0 Copyrbht 1983 American Chemical Society
Volume 2 Number 2 February 1983
He I and He I 1 Photoelectron Studies of Bis(cyclopentadienyl)vanadium( I I I ) Complexes Jennifer C. Green" and Martin P. Payne Inorganic Chemistry Laboratory, Oxford OX1 3QR, England
Jan H. Teuben Department of Inorganic Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands Received June IO, 1982
He I and He I1 photoelectron spectra have been obtained for a series of bis(q5-cyclopentadieny1)vanadium(II1) halides, alkyls, and aryls, V(q-C5H5)zX, where X = C1, Br, I, Me, CH2SiMe,, CHzCMe3,C6F5, Cd-15,o-C6H4Me,m-C6H4Me,and 2,6-C6H3Me2Assignments are made principally on the basis of variations of band intensity with photon energy, ionization energy trends, and a generalized molecular orbital scheme. Evidence is presented for weak but significant ?r interactions between metal d and halogen p orbitals in the halide complexes and a ?r-donor effect in the alkyls.
Introduction The electronic structure of bent metallocene complexes had been extensively investigated in the last 10 years by a wide variety of techniques, including electron paramagnetic resonance (EPR) and photoelectron (PE) spectroscopy and X-ray crystallography, as well as by Fenske-Hall, SCF, and extended Huckel calculations. The earliest bonding model for bent metallocenes was Ballhausen and Dahl's hybridization scheme1 which employed equal contributions of the five ( n - l)d, three np, and one ns orbitals to the metal-ligand bonding system and predicted for d2 systems, e.g., Mo(q-C5H5),H2, a lone-pair orbital directed between the two a-bonded ligands. Crystallographic work, originally by Alcock,2 but considerably extended by the systematic studies of Green, Prout et al.,3-5 and Dah16 revealed that the L-M-L bond angle in d2 M(q-C5H5)zX2 complexes was too small to accommodate a lone pair between the X ligands, and furthermore the bond angle decreased with increasing number of d electrons, do-d2. Thus the crystallographic data supported a model in which the "lone-pair" orbital was (1) Balhausen, C. J.; Dahl, J. P. Acta Chem. Scand. 1961, 15, 1333. (2) Alcock, N. W. J . Chem. SOC. A 1967, 2001. (3) Green, J. C.; Green, M. L. H.; Prout, C. K. J. Chem. SOC. Chem. Commun. 1972, 421. (4) Green, M. L. H. Pure Appl. Chem. 1972, 30, 373. ( 5 ) Prout, C. K.; Cameron, T. S.; Critcheley, S. R.; Denton, B.; Rees, G. V. Acta Crystallogr. Sect. B 1974, B30, 2290. (6) Dahl, L. F. 4th International Conference on Organomettalic Chemistry, Bristol 1969.
0276-733318312302-0203$01.50/0
primarily directed normal to the plane bisecting the ML2 fragment. EPR spectroscopy has enabled the electron distribution in the highest occupied molecular orbital (HOMO) of bent dl metallocene complexes to be determined. Thus the single-crystal EPR studies of Peterson and Dah17 on the complexes V ( V - C ~ H ~and ) ~ V(+25H4Me)2C12 S~ revealed an AI electronic ground state, with the unpaired electron residing in an orbital of ca. 80% d character, which is principally directed along the molecular z axis (using the coordinate system of Figure l a ) but has a small contribution along the x and y axes. If small ligand orbital contributions are neglected $HOMO
= aldaz)
+ bld,2-p)
and a2/b2 is 20 for V ( T ~ C ~ H , M ~and ) ~ C12.7 ~ , for V(qC5H5)2S5.Comparison of calculated and observed spin polarization values (x) showed that the HOMO had a neglibibly small 4s contribution, illustrating a further difference between the observed and predicted electron distributions based on the Alcock or Ballhausen-Dah1 models. These EPR results were conclusively supported by the nonparameterized Fenske-Hall molecular orbital (MO) calculations of the same austhors? who also showed previous EPR studies of related d' complexes to have a (7) Petersen, J. L.; Dahl, L. F. J.Am. Chem. SOC.1974,96 2248; 1975, 97, 6416,6422. (8)Petersen, J. L.; Lichtenberger, D. L.; Fenske, R. F.; Dahl, L. F. J. Am. Chem. SOC. 1975,97, 6433.
