2804
Organometallics 1989,8, 2804-2808
Chemischen Industrie, Frankfurt, and the BASF AG, Ludwigschafen, Germany, which is gratefully acknowledged. We thank Dr. K. Steinbach, University of Marburg, for FD-mass spectra. Registry No. 4a, 112681-58-0;4b,112681-557;6a, 122474-90-2; 7a, 122474-91-3;7a.0.5C4Hlo0,123122-66-7;7b, 123053-96-3;
2,4,6-t-Bu,C6H2AsCl2, 117184-75-5;((Z)-C8H14)Cr(CO)5, 9288973-1; [2,4,6-t-Bu,CsH,As]z,117184-76-6. Supplementary Material Available: A table of anisotropic thermal parameters (1page); a listing of observed and calculated structure factors (53 pages). Ordering information is given on any current masthead page.
Effective Electronegativities of Phosphorus, Arsenic, and Antimony in a 7r System. Evidence from Magnetic Circular Dichroism Jacek Waluk,laVb Heinz-Peter Klein,la Arthur J. Ashe 111,'' and Josef Michl*-la Center for Structure and Reactivity, Department of Chemistty, The University of Texas at Austin, Austin, Texas 78712-1167, and Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48 109 Received April 20, 1989
The UV absorption and magnetic circular dichroism of phosphabenzene, arsabenzene, and stibabenzene are reported and analyzed. In each molecule, the lowest energy transition is attributed to an na* state. Three aa* states are also assigned and related to the Lb, La, and Bbstates of the aromatic six-electron perimeter. The analysis of orbital splitting5 indicates that the effective a-orbital electronegativities of P, As, and Sb are higher than that of carbon.
Introduction Table I. Valence State Ionization Potentials (ZX)a n d Electron Affinities (E,) for t h e T OrbitallSm a6 a Function oP t h e Since their synthesis in the seventies,2-6 heterocyclic Valence Ande a (C-XC) derivatives of benzene containing the heavier heteroatoms a = 1200 (trstrtrr) a = goo (s2ppr) of the group 15, P, As, Sb, and Bi, have been investigated Ix Ex Ix + Ex Ix + Ex a,Odeg Ix Ex IX + Ex by electron diffra~tion,~J NMR,8 photoelectron (PE)? 11.33 120 11.27 0.34 11.60 electron transmission (ETS),l0 and m i c r o ~ a v e ~ J ~ - ' ~Cb 11.16 0.17 11.33 13.94 0.84 14.78 15.83 117 N 14.12 1.78 15.90 spectroscopy. It was found that the "aromatic character" 10.73 1.40 12.13 P 11.64 1.78 13.42 12.75 101 of benzene and benzene-like molecules such as pyridine 11.89 97 9.36 1.49 10.85 As 11.24 2.80 14.04 (1) is qualitatively preserved in these heterocycles, even 9.90 8.75 1.15 10.40 93 Sb 10.51 2.74 13.25 in the antimony and bismuth derivatives.14-16 These "The experimental valence angles for 1-4.'' The IX + EX values molecules can be viewed as the prototypes of a large class were interpolated from those at a = 120° (trqrtrr) and a = 90' ( s $ p ~ ) of heteroaromatic systems. It is of interest to ascertain ) ~(tan-' 60°)2]/[(tan-' 45°)2 - (tan-' 60°)2] as a using [(tan-' ( ~ x / 2 ) fraction of the sZpprconfiguration. *In this case, the atomic configuthe basic characteristics of these heavier heteroatoms with rations are trtrtrr and sppr. respect to participation in a conjugation. The two main factors to be established are (i) changes in the carbon-heteroatom resonance integral Pcx relative to pCc and (ii) differences in the effective heteroatom electronegativities, xx = (IFx+ EAx)/2. Here, IPX stands (1) (a) The University of Texas. This project was initiated a t the University of Utah. (b) On leave from The Institute of Physical Chemfor the ionization potential and EAx for the electron afistry,Polish Academy of Sciences, Kaspnaka 44,Ol-224, Warsaw, Poland. finity of the a-symmetry atomic p orbital of element X in (c) The University of Michigan. its valence state. PE9 and ETS'O studies of the energies (2) Ashe, A. J. 111 J. Am. Chem. SOC.1971, 93, 3293. and ordering of a orbitals in the P, As, and Sb derivatives (3) Ashe, A. J. 111, J. Am. Chem. SOC.1971, 93, 6690. (4) Ashe, A. J. 111, Tetrahedron Lett. 1976, 415. leave little doubt that the resonance integrals PCX are (5) Ashe, A. J., 111; Gordon, M. D. J. Am. Chem. SOC.1972,94,7596. substantially less negative than PCc as might be surmised (6) Wong, T. C.; Bartell, L. S. J. Chem. Phys. 1974, 61, 2840. from the greatly increased bond lengths. Due to its low (7) Wong, T. C.; Ashe, A. J., 11%Bartell, L. S. J.Mol. Struct. 1975,25, 65. stability, bismabenzene has received less attention. In (8) Ashe, A. J., 111; Sharp, R. R.; Tolan, J. W. J . Am. Chem. SOC.1976, these studies, the a-orbital electronegativities of the het98, 5451. eroatoms were assumed to be larger than that of carbon (9) Batich, C.; Heilbronner, E.; Hornung, V.; Ashe, A. J., 111;Clark, D. T.: Cobley, U. T.; Kilcast, D.; Sanlan, I. J.Am. Chem. SOC.1973,98,928. for N but smaller for P, As, and Sb. This may have been (10) Burrow, P. D.; Ashe, A. J., 111; Bellville, D. J.; Jordan, K. D. J . natural in view of the low Pauling electronegativities of Am. Chem. SOC.1982, 104, 425. these elements,17but we now provide evidence that it is (11) Kuczkoweki, R. L.; Ashe, A. J., 111J.Mol. Spectrosc. 1972,42,457. (12) Lattimer, R. P.;Kuczkowski. R. L.: Ashe. A. J.. 111: Meinzer, A. not correct. L. J. Mol. Spectrosc. 1975, 57, 428. One indication that at least the effective a electroneg(13) Kuczkowski, R. L.; Fong, G.; Ashe, A. J., 111 J. Mol. Spectrosc. ativities of P and As are higher than that of C has been 1978, 70, 197. (14) Ashe, A. J. 111 Acc. Chem. Res. 1978, 11, 153. (15) Ashe, A. J. I11 Top. Curr. Chem. 1982, 105, 125. (16) Ashe, A. J., III; Diephouse, T. R.; El-Sheikh, M. Y. J . Am. Chem. SOC. 1982,104,5693.
0276-7333/89/2308-2804$01.50/0
(17) Pauling, L. in The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY; 1960.
