn and (c) refer to the Walsh orbltals m III
(a), (b)
218
has already been pointed out [ 111, but the relative worth of this model for CZ H, Fe{ CO), versus the Chatt-Dewar-Duncanson n-bonding, R* -backbonding model has been the subJect of considerable dlscusslon [12] The correlatron m Fig 2 does not discredit the latter model, as the question 1s more one of point of view than of lnght or wrong That is, the latter model emphasizes the relation of the bonding orbltals to those of the separated fragments, C,H4 and Fe(CO), , whereas the former emphasizes the relatxon of the bondmg orbltals to those of the homonuclear tnangular fragment The bonding m CH2 Fe, (CO), can also be examined from the alternative fragments In terms of point of view of the separated CH, and Fe* (CO), singlet CH, the bonding would result from donation of electron density from orb&l IIb to the metal fragment and back-donation from the metal fragment mto the empty orbital IIc The importance of the latter interaction may be Judged by comparison of the stablllzatlon of the CH3 donor orbital u?th that of a sigma-only hgand From the known IP of CH2 [ 131 and that of band 2, the orbital IIb 1s apparently stabilized by 0 7 eV on being bound to Fe,(CO)s Calculations indicate that the magnitude of this stablhzatlon will be roughly mdependent of the ground state of CH2 [lo] For (CH, )3 N, the measured stablllzatlon of the lone pair on being bound to Fe(CO), IS 3 1 eV [ 14,151 The difference m the two apparent stablllzatlons demonstrates the importance of the backbondmg interaction m increasing the electronic charge of CH2 and thereby reducing the net observed stablhzatlon of orbital IIb This conclusion concerning the charge on CH2 1s consistent urlth the theoretical results [ 23 Vleunng the PE spectrum of CH,Fe,(CO), m terms of CH* and Fe,(CO)s fragments has an additional benefit For example, the reaction of C,H, with vaslous NI surfaces has been exammed usmg UV PE techmques as well as thermal desorptlon spectroscopy [ 161 A rather extended chain of reasomng, relymg heavily on model calculations [ 171,~ used to Identify CH, as one of the species @vmg nse to the photoemlsslon bands resulting from the reaction of Cz HZ on Nl(110) The spectrum of CH, Fe, (CO)s reported here suggests that such a CH2 species urlll be characterized by two bands m the low-IP re@on splxt by 1 6 eV In fact, the features of the Nl(110) spectra assigned to CH, consist of two bands m the low-IP reaon (relative to the lowest IP’s of bound &Hz ) vvlth a sphttmg of 1 5 eV Clearly the photoemlsslon behavior of CH2 and other hydrocarbon prototypes will be useful m the mterpretatlon of surface spectra Such spectra, as well as further details of this work, til be published m due course (a PE study of pz-CH2 [CpMn(CO), ] 2 has recently been reported by two groups [ 18,191) ACKNOWLEDGEMENTS
The support of the National 15220) IS gratefully acknowledged
Science
Foundation
(Grant
No
CHE79-
219 REFERENCES 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19
W A Herrmann, J Plank, D Rledel, M L Ziegler, K Weldenhammer, E Guggolz and B Balbach, J Am Chem Sot , 103 (1981) 63 P Hofmann, Angew Chem , Int Ed Engl , 18 (1979) 554 A R Pmhas, T A Albnght, P Hofmann and R Hoffmann, Helv Chum Acta, 63 (1980) 29 E L Muettertles and J Stem, Chem Rev, 79 (1979) 479 E Summer, Jr, P E Riley, R E DavLsand R Pettlt, J Am Chem Sot ,102 (1980) 1752 H Basch, M B Robm, N A Kuebler, C Baker and D W Turner, J Chem Phys , 51(1969) 52 H Van Dam and A Oskam, J Electron Spectrosc Relat Phenom ) 17 (1979) 357 K W Wong, T K Dutta and T P Fehlner, J Organomet Chem , in press R C Brady III and R Pettlt, J Am Chem Sot, 102 (1980) 6182 W L Jorgensen and L Salem, The Organic Chemist’s Book of Orbltals, Academic Press, New York, 1973, p 153 J D Green, D M P Mmgos and E A Seddon, J Organomet Chem , 185 (1980) c20 S D Ittel and J A Ibers, Adv Organomet Chem , 14 (1976) 33 G Herzberg, Can J Phys, 39 (1961) 1511 G Boxhoom, M B Cerfontam, D J Stufkens and A Oskam, J Chem Sot , Dalton Trans , (1980) 1336 T P Fehlner, unpubhshed data J E Demuth, Surf Scl , 93 (1980) 127 A B Anderson, J Am Chem Sot, 100 (1978) 1153 G Granozzl, E Tondello, M Casarm and D A& Inorg Chum Acta, 48 (1981) 73 D L Llchtenberger, D C Calabro and W A Herrmann, J Am Chem Sot , m press