0 1983 American Chemical Society
204 Organometallics, Vol. 2, No. 2, 1983
Green, Payne, and Teuben Table I. Measurement Conditions for Obtaining Photoelectron Spectra helium I helium I1 compd
count count temp, rate, temp, rate, "C cis "C cis
118 128 V(?-CsHs)21 125 ia; (b! v(v-c j H j )7.( ) 95 Figure 1. Structure and coordinate schemes for bent bis(cyV(vI-CjHS),(2-C6H4Me) 84 V(?-C,Hs)2(3-C€.H,Me) 90 clopentadieny1)metalcomplexes. V(q-CsHs),(2,6-C6H3Me,)1 2 9 ~ ~ v - ~ S ~ s ~ 7 . ~ ~ 1. 2 3 5 parallel interpretation. The frozen solution EPR work of Vv-CjHs)zMe 40 Evans et al.9 on the d' complexes V ( T ~ C ~ Hand ~ )V-~ E ~ ~ V(7j-CjHs)ZCHISiMe3 59 ( V - C ~ H ~ ) ~ ( C Hgave ~ S very ~ M similar ~ ~ ) ~ results, with a2/b2 V(ri -csH, ),CH,CMe 3 63
V(v-CsHj)zCl V(t7-CSHs ),Br
= 17.4 and 14.1, respectively. More recent frozen solution
~
1000 1000 1000 1000 1000 1000 1000 1000 j ~ 1400 800 1000
122 132 129 96 103 95 119 119 51 62
100 100 100 100 100 100 80 100 100 200
EPR studies of Ta(q-C5H5)2C1210 and Ta(.rl-C5H5)2(uC5H5)2" have yielded the very suprising result that the HOMO in these complexes has a metal contribution almost entirely described by dXz+ with a = 0.001 and b = 0.999. A qualitative molecular orbital bonding scheme for bent metallocenes was put forward by Green et al.;3J2this was obtained by bending the "straight" D M metallocene system to CZusymmetry, the eZg,alg, and elg orbitals transforming as a l + bl, al, and a2 + b2 orbitals respectively, using the coordinate scheme of Figure lb. This approach was put on a more quantitative basis by the extended Huckel calculations of Hoffmann and Lauher13 who examined the variation of metal orbital energies as a function of bending angle for the hypothetical [ T ~ ( V - C ~ Hspecies. ~ ) ~ ] + These authors calculated electron density distributions for all the predominantly metal orbitals. M(T~C,H,),X, complexes have been extensively studied by PE s p e c t r o s ~ o p y . ~ J ~In J ~an - ~account ~ of bis(cyc1opentadieny1)metal dihalides a classification of bonding type was proposed on the basis of the relative energies and extent of mixing of the halogen p~ and cyclopentadienyl el orbitals, He I and He I1 intensities providing a criterion for determining orbital character. The relative energies of the halogen and cyclopentadienyl orbitals were found to be dependent on the metal as well as on the halogen (assuming differential relaxation effects to be minimal). Bis(cyclopentadienyl)vanadium(III) halides, alkyls, and aryls were first synthesized by Fischer et al.18 and de Liefde Meijer et al.19r20 Epr,21,22magnetic measurement^,^^ and more recently NMR s t ~ d i e s *have ~ * ~demonstrated ~ these (9) Evans, J. C.; Evans, A. G.; Espley, D. J. C.; Morgan, P. H.; Mortimer, J. J . Chem. Soc., Dalton Trans. 1978, 57. (IO) Al-Mowali, A. H.; Kuder, W. A. A. J . Mol. Struct. 1979,57, 141. (11)Al-Mowali, A. H. J . Chem. SOC.Dalton Trans. 1980, 426. (12) Green, J. C.; Higginson, B.; Jackson, S. E. J . Chem. SOC. Dalton Trans. 