0 1989 American Chemical Society
-
Effective Electronegativities of P, As, and Sb available for a long time. Standard tables of valence state ionization potentials and electron affinities derived from atomic contain xp, xh, and XSb values higher than Xc for trigonally (sp2) hybridized heteroatoms (tr2trtra in Table primarily due to large electron affinities. Since the C-X-C valence angles are not 120' but 117' in pyridine, 101O in phosphabenzene, 97" in arsabenzene, and 93' in ~tibabenzene,'~ it is clear that the appropriate electronegativities are intermediate between these values and those for unhybridized heteroatoms (s2ppain Table I), which correspond to a C-X-C angle of 90'. These are readily estimated for the experimental bond angles (Table I), and it is seen that xp and xAsstill are larger than xc, though XSb no longer is. The electronegativity values given in the standard tables pertain to atoms undisturbed by u bonding to their neighbors. The effective electronegativities may be different in a real molecular environment in which more or less strongly electronegative neighbor atoms may polarize bonds in the u skeleton. However, in the u system the heavier elements of group 15 will act as electron donors toward carbon, decreasing the shielding of their nuclear charge and thus increasing their a electronegativity. We should therefore certainly expect the effective a electronegativities of P and As, and possibly even Sb, to be higher than that of carbon, although it may appear counterintuitive. A simple way to estimate the a-resonance integrals and effective a electronegativities within the framework of a simple model is to compare the estimates of molecular orbital energies of benzene with those of the heterobenzenes, using first-order perturbation theory. This is obviously only an approximate procedure but should be qualitatively significant and should permit the estimation of properties of other a systems containing these heteroatoms. It is indeed the procedure that was used in the PE9 and ETSO studies to deduce the fact that PCxis closer to zero than pee. However, for definitive conclusions concerning the electronegativities xx the relative energies of all four frontier orbitals are needed. Since the LUMO is bound, its energy was not determined by ETS and the xx values remained unknown. Magnetic circular dichroism (MCD) of the L bands of a cyclic a system with a (4N + 2)-electron perimeter such as the heterobenzenes is determined primarily by the relative magnitude of the splitting of the energies of the MO's derived from the top two bonding and the bottom two antibonding orbitals of the a perimeter and thus provides a direct measure of the quantity necessary for the evaluation of the relative magnitudes of xx and xc. The disadvantage of the MCD approach is that it requires an understanding of the electronic spectrum of the molecule, and the advantage is that it provides information on the orbital energies in the neutral parent molecule, and not energies relating to ions as PE and ETS do, and thus does not need to invoke Koopmans' theorem. In cases where comparison is possible, such as pyridine (l),the three methods yield compatible results. One could expect that the optical spectra of the heterobenzenes display recognizable L b and La absorption bands characteristic of perturbed uncharged (4N + 2)-electron perimeters, in addition to possible bands of the na*-type, and that MCD measurements might provide information (18)Hinze, J.; Jaff6, H. H. J. Am. Chem. SOC.1962,84,540. (19)Hinze, J.;Jaff6, H. H. J. Phys. Chem. 1963,67, 1501. (20)Mead, R. D.;Stevens, A. E.; Lineberger, W. C. Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York; 1984;Vol. 3, p 213.
Organometallics, Vol. 8, No. 12, 1989 2805
0.56
--I
--2 I
.
. . .
l
.
,
.
.
I
.
.
.
.
L
.
.
E
30
35
45
40
i (io3cm-')
Figure 1. Phosphabenzene in cyclohexane at 293 K: bottom, absorption spectrum (oscillator strengths given); top, MCD spectrum (magnetically induced molar ellipticit in units of deg G-' M-l, B terms in units of 10-3&D2/cm-K). Chart I
1
3
b
3
4
on the effective a electronegativity of P, As, and Sb. We have measured the absorption and MCD spectra of phosphabenzene (2), arsabenzene (31, and stibabenzene (4). In addition, to assure ourselves that the transition assignments are correct, we have also measured the spectra of 2-methylarsabenzene (2-Me-3)and 4-methylarsabenzene (4-Me-3). The results are compared with the data for the simplest compound of this class, pyridine (I).
Results and Discussion MCD and absorption spectra of 2-4,2-Me-3, and 4-Me-3 are shown in Figures 1-3. Three different electronic transitions, So S1, Sz, SB,are readily distinguished in the MCD curves in the region of the broad lowest energy absorption band. In the absorption spectra, one or both of the two lowest transitions are seen only as weak shoulders (2,3, 2-Me-3,4-Me-3) or not at all (4). The same pattern of MCD signs (-,+,-, corresponding to a +,-,+ sequence of B term signs) is preserved in all three parent compounds. In 4, a fourth transition with a positive B term is observed (So S4). The shapes of the MCD curves of 2 and 3 suggest that also in these molecules the B term of the So S4 transition is positive. The presence of three transitions in the low-energy range is intriguing. In pyridine, only two excited states are observed in this range. I t is now well established that they correspond to an na* level, S1( B < 0), and the lowest aa* level, s&,) ( B > O).21*22
-
-
-
(21)Kaito, A.; Hatano, M.; Tajiri, A. J. Am. Chem. SOC.1977,99,5241. (22)Castellan, A.; Michl, J. J. Am. Chem. SOC.1978,100, 6824.
