J. Phys. Chem. B 2001, 105, 9239-9244
9239
Theoretical Study of Germaneselone HXGedSe (X ) H and F): Thermodynamic and Kinetic Stability Hsin-Yi Liao,† Ming-Der Su,*,‡ and San-Yan Chu*,† Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30043, Taiwan, R.O.C., and School of Chemistry, Kaohsiung Medical UniVersity, Kaohsiung 80708, Taiwan, R.O.C. ReceiVed: March 15, 2001; In Final Form: June 7, 2001
To extend knowledge of germanium-selenium double bonds, unimolecular reactions of HXGedSe (X ) H and F) have been investigated and compared with those of HXCdSe, by means of ab initio calculations including polarization functions and electron correlation. Five different reaction mechanisms are proposed: (A) 1,1-HX elimination, (B) 1,2-H shift, (C) 1,2-X shift, (D) H and XGeSe radical formation, and (E) X and HGeSe radical formation. Our theoretical findings suggest that HXGedSe is kinetically stable with respect to the unimolecular reactions given above. Moreover, these comparisons emphasize that selenium is more reluctant to form double bonds with germanium than with carbon. We also report theoretical predictions of molecular parameters and vibrational IR spectra of HXGedSe and its derivatives, which should be useful for further experimental observations.
I. Introduction For many years, it was considered that compounds featuring double bonds between heavier main-group elements would not be stable because of weak pπ-pπ bonding.1 Recently, remarkable progress has been made in the chemistry of organogermanium compounds containing multiple bonds to group 16 elements, i.e., GedO,2 GedS,3 GedSe,4 and GedTe.5 The original experimental difficulties arising from the instability of such germanium-chalcogen double-bonded species have been overcome by taking advantage of kinetic stabilization with bulky substituents. For instance, Tokitoh, Okazaki, and co-workers have recently succeeded for the first time in isolating the kinetically stable diaryl-substituted germaneselone, Tbt(Tip)GedSe and Tbt(Dis)GedSe, by taking advantage of very effective steric protection groups.4 The chemical and physical properties of GedSe doublebonded species has been a subject of growing interest to experimentalists and theoreticians in the past decade.4,6 The interest is partly due to the ability of selenium to act as a redox catalyst, which has been demonstrated to be an important factor in understanding the biological function of several selenoproteins.7 Nevertheless, experimental determinations of physical parameters for these generally short-lived, reactive GedSe double-bonded species are difficult to obtain and hence largely nonexistent. Such systems are, however, amenable to accurate theoretical investigations, and it is desirable to explore the possibility and opportunity to obtain reliable information in this manner. In view of the situation, theoretical information is of great help for further advance in germaneselone chemistry. Thus, we have undertaken ab initio calculations of the properties of the ground state of the parent compound H2GedSe and its fluoride (HXGedSe) in order to extend our knowledge of germaniumselenium double bonds. To the best of our knowledge, theoreti* Corresponding authors. † National Tsing Hua University. ‡ Kaohsiung Medical University.