1975, 403. (13) Hoffman, R.; Lauher, J. W. J . Am. Chem. SOC.1976, 98, 1729. (14) Fragala, I.; Ciliberto, E.; Thomas, J. L. J. Organomet. Chem. 1979, 175 C25. (15) Condorelli, G.; Fragala, I.; Centineo, A. J . Organomet. Chem. 1975, 87, 311. (16) Clark, J. P.; Green, J. C. J . Less-Common Met. 1977, 31. (17). Green, J. C.; Cauletti, C.; Clark, J. P.; Jackson, S. E.; Fragala, I. T.; Ciliberto E.; Coleman, A. W. J . Electron Spectrosc. Relat. Phenom. 1980, 18, 61. (18) Fischer, E. 0.;Vigoureux, S.; Kuzel, P. Chem. Ber. 1960,93,701. (19) de Liefde Meijer, H. J.; Janssen, M. J.; van der Kerk, G. J. M. Recl. Tray. Chim.Pays-Bas 1961, 80, 831. (20) de Liefde Meijer, H. J.; Janssen, M. J.; van der Kerk, G.J. M. Chem. Ind. (London) 1960, 119. (21) Chien, J. C. W.; Boss, C. R. J . Am. Chem. SOC.1961,83, 3767. (22) Dessy, R. E.; King, R. R.; Waldrop, M. J. J . Am. Chem. Soc. 1966, 88, 5112. (23) Bouwman, H.; Teuben, J. H. J. Organomet. Chem. 1976,110,327. (24) Fieselmann, B. F.; Stucky, G. D. J . Organomet. Chem. 1977,137, 43.
Figure 2. Qualitative energy level scheme for bis(cyc1opentadieny1)vanadiumhalides. complexes to have spin triplet ground states, and hence their d orbital configuration is lalllbll. We report below PE studies on a series of these molecules. Experimental Section The compounds were prepared by literature method^.'^^^^,^^ He I and He I1 spectra were obtained by using a Perkin-Elmer PS 16/18 spectrometer fitted with an Helectros lamp capable of supplying both He I and He I1 radiation. Measurement conditions are given in Table I and vertical ionization energies and some band intensity data in Table 11. The spectra were calibrated by admission of N, and Xe and by reference to the He self-ionization band. This has an apparent IE (ionizationenergy) in the He I spectra of 4.99 eV (see Figure 3).
Discussion Bis(cyclopentadienyl)vanadium(III) Halides. A qualitative molecular orbital diagram for these halides may be constructed, by considering orbital interactions between a bent V ( V - C ~ H ~unit ) ~ +and a halide ion, X- (Figure 2). This MO diagram is similar to but more detailed than that proposed by Stucky and Fiesel~tein,~~ whose X-ray crystal study confirmed an approximate CZusymmetry, the halogen atom residing symmetrically in the open face created by tilting the cyclopentadienyl rings, which themselves are in a staggered conformation. Figure 2 ignores the effects of spin-orbit coupling and excludes any cyclopentadienyl-halogen mixing. The He I and He I1 P E spectra of the chloride, bromide, and iodide are illustrated in Figure 3 and vertical ionization energies (IE) band intensities and proposed assignments in Table 11.1-3. For V(q-C5H,)21the intensity data are difficult to obtain accurately due to overlap of the second and third spectral bands. The spectra may be divided into (25) Kohler, F. H.; Hoffmann, P.; Prossdorf, W. J . Am. Chem. Soc. 1981, 103, 6359.
Organometallics, Vol. 2, No. 2, 1983 205
Bis(cyclopentadienyl)uanadium(III)Complexes
A"
tic
- II
A A
A"
n
At
H Q - II
cs v-I
HQ - I
1
5
.
I
I
I
I
10
I
.
IEIQV
.
I
5
.
.
I
10
.
IEIeV
I 5
.
.
.
.