2806 Organometallics, Vol. 8, No. 12, 1989
Waluk et al.
3 30
35
2-Me- 3
4-Me- 3
45
40
a As
Me
- 9 x IO3
& -6
-3
--
30
35
i (lOJcm")
Figure 2. Arsabenzene and its methyl derivatives in cyclohexane at 293 K. See caption to Figure 1. 25
2x16'
3 3 3 5
40
45
0
-0.095 I
A
[*IN
Lb and La states, lying much closer to each other than in pyridine. In stibabenzene, these three states lie even closer; the resulting values of the B terms seem distorted by mutual cancellation of overlapping positive and negative MCD signals. Such an assignment seems plausible already upon inspection of the absorption spectra: the exctinction coefficients of the maxima of the first absorption band of 2 (8500) and 3 (11OOO) are roughly equal to the sum of and €(La)in pyridine (2000 + 6300). MCD signs and relative magnitudes of hundreds of the lowest few lmr* transitions of aromatic compounds derived from a (4N 2)-electron perimeter have been successfully interpreted in terms of the perimeter m0de1.~"~~The crucial quantity in this approach is the difference AHOMO - ALUMO (AHOMO is the energy difference between the two highest occupied and ALUMO the energy difference between the two lowest unoccupied *-symmetry orbitals derived from the frontier orbitals of the perimeter). The inequality AHOMO # ALUMO leads to the appearance of relatively large contributions to the MCD B terms. For AHOMO > ALUMO, these so-called p+ contributions predict a +,-,+ sequence of B term signs for the three lowest lxr* states; the opposite signs are predicted for AHOMO C ALUMO. When AHOMO = ALUMO, p+ contributions vanish and the MCD signal is predicted to be small and determined only by the p- contributions, which are always present but are normally much smaller than p' for the Lb and La bands (for the higher energy Bb and B, transitions they frequently dominate and are positive and negative, respectively). In case of molecules for which AHOMO N ALUMO, the MCD sign of the two L bands may easily be determined even by a small addi-
+
.9x103
& -6 -3
i;
(10~cm'~)
Figure 3. Stibabenzene in cyclohexane at 293 K. See caption
to Figure 1.
Among the So -,S1, So -,Sz, and So -,S3transitions, So -,S3has the most intense MCD band in 2 and 3, with a positive B term, about five times stronger than that of the Lb transition in pyridine. The same intensity difference is observed in the absorption spectra: f(Lb) = 0.04 in pyridine and f N 0.2 in 2-4 for the sum of So S1, Sz, Ss. The values of the extinction coefficient of the n?r* transition in 1 and of the So -,S1 transition in 2-4 are similar. This suggests an assignment of SI to an nx* transition. Sz and S3levels would then be assigned as KA*
-
(23)Michl, J. J. Am. Chem. SOC.1978,100,6801. (24)Michl, J. J. Am. Chem. SOC.1978,100,6812. (25)Michl, J. J. Am. Chem. SOC.1978,100,6819. (26)Michl, J. Tetrahedron 1984,40,3845.