cal investigations of germaneselones are as scarce as experimental work. Only the parent H2GedSe species has been the subject of previous theoretical treatments.4 The most recent and comprehensive study has been that by Nowek, Sims, Babinec, and Leszczynski.4e These authors used the QCISD(T)/ TZP++(2df,2pd) level of theory to investigate three of the singlet isomers considered here, H2GedSe, trans-HGeSeH, and cis-HGeSeH. On the other hand, no information is currently available about the molecular structure and vibrational IR spectra of its halide (such as, HFGedSe). The aim of the present work is therefore to investigate the structures, stabilities, and unimolecular decomposition reactions of various GedSe double bonded forms by employing a high level of theory to furnish reliable quantitative data on the energies of the HXGedSe species. In addition, since the reported QCISD(T) theoretical data are in very good agreement with the previous work,4e we believe that the predicted geometries and vibrational IR spectra of the asymmetric halides of germaneselone are also accurate and might be helpful to experimentalists in identifying the species for which experimental data are still elusive. II. Methodology Geometry optimizations were performed in the full space of coordinates by analytical gradient based techniques,8 employing second-order Møller-Plesset perturbation (MP2) theory9 with the 6-311++G(d,p) basis set. In these calculations, inner-shell molecular orbitals were not included for computing electron correlation energies (frozen-core approximation). The geometries obtained in this way are denoted by MP2(fc)/6-311++G(d,p). The vibrational frequencies, at the same level of theory, were computed for all of the H2GedSe and HFGedSe species, to characterize them as true minima or transition structures on the corresponding potential energy hypersurfaces. Unscaled vibrational frequencies were employed in order to account for zeropoint vibration energies (ZPVE) of all the investigated species. On the MP2 geometries we performed QCISD(T) calculations,10 that is, quadratic CI calculations with single and double
10.1021/jp010999s CCC: $20.00 © 2001 American Chemical Society Published on Web 08/29/2001
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TABLE 1: Calculated Harmonic Vibrational Frequencies, Rotational Constants, Dipole Moments, Atomic Charges, and Relative Energies of the Species in H2GeSe Decomposition Reactions at the MP2/6-311++G(d,p) Level of Theory
species H2GedSe
frequencya (cm-1) 2196(2208)b, 2193(2207)c, 899(917)d, 548(561)e, 528(537)f, 403(416)g
rotational constants (MHz)
dipole moment (Debye)
q(Ge)
q(Se)
A 157213.00
3.706
0.5231
-0.3858
q(H) -0.06866
relative energies (kcal/mol)h 0.0
B 2674.47 C 2629.73 A-TS B-TS-1 trans-HGeSeH
2158, 1463, 482, 475, 373, 1552i 2033, 1777, 672, 402, 333, 1493i 2494, 1990, 809, 565, 557, 290
B-TS-2 cis-HGeSeH
2452, 1970, 704, 463, 252, 600i 2502, 1990, 726, 587, 495, 283
B-TS-3 HGeSe
1694, 1545, 956, 920, 331, 1372i 1980, 742, 426
A 109785.89 B 2325.02 C 2276.81
1.146
0.3563
-0.2581
-0.1426(Ge) 0.04442(Se)
59.17 41.73 -0.9662 (-0.5)i
A 110807.27 B 2291.56 C 2245.13
1.354
0.3552
-0.2496
-0.1446(Ge) 0.03895(Se)
14.01 0.9078 (1.5)i
A 292114.92 B 2739.28 C 2713.83
2.824
0.3958
-0.3247
-0.07110
28.04 69.43j
a Values in parentheses are taken from ref 4e. b GeH asym str; B sym. c GeH sym str; A sym. d H GeSe sciss; A sym. e H GeSe wag; B 2 2 2 1 2 1 2 1 sym. f H2GeSe rock; B2 sym. g GedSe str; A1 sym. h At the QCISD(T)/6-311++G(3df,3pd)//MP2(fc)/6-311++G(d,p) level of theory. i Value in j parentheses is taken from ref 4e at the QCISD(T)/TZP(2df,2pd) level of theory. The relative energy of two radicals (i.e., HGeSe and H) with respect to H2GedSe.