, 10
. IE/QV
Figure 3. He I and He I1 photoelectron spectra of V ( T ~ C ~ H V ~ )( ~VC- C~ ~, H ~and ) ~ BV(V-C,H,)~I. ~, Ionization bands at 4.99 eV are due to He (ls)-l excited by He I1 radiation.
the following regions: A', 6-7.5 eV; A", 7.5-11 eV; p, 11-16 eV; C, 16-20 eV, and D, >20 eV. Above 11eV the spectra are similar for all the halides, with a large amorphous band centered a t -13 eV and a less intense one a t -17.5 eV, resembling those of other metallocene ~ y s t e m s ,and ~ ~ ?may ~ ~ be assigned to ionizations of the lowest a level and the a-orbitals of the cyclopentadienyl rings. A common feature of all PE spectra of the V(V-C~H,)~X compounds is the presence of two relatively small narrow bands in region A' which become relatively more intense relative to the central band B on changing from He I to He I1 radiation. These bands are thereby assigned to ionizations of principally metal localized d orbitals. The first band however shows a smaller intensity increase than the second. In addition it is broader than the second and is more intense in the He I spectrum. These features are consistent with the first and second bands involving ionization from the metal-halogen antibonding 3bl orbital and the essentially nonbonding metal 3al orbital respectively in agreement with the predictions of the MO diagram (Figure 2). The 3bl orbital ionization band gains intensity in the He I spectrum from its halogen content and gives rise to a broader band than the 3al orbital as a result of its antibonding nature. As further support for bl metal halogen a interactions we note that the separation of the first two bands is greater in the halides than in the aryls where the evidence suggests metal a interaction with the ring to be small. On the basis of the characteristic decrease of halogen orbital ionization cross sections with increasing photon energy compared with C-C and C-H ionization bands, the subsequent bands a t 8.29 eV for V(V-C~H,)~C~, 8.14 eV for V ( T - C ~ H ~ )and ~ B 7.69 ~ , and 8.30 eV for V(T-C?H,)?Imay be assigned to the halogen p a (2bl + 2b2) orbital ionizations. For the iodide two bands are observed due to the much larger spin-orbit coupling constant of iodine (0.63 (26) Evans,S.; Green, M. L. H.; Jewitt, B.; Orchard, A. F.; Pygall, C. F. J. Chem. Sac., Faraday Trans. 2 1972,68, 1847. (27) Green, J. C. Struct. Bonding (Berlin) 1981,43, 37.
eV) compared with bromine (0.305 eV) or chloride (0.073 eV). The splitting of the bands is very close to the atomic spin-orbit coupling constant of iodine, and the higher IE band is noticably broader than the first as demonstrated by the half-width values quoted in Table 11. This is consistent with a relatively weak interaction between the 3bl metal orbital and the 2bl halogen p orbital, as occurs for example in alkyl halides. Brogli and HeilbronneP showed that strong mesomeric interactions with bromine or iodine p orbitals, as occurs for example in aryl or vinyl halides, are required to increase the splitting of the p orbital energies much above the free atom values ({). In further support of only a weak p a interaction, the -0.3-eV increase in the energy of the 3bl orbital, calculated by comparison of the d orbital splittings of the halides and aryls, is small compared with the >l.O-eV lowering of the 3bl orbital energy found in bis(cyclopentadieny1)metalalkenes or carbonyl compounds12 such as Mo(v-C,H,)~COor W(T-C,H~)~(V-C~H~), due to interaction with the high energy acceptor bla* ligand orbitals. The very small He II/He I intensity ratios of the halogen p bands also indicate a small degree of metal-halogen a orbital mixing. Broadening of the bromine and chlorine pa bands relative to the two iodine p a bands, as demonstrated by their larger half-widths, is a result of the combined effect of spin-orbit coupling and mesomeric interactions with the metal. Identification of the halogen pa (lal) ionization is more difficult. The broad band in the region 8.5-10.5 eV for all three halides, by analogy with the PE spectra of other bis(cyclopentadieny1) c ~ m p l e x e smust , ~ ~ represent ionization of the cyclopentadienyl orbitals la2 + 1b2+ 2al + lbl in addition to the V-X la, a orbital ionization. The halogen character of this latter orbital should affect its band intensity sufficiently for it to be detected by changes in band profile with photon energy. For the chloride a significant change in band profile is observed. The low IE shoulder in the He I spectrum becomes the maximum of the band in the He I1 spectrum and is thereby assigned to cyclopentadienyl a ionizations. The peak a t 10.35 eV (28) Heilbronner, E.; Brogli, F. H e l a Chim. Acta 1971, 54, 1423.