Organometallics, Vol. 8, No. 12, 1989 2807
Effective Electronegativities of P, As, and S b
4
-a
LUMO
-S
i
S
a Figure 4. Frontier orbitals of heterobenzenesand the predicted
changes of their energies upon methyl substitution. The energy of the -9 orbital in phosphabenzene has been taken from INDO/S calculations and the other energies from e~periment.~J~ tional perturbation such as methyl substitution. Therefore, such molecules have been labeled "soft MCD chromophore^".^^ Before interpreting the MCD spectra of the heterobenzenes 2-4, we need to analyze the changes in the HOMO and LUMO energies of benzene expected to be introduced upon heteroatom substitution (Figure 4). In benzene, HOMO and LUMO form degenerate pairs of orbitals. Within each pair, there is an orbital with a nodal plane passing through atoms 1and 4 (a and -a orbitals) and an orbital with antinodes at these positions (s and 3). The orbitals s and s and a and -a are paired, so that c k g 2 = ck,*2 and C k . 2 = Ck,-: for each position k. The effect of aza substitution may be treated as a +I (electron-withdrawing)inductive perturbation, which will stabilize the orbitals s and -s by amounts that are equal to first order in perturbation theory. The energy of the orbitals a and -a remains unchanged to the first approximation, as they have a node at the position of substitution. This leads to the orbital ordering s, a, -5, -a and, to first order in perturbation theory, to the equality AHOMO = ALUMO (soft chromophore). It has been shoWn21*22 that pyridine actually is an approximately soft chromophore. The B term of its Lb transition is weakly positive, being a sum of the small and positive p- and w+ contributions. The latter arises because AHOMO becomes slightly greater than ALUMO when one goes beyond the first order in perturbation theory. The sign is easily changed by sub~titution.~'The orbital energies derived for 1 from PE9 and ETS'O spectra through the use of Koopmans' theorem are -10.5 (s), -9.7 to -9.8 (a), 0.59 (4,and 1.16 eV (-a). Upon passing from benzene to 1-4, one has to take into consideration two different effects that can be expected to cause orbital energy shifts: (i) First, an inductive perturbation of the pz orbital in position 1,which would either stabilize the energy of the s and -s orbitals (+I, electronegativity xx increased relative to xc) or destabilize it (-1, xx decreased relative to xc). By itself, this would lead to the orbital sequence s, a, -s, -a in the former case and to a sequence a, s, -a, s in the latter case, with AHOMO N ALUMO to first order in perturbation theory. (ii) Second, in 2-4, but not in 1, a large increase of C-X bond lengths is observed (1.37, 1.73, 1.85, and 2.05 A, respe~tively),'~ which will significantly lower the absolute value of the carbon-heteroatomresonance integral &x. This factor will (27) Wallace, S. L.; Castellan, A.; Muller, D.; Michl, J. J.Am. Chem. SOC.1978,100,6828.
cause the energy of the orbital s to increase and the energy of the orbital -s to decrease in 2-4; the energies of the a and -a orbitals will again remain unchanged in the first approximation. By itself, this leads to the orbital sequence a, s, s,-a, again with AHOMO ALUMO to first order. The two effects on the s and -s orbitals are additive to first order in perturbation theory. If the electronegativity perturbation is of +I type, they are opposed for s and act in the same direction for -s; we would then expect AHOMO < ALUMO in the heterobenzenes 2-4, making them quite distinct from pyridine (1). If a -I electronegativity perturbation occurs, the two effects act in the same direction for s and are opposed for -s. In this case, one would expect AHOMO > ALUMO. Thus, the knowledge of the sign of AHOMO - ALUMO provides information about the orbital electronegativities of the heteroatoms relative to carbon. In order to determine the sign of AHOMO - ALUMO, we consider the expected form of the MCD and absorption spectra for both possible relative magnitudes of the critical factors AHOMO and ALUMO. (a) AHOMO < ALUMO. In this case, the heterobenzenes should reveal a -,+ sequence of B terms for the first two m*transitions. This is exactly what is observed in the spectra (Figures 1-3), if one accepts the above proposed assignment of the first excited