Short commumcatlon
IMPLICATIONS OF THE PHOTOELECTRON SPECTRUM OF A METHYLENE-BRIDGED, DINUCLEAR TRANSITION-METAL COMPLEX
J VITES and T P FEHLNER Department of Chemrstry, Unzversrty of Notre Dame, Notre Dame, IN 46556 (Fwst received 1 Apr111981,
(US A )
in final form 10 June 1981)
Bndged methylene transltlon-metal complexes of the type I have generated considerable chemical [l] and theoretical interest [ 2,3] as they model bonding features thought to play a malor role m the catalytic hydrogenation of carbon monoxide [4] These compounds have been termed &metallacyclopropanes, and the qualitative features of the bondmg have been analyzed from this point of mew [Z] Recently, CH, Fe, (CO)*, an example of I I
that 1s particularly simple from the pomt of view of photoelectron (PE) spectroscopy, has been prepared and characterized [ 51 As the PE spectra of the other members of the cyclopropane series including this compound have been measured (C3H6 [6], CzHqFe(C0)4 [7], and Fe3(C0)12 IS]), the PE spectrum of CH2 Fez(CO)s allows an emplrlcal correlation of the radical cation states correspondmg to the Walsh orbltals of the C,Fes_, rmg In addition it provides an example of the PE spectrum of the CH2 fragment bound to a dmuclear metal moiety As surface-bound CH2 fragments appear to be active mtermedlates m the Fischer-Tropsch reaction [ 91, the electronic charactemstlcs of this species are of great interest The gas-phase He(I) PE spectrum of CH2 Fez(CO)s 1s displayed in Fig l(a) Band assignment has been cmed out by usmg relative band mtensltles, relative band mtensltles as a function of photon energy, the spectra of model compounds, and the calculated properties of CH2, CZH4 and C3Hs The analysis suggests that band 1 contams eight lomzatlon potentials (IP’s), mcludmg the SK Fe 3d “lone pars”, band 2 contams one IP correlating with
0368-2048/81/0000-0000/$02
50
0 1981
Elsevler Scientific Pubhshmg Company
216
I
14
12
8
16
lonlzatlon potentlol (EV) Fig 1 (a) The He(I) PE spectrum of CH2Fe,(CO)s at 30°C with Xe,Ar cahbratlon Vaporlzatlon of molecular CHz Fez (CO) s was confirmed mass-spectrometmcally (b) He(I)--Ne(I) difference spectrum with mtensltles adlusted such that the band 2 difference intensity 1s zero (c) CHZ Fez (CO)s-Fe3 (CO) 12 He(I) difference spectrum mth mtenslttes adlusted such that the CO band (4) difference mtenslty IS zero
the 3a, orbital of CH, (see IIb), band 3 contains one IP correlatmg wth the lb2 orbital of CH2 (see IIa), and band 4 contams twenty-four IP’s correlatmg mth the 50 and ?r orbltals of the CO hgands In terms of the cyclopropane analogy, band 2 corresponds to the lonlzatlon of the Walsh orbital IIIa [ 101.
a
b
Ionlzatlons correspondrng to the other two Walsh orbltals of CH2 Fez (CO)s are expected to have high Fe 3d character and to he m band 1 IIIb will have high Fe 3d character as it correlates with the double bond of the Fe, (CO), fragment, and Ilk because It correlates with the high-lymg empty 1 b l orbltal of CH, (11~) Partial resolution of band 1 1s achieved by correlatmg two observations First, the He(I)-Ne(1) difference spectrum (Fig l(b)) demonstrates that the two low-IP shoulders on band 1 have higher carbon 2~ character than the rest
217
of the band Second, we have noted that the PE spectra of metal carbonyl clusters m general exhibit d-band IP’s that are amazmgly msenskve to cluster size, structure or even metal Such bemg the case, the difference spectrum
a
b
(Fig l(c)) between CH2Fez(C0)s and Fe3(C0)12 normahzed to match CO band mtensltles demonstrates “extra” mtenslty m band 1 over that expected for a metal carbonyl cluster alone Hence, although the latter difference spectrum does not provide the precise location of the two IP’s, It does suggest that the higher carbon 2~ character observed for the shoulders of band 1 m the He(I)-Ne(1) difference spectrum may be associated with the two other Walsh IP’s This assignment permits lme correlation of the PE spectra of C, H2,,Fe3_,, (CO) 12-4n molecules and this 1s gwen m Fzg 2 The observed correlation demonstrates the appropnateness of the cyclopropane analogy, at least m terms of a molecular-orbltal descnptlon of the bondmg of the tnangular fragment The usefulness of the metallacyclopropane analogy for M,(CO) 12
Fig 2 Line correlation diagram of the Walsh orbltals for C,HznFe3_n(CO)1z-sn and (c) refer to the Walsh orbltals m III