Green, Payne, and Teuben
206 Organometallics, Vol. 2, No. 2, 1983
band
Table 11. Vertical Ionization Energies, Relative Band, Intensities, and Assignments for Bis(q'-cyclopentadienyl)vanadium( 111) Halides, Alkyls, and Aryls intensities intensity ratio IE, eV w1/2 He I He I1 He I/He I1 assignt 1
A A'
6.80 7.42 8.29
0.36 0.28 0.42
(r15-CP),VC1 1.07 0.93 4.1
1.03 0.97 0.95
22.4
0.96 1.04 0.23
3bl
0.41
Cp n
+
C1 pa
0.51
Cp a
+
Cp H
+
Cp n
3a1
c1 p n
10.35
B C D
A' A' '
12.8 13.2 17.5 20.7 22.1
)
64
32.6
CP 2. (qS-Cp),VBr 1.OB 0.89 0.92 1.11 3.9 1.3
0.33 0.25 0.37
6.81 7.43 8.14
0.82 1.21 0.34
16.3
9.9
0.61
47.6
28.5
0.55
Cp a CP CI
0.84 1.17 0.17 0.1 2 0.48
3b 1 3a I I pn "Zb," I pn "2b1" c p n + Ipo
10.07
B C A' A'
13.5 12.9 17.6 6.71 7.33 7.69 8.30 9.34 9.02
B C D
t
13.3 12.85 17.4 21.1 23.3
3. (q'-Cp),VI 1.07 0.93 3.6 3.1 13.1
0.31 0.21 0.22 0.28
I t
49.8
36.4
intensities band A' A" A", A", A",
B C D
A'
C
D
IE. eV
He I
A," A," A," A," B B C
D
0.73
intensity ratio He II/He I
He I1
cp
o t
CP
0
cp n
assiant
4. (q5-Cp),VPh 6.52 6.83 7.67
1 13.5 12.6 t 17.55 9.13 11.02
-1 -1
0.86 1.14 3.19 16.9
13.9
0.82
8.2 64.8
5.1 38.9
0.62 0.60
2.49
0.78
2b, 4a V-C a Arn CP Arn
+
Ara
+
Arc Cpo
Ar a
+
Cp a
+
Cpn
21.7 6.51 6.86 5.55 8.23 8.99 10.83 13.0 15.4 17.2 21.5
0.84 1.16 3.25 6.0 12.9 8.4 79
5. ( r15-Cp),V(2-MeC6H,) -1 -1 2.45 0.75 0.68 4.1 10.3 0.80 5.3 0.63 45
6.52 6.83 7.56 8.18 8.99
10.83
0.69 1.31
2.7 5.5 12.6 8.0 124
15.6 17.2 21.4 22.1
2bl 4a, V-C a Ar n CP Arn + Ara AratCpatCpn+Meo A r o t Cpo + M e a
6.