singlet state to an na* transition, as in pyridine. Much stronger arguments for this state of affairs, and for an a, s orbital sequence, emerge from the comparison of the predicted and observed responses of MCD and absorption spectra to substitution in positions 2 and 4 by the methyl group, a weak a donor (Figures 2 and 4). Substitution in position 2 should influence mostly the orbital a, raising its energy. In contrast, substitution in position 4 should destabilize the orbital s, leaving the orbital a at the same energy. The effects on the antibonding orbitals -s and -a should be much smaller since these are far removed in energy from the methyl donor orbital. Thus, if the orbital ordering in 3 is a, s, one should expect AHOMO - ALUMO to become more negative in 2-Me-3 than in 3 itself; the opposite should be true for 4-Me-3. The MCD intensity of the lowest a** transitions should increase in the former derivative and decrease in the latter. This is exactly what is observed and is particularly well seen for the strongest transition, So S3. For the absorption intensity of the L, (So S,) transition, which is also determined by AHOMO and ALUMO, one expects an effect opposite to that of MCD: the oscillator strength should increase in 4-Me-3 and decrease in 2-Me-3. For the Lb band, the reverse should hold. Again, these expectations agree with experiment. (b) AHOMO > ALUMO. Now, a +,- sequence of MCD B terms would be expected for the first two TU* transitions. This is impossible to reconcile with the experimental findings, unless one makes the completely unacceptable hypothesis that the So S3transition is of na* type and yet carries much larger absorption intensity than the s u m of the T?T*transitions I+,and La.This would be without precedent and can be safely rejected. Thus, we are left with AHOMO < ALUMO and the orbital ordering a, s, -5, -a for the heterobenzenes. Note that the positive B term of the next higher transition, So s4,is as expected for a transition into the Bb state (hcontribution), and we propose such an assignment. The above expectations, based on simple orbital symmetry considerations, were confirmed by the INDO/S calculations of state and orbitals energies, intensities, and B term values, which we carried out for 1,2,2-Me-2, and
-
-
-
-
2808 Organometallics, Vol. 8, No. 12, 1989
Waluk et al.
Table 11. Results of INDO/S Calculations t,1 0 3 ~ ~ 4
1
2
2-Me-2
4-Me-2
32.0 37.5 42.2 47.5 34.3 37.9 39.5 40.6 47.4 32.5 38.4 39.2 39.3 47.1 34.3 38.5 39.4 41.2 47.4
f 0.01 0.04 0.00 0.0002 0.04 0.08 0.22 0.00 0.53 0.07 0.07 0.19 0.01 0.64 0.01 0.08 0.36 0.00 0.53
B, 10-3@eD2/cm-1 assignt 0.39 0.04 0.00 0.06 -0.83 4.05 -0.82 0.00 -2.25 -0.29 5.97 1.15 -4.41 -2.21 -0.61 9.22 -6.57 0.00 -1.34
nr* *r*(Lb) nr* ur*(La) ***(Lb) nr*
*?r*(L,) UR*
*r*(Bb) r**(Lb)
nr*
R?r*(L.¶) au*
Tr*(b,) ***(Lb) nr*
*a*(L,) UT*
rr*(Bb)
4-Me-2 (Table 11). It is noteworthy that the calculations confirm a large stabilization of the La state upon passing from pyridine to phosphabenzene, about 8000 cm-'; the calculations also predict a large increase of the oscillator strength for this transition. The Lb state is predicted to be stabilized by about 3000 cm-' and unchanged in intensity upon going from 1 to 2, in very good agreement with experiment. The calculations predict that the na* transition energy should increase upon going from 1 to 2, but the computed energy change is larger than observed (6000 cm-' vs observed 2000 cm-'). The calculated values of the B terms of the Lb transition are positive for pyridine and negative for phosphabenzene, exactly as observed. On the other hand, the values of the B terms of the La transition are not computed reliably because the calculations predict an n?r* and a a** transition to lie in close vicinity. Their mixing with the La state provides the main computed contribution to the B term; it also gives an unrealistically large value for the B term of the n?r* transition. In reality, it is observed that the Lb state that lies closer to La than the na* transition: this can be deduced from the absorption and MCD spectra. We believe that the main contribution to the B term of the La state is actually provided by the La+, mixing. This is predicted to be positive by both perimeter theory and the INDO/S calculations.