substituents followed by a perturbative treatment of triple substitutions, with the 6-311++G(3df,3pd) basis set. Unless otherwise noted, the relative energies given in the text are those determined at QCISD(T)/6-311++G(3df,3pd)//MP2(fc)/ 6-311++G(d,p) (hereafter designated QCISD(T)) and include ZPVE corrections determined at MP2(fc)/6-311++G(d,p). All calculations were performed on an IBM397 in our laboratory, with the GAUSSIAN 94 programs.11 III. Results and Discussion (1) H2GedSe Decomposition Reactions. In the case of H2GedSe, there are three kinds of possible dissociation pathways, i.e., (A) 1,1-hydrogen elimination, (B) 1,2-hydrogen shift, and (C) radical dissociation. The predicted geometrical parameters (bond lengths and bond angles) for H2GedSe and its isomers as well as the calculated vibrational frequencies, dipole moments, atomic charges, and rotational constants are collected in Table 1, where they are compared with previous theoretical calculations.4e Also, the calculated molecular parameters of H2GedSe and its isomers at the MP2 level of theory are shown in Figure 1. The relative energies obtained by MP2 and QCISD(T) calculations are summarized in Figure 2. Unfortunately, at present no experimental data for H2GedSe and its derivatives are available for comparison. For this reason, the reliability of the predicted geometries can be assessed only by comparison between different levels of theory. As mentioned earlier, the most recent theoretical values (QCISD(T)/ TZP++(2df,2pd)) for H2GedSe, trans-HGeSeH, and cisHGeSeH species were obtained by Nowek, Sims, Babinec, and Leszczynski.4e As can be seen in Table 1 and Figure 1, the molecular parameters of H2GedSe for our MP2 calculations compare well with the QCISD(T) results. The bond lengths and bond angles are in agreement to within 0.01 Å and 0.20°, respectively. Moreover, our energies based on the QCISD(T)/ 6-311++G(3df,3pd) calculations are in reasonable agreement with theirs. The relative energies for H2GedSe, trans-HGeSeH, and cis-HGeSeH are 0.0, -0.97, +0.91 kcal/mol from our results in comparison with their values of 0.0, -0.50, +1.5 kcal/ mol. It should be mentioned here that our calculated GedSe
Figure 1. MP2/6-311++G(d,p) optimized geometries (in Å and deg) for the H2GedSe isomers. Values in brackets are at the QCISD(T)/ TZP(2df,2pd) level of theory taken from ref 4e.
double bond length (2.171 Å for H2GedSe) is in good agreement with the experimental one (2.180 and 2.173 Å for Tbt(Tip)GedSe and Tbt(Dis)GedSe, respectively).4d Additionally, no experimental values are available for transand cis-HGeSeH. As seen in Figure 1, our calculated geometries of trans- and cis-HGeSeH compare well with QCISD(T)/TZP(2df,2pd) calculations done by Leszczynski et al.4e The GeSe and GeH bond lengths in their work are slightly shorter (∼0.02 Å) than we obtained, while our SeH bond length is nearly the
Thermodynamic and Kinetic Stability of HXGedSe
J. Phys. Chem. B, Vol. 105, No. 38, 2001 9241
Figure 2. Energy level diagram for the unimolecular decomposition reactions of H2GedSe. The relative energies are taken from the MP2 level as given in Table 1. The MP2-optimized structures of the stationary points see Figure 1. The quantities in brackets are at the QCISD(T) level of theory (see the text).
same as theirs. Furthermore, our theoretically predicted groundstate vibrational frequencies for H2GedSe are in agreement with previous computational results.4e The average deviation between our and their theoretical values for all fundamental frequencies is about 1.7%, well within the established error range.12 In any event, as a result of these good agreements on the known singletstate features, we are confident that the computational methods used in this study should be reliable and useful for comparison with possible future experimental studies. As one can see in Figures 1 and 2, H2GeSe-A-TS is the transition state for 1,1-hydrogen elimination leading to H2 + GeSe. Our MP2 calculations show that this transition structure is planar with both hydrogens on the same side of the GeSe bond axis. The QCISD(T) results predict that the reaction path A is exothermic (-9.1 kcal/mol) and possesses a considerable energy barrier (59 kcal/mol). For reaction path B, H2GeSe-B-TS-1 and H2GeSe-BTS-2 are the transition states for the trans-cis isomerization of Η2GedSe, while H2GeSe-B-TS-3 is the transition state for the dissociation of cis-HGeSeH to H2 + GeSe. Two planar trans and cis isomers can be distinguished in the case of HGeSeH, i.e., trans-HGeSeH and cis-HGeSeH. Both of these forms are found to be minima on both MP2 and QCISD(T) potential energy surfaces. The bonding parameters of the trans conformer are very different from those of the cis conformer. For example, the main difference between the computed geometries concerned the bond angles HGeSe and GeSeH, respectively. They are both consistently larger for the cis isomer by 4°. Differences were also found for the bond distances. With regard to the energies, the trans isomer is estimated to be ca. 1.9 kcal/mol (QCISD(T)) more stable than the cis one. It should be stressed that there exists a high activation barrier (42 kcal/ mol at QCISD(T)) between H2GedSe and the HGeSeH isomers. Consequently, if H2GedSe and/or HGeSeH isomers were formed directly by the appropriate gas-phase experiments, they would not easily interconvert. In addition to this, according to our QCISD(T) results, the barrier height for the dissociation cis-HGeSeH f H2 + GeSe is 28 kcal/mol above germaneselone, but still lower than the barrier height (42 kcal/mol) for the reaction H2GeSe f trans-HGeSeH. The internal rotation transition state (H2GeSe-B-TS-2) lies ∼14 kcal/mol above the H2GedSe, trans-HGeSeH, and cis-HGeSeH species. All these findings strongly imply that when the 1,2-hydrogen occurs, it is likely to be followed by the generation of H2 and GeSe via the transience of HGeSeH. It is worth noting that when comparing the relative stabilities of H2GeSe isomers at the highest precision QCISD(T) level, the trans-HGeSeH isomer
Figure 3. MP2/6-311++G(d,p) optimized geometries (in Å and deg) for the HFGedSe isomers.