A'
0.90 1.10 0.61 0.37 6.3
(rl '-CP ),V(
0.71 1.29 2.0 3.1 9.2 4.4 47
3-MeC, H, 1 1.03 0.98 0.74 0.57 0.73 0.55 0.38
2b I 4al
v-c u
Arn CP n Arn+Ara
Ara+Cpa+C~n+Meu AratCpa+Mea
Organometallics,Vol. 2,No.2, 1983 207
Bis(cyclopentadienyl)vanadium(III)Complexes Table I1 (Continued ) intensities band
IE, eV
He I
A'
6.51 6.84 7.55 8.02 8.98 10.62
0.84 1.16 3.32 6.83 13.45 10.0
A," A," A," A," B
151 C
D
A'
A," A," A,"
A,"
He I1
intensity ratio He II/He I
7. (s'-Cp),V( 2,6-Me,C6H,) 0.83 0.99 1.17 1.01 3.1 5 0.95 4.4 0.65 .10.2 0.76 5 .O 0.50 53
0.35
7.15 7.43 8.35 . 9.39 9.93 11.65
5
0.82 1.18 3.02 18.5
8. ( ~ ~ ' - C P ) , V C ~ F S 0.84 1.02 1.16 0.98
A'
6.42 6.85
0.99 1.01 14.4
C
D
A A'
B C
D
A' A' '
8.98 11.68
12*4 13.2 17.1 21.3 21.8 22.4
+ Cpn
ArutCpo+Cpn+Meo
2b I 4a 1
v-c u A r n + Cpn Arc7
9. (q5-Cp),VMe 0.89 1.11
0.89 1.10
I
5 77
+ c p u + Cpn
v-c u
9.9
0.69
CP =
2.3 34
0.46 0.44
Me u CPU t Cpn + M e 0
10. (qWp),VCH,SiMe, 6.32 6.82 7.90 10.3 13.3 17.0 21.9 23.9 6.38 6.78 7.86
B C
Arn CP n Arn
5.1 16.35 17.3 20.8
A'
B
v-c u
Arn+Aru
3.02 95.1
D
2b I 4a1
14.9 17.1 5 20.6 21.9
B C
assignt
12.4 16.65
}
0.91 1.09 2.27
0.85 1.15 0.81
0.93 1.06 0.36
20.1
10.6
0.53
98
34
0.35
0.44 1.56 1.32 8.09 51.1
11. ( q5-Cp),VCH,CMe, 0.49 1.10 1.51 0.97 0.70 6.3 0.78 40.3
shows a large decrease in intensity in the He I1 spectrum, indicating predominantly halogen character; it is assigned to the lal V-Cl u ionization. That the peak at 9.81 eV also suffers intensity loss at higher photon energies indicates that there is some mixing of cyclopentadienyl and halogen p orbitals, similar to that observed for Ta(&SHs)2Cl~17 For V(v-C&J2Br and V(T&,H,),I the 8.5-10.5eV bands show a very similar profile in the He I and He I1 spectra, with the exception of the 8.9-eV peak in the spectrum of the bromide which shows a slight drop in intensity relative
0.78
v-c u CP Si-C u Cp u + Cp n t CH,SiMe, u
bl a1
v-c 0 CP n Cp u Cp u
+ Cp n + alkyl u + alkyl u
to the rest of the band in the He I1 spectrum, suggesting ita assignment to a V-Br u orbital ionization. However, since it has been observed that metal-halogen u ionization energies show a continuous decrease C1> Br > I in most transition-metal complexesmand there is no band assignable to V-I u orbital ionization below an IE of 8.9 eV in the spectrum of the iodide, this assignment is rather du(29) Hall, M.B. J. Am. Chem. SOC.1975,97, 2057.
Green, Payne, and T e u b e n
208 Organometallics, Vol. 2, No. 2, 1983
x ‘a. bious and an alternative assignment as a cyclopentadienyl 32 x orbital is more likely. In this context it should be recalled \ that the cyclopentadienyl x band of vanadocene shows ~ ~ . . ~ . exchange splitting, and owing to the open-shell nature of a. .-- - A ::; these compounds there is a possibility of similar structure 2a. in these vanadocene derivatives. The vanadium-halogen < z a 2 .2?2-=:-.-.:::l= a2 . : 2 la, a orbital ionizations for the bromide and the iodide a. ___....... ___ cannot therefore be assigned with certainty to a single peak ‘a2 a i . ? 2 ___ .......___ but must lie under the 8.5-10.5-eV bands. Unlike the chloride, no evidence can be found from the He II/He I < band profile comparisons for cyclopentadienyl-halogen