Conclusions The analysis of absorption and MCD spectra of heterobenzenes in terms of the perimeter model leads us to propose that the first broad electronic absorption band in solutions of phosphabenzene (2), arsabenzene (3), and stibabenzene (4) is composed of three electronic transitions, nn*, aa*(Lb), and a?r*(L,), with another m*transition (Bb)at a higher energy. The signs of the B terms and the spectral changes observed upon methyl substitution demonstrate that AHOMO < ALUMO and agree with the previously deducedgJOorbital ordering in 2-4: a, s, followed by -s, -a. This differs from the ordering s, a, -s, -a encountered in pyridine. This difference can be understood if the orbital energy shifts in 2-4 are mainly caused by
variation of the resonance integrals of the C-X bonds caused by the increase of the carbon-heteroatom distance, as has been concluded earlier from the results of electron transmission spectroscopy experiments.l0 In this latter work, the authors were not able to determine the energy of the LUMO (-5) orbital, since it is bound. Our conclusion that AHOMO < ALUMO reduces the uncertainty in the position of this orbital as the value of AHOMO is relatively well known from photolectron spectroscopy measurements (0.6-0.8 eV in 2,0.8-1.1 eV in 3, 1.1-1.3 eV in 4).9 Combining these values with the energy of the -a orbital in 2, 3, and 4 (0.6 eV,'O we obtain an upper limit for the energy of the -s orbital: 0.0 eV in 2, -0.3 eV in 3, and -0.5 eV in 4. By far the most interesting implication of the negative sign of AHOMO - ALUMO is that, against "chemical intuition", P, As, and Sb are all, in a ?r-electronsense, more electronegative than carbon. The use of atomic parameters of Table I, which reflect this fact, in a PPP calculation indeed produces the expected results: excess *-electron density on the heteroatom and deficiency thereof in the positions 2, 4, and 6, albeit to a smaller extent than in pyridine. A similar distribution of ?r charges is observed in INDO/S results for phosphabenzene (-0.21 on P, +0.11 on C2 and C6, +0.06 on C4). Inspection of the a-electron distribution calculated by the INDO/S method indicates a a-electron deficiency on the heteroatom (+0.21) and excess (-0.11) at the neighboring carbons 2 and 6 (-0.11 on each). Thus, the simplest description of the effect of the heteroatom is that it acts as a a-electron acceptor and a a-electron donor, with obvious chemical consequences. For example, the 2-phosphaphenyl group should act as a weak a-electron acceptor etc.
Experimental and Computational Part The synthesesof 2 and 32and of the methyl derivatives" have been described. Stibabenzene was prepared just prior to meaand ansurements from 1,4-dihydro-l,l-dibutylstannabenzene timony trichloride? All spectra were measured in spectral grade cyclohexane at 293 K. Special precautions were taken to ensure the absence of oxygen and moisture in the samples: the solutions were prepared in a drybox under argon atmosphere, and the samples were immediately transferred to a cell designed for absorption and MCD measurements. MCD spectra were taken on a JASCO 5-500spectropolarimeter equipped with a 15-kG electromagnet. Absorption spectra were run on a Cary 17 spectrophotometer. Calculations were performed by the INDO/SW and PPP31 methods, using the experimental geometry of phosphabenzenes and the lowest 100 singly excited configurations in the INDO/S CI procedure. Acknowledgment. This work was supported by the US. Public Health Service (GM 37929-03). Registry No. 2, 289-68-9; 3, 289-31-6; 4, 289-75-8; 2-Me-3, 56577-93-6; 4-Me-3, 57242-07-6. ~
(28) Ashe, A. J., III; Chan,W . T. Tetrahedron Lett. 1975,2749. (29) Ashe, A. J., 111; Chan, W. T.; Perozzi, F. Tetrahedron Lett. 1975, 1083. (30) Ridley, J. E.; Zerner, M. Z. Theor. Chim. Acta 1973, 32, 111. (31) Pariser, R.; Parr, R. G. J . Chem. Phys. 1953, 21, 466. Pople, J. A. Trans. Faraday SOC.1953,49, 1375.