(a), (b)
218
has already been pointed out [ 111, but the relative worth of this model for CZ H, Fe{ CO), versus the Chatt-Dewar-Duncanson n-bonding, R* -backbonding model has been the subJect of considerable dlscusslon [12] The correlatron m Fig 2 does not discredit the latter model, as the question 1s more one of point of view than of lnght or wrong That is, the latter model emphasizes the relation of the bonding orbltals to those of the separated fragments, C,H4 and Fe(CO), , whereas the former emphasizes the relatxon of the bondmg orbltals to those of the homonuclear tnangular fragment The bonding m CH2 Fe, (CO), can also be examined from the alternative fragments In terms of point of view of the separated CH, and Fe* (CO), singlet CH, the bonding would result from donation of electron density from orb&l IIb to the metal fragment and back-donation from the metal fragment mto the empty orbital IIc The importance of the latter interaction may be Judged by comparison of the stablllzatlon of the CH3 donor orbital u?th that of a sigma-only hgand From the known IP of CH2 [ 131 and that of band 2, the orbital IIb 1s apparently stabilized by 0 7 eV on being bound to Fe,(CO)s Calculations indicate that the magnitude of this stablhzatlon will be roughly mdependent of the ground state of CH2 [lo] For (CH, )3 N, the measured stablllzatlon of the lone pair on being bound to Fe(CO), IS 3 1 eV [ 14,151 The difference m the two apparent stablllzatlons demonstrates the importance of the backbondmg interaction m increasing the electronic charge of CH2 and thereby reducing the net observed stablhzatlon of orbital IIb This conclusion concerning the charge on CH2 1s consistent urlth the theoretical results [ 23 Vleunng the PE spectrum of CH,Fe,(CO), m terms of CH* and Fe,(CO)s fragments has an additional benefit For example, the reaction of C,H, with vaslous NI surfaces has been exammed usmg UV PE techmques as well as thermal desorptlon spectroscopy [ 161 A rather extended chain of reasomng, relymg heavily on model calculations [ 171,~ used to Identify CH, as one of the species @vmg nse to the photoemlsslon bands resulting from the reaction of Cz HZ on Nl(110) The spectrum of CH, Fe, (CO)s reported here suggests that such a CH2 species urlll be characterized by two bands m the low-IP re@on splxt by 1 6 eV In fact, the features of the Nl(110) spectra assigned to CH, consist of two bands m the low-IP reaon (relative to the lowest IP’s of bound &Hz ) vvlth a sphttmg of 1 5 eV Clearly the photoemlsslon behavior of CH2 and other hydrocarbon prototypes will be useful m the mterpretatlon of surface spectra Such spectra, as well as further details of this work, til be published m due course (a PE study of pz-CH2 [CpMn(CO), ] 2 has recently been reported by two groups [ 18,191) ACKNOWLEDGEMENTS
The support of the National 15220) IS gratefully acknowledged
Science
Foundation
(Grant
No
CHE79-
219 REFERENCES 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19
W A Herrmann, J Plank, D Rledel, M L Ziegler, K Weldenhammer, E Guggolz and B Balbach, J Am Chem Sot , 103 (1981) 63 P Hofmann, Angew Chem , Int Ed Engl , 18 (1979) 554 A R Pmhas, T A Albnght, P Hofmann and R Hoffmann, Helv Chum Acta, 63 (1980) 29 E L Muettertles and J Stem, Chem Rev, 79 (1979) 479 E Summer, Jr, P E Riley, R E DavLsand R Pettlt, J Am Chem Sot ,102 (1980) 1752 H Basch, M B Robm, N A Kuebler, C Baker and D W Turner, J Chem Phys , 51(1969) 52 H Van Dam and A Oskam, J Electron Spectrosc Relat Phenom ) 17 (1979) 357 K W Wong, T K Dutta and T P Fehlner, J Organomet Chem , in press R C Brady III and R Pettlt, J Am Chem Sot, 102 (1980) 6182 W L Jorgensen and L Salem, The Organic Chemist’s Book of Orbltals, Academic Press, New York, 1973, p 153 J D Green, D M P Mmgos and E A Seddon, J Organomet Chem , 185 (1980) c20 S D Ittel and J A Ibers, Adv Organomet Chem , 14 (1976) 33 G Herzberg, Can J Phys, 39 (1961) 1511 G Boxhoom, M B Cerfontam, D J Stufkens and A Oskam, J Chem Sot , Dalton Trans , (1980) 1336 T P Fehlner, unpubhshed data J E Demuth, Surf Scl , 93 (1980) 127 A B Anderson, J Am Chem Sot, 100 (1978) 1153 G Granozzl, E Tondello, M Casarm and D A& Inorg Chum Acta, 48 (1981) 73 D L Llchtenberger, D C Calabro and W A Herrmann, J Am Chem Sot , m press