is more stable by ∼1.0 and ∼1.9 kcal/mol than H2GedSe, and cis-HGeSeH, which is in good agreement with previously reported results (see Table 1).4e For isolating germaneselone, the magnitude of the energy barrier between H2GedSe and the more stable trans-HGeSeH is important. The barrier is 38 kcal/mol smaller than that between H2CdSe and trans-HCSeH, but is still as large as 42 kcal/mol. This finding suggests that germaneselone is kinetically stable with respect to 1,2-hydrogen shift. However, there still remain two possible pathways for the unimolecular destruction of germaneselone, i.e., the molecular (reaction A) and radical (reaction C) dissociations which lead to H2 + GeSe and H + HGeSe, respectively. As Figure 2 shows, the former reaction must proceed with a sizable barrier of 59 kcal/mol, while the energy required for the latter reaction is expected to be more than 69 kcal/mol.13 These results confirm that germaneselone is stable in a kinetic sense despite its thermodynamic instability. We return to compare the 1,2-hydrogen shift and molecular dissociation for the cases of H2GeSe and H2CSe.14 Although the energies required for the two reactions are both sizable (42 and 59 kcal/mol), the former pathway leading to trans-HGeSeH is available at a lower energy than is the latter to H2 + GeSe. As can be seen in Figure 2, the reactions after the more facile 1,2-hydrogen shift are trans-HGeSeH f cis-HGeSeH f H2 + GeSe. The transition states for the subsequent reactions lie considerably lower in energy than for the 1,2-hydrogen shift. In contrast, the 1,2-hydrogen shift and molecular dissociation of H2CdSe seem to compete with each other, since the barriers for those reactions are comparable (80 and 88 kcal/mol, respectively).14 In addition, even if the 1,2-hydrogen shift takes
9242 J. Phys. Chem. B, Vol. 105, No. 38, 2001
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TABLE 2: Calculated Harmonic Vibrational Frequencies, Rotational Constants, Dipole Moments, Atomic Charges, and Relative Energies of the Species in HFGeSe Decomposition Reactions at the MP2/6-311++G(d,p) Level of Theory
species
frequency (cm-1)
HFGedSe
2239, 761, 698, 427, 409, 172
A-TS B-TS-1 trans-FGeSeH
1910, 490, 415, 401, 127, 1392i 1771, 667, 365, 287, 149, 1262i 2486, 658, 614, 333, 289, 159
B-TS-2 cis-FGeSeH
2459, 659, 471, 264, 168, 348i 2492, 653, 546, 314, 284, 177
B-TS-3 C-TS-1 trans-HGeSeF
1370, 797, 549, 365, 327, 1167i 1985, 578, 497, 404, 222, 945i 1918, 618, 550, 512, 279, 122
C-TS-2 cis-HGeSeF
2016, 691, 573, 364, 249, 208i 2027, 611, 590, 408, 282, 156
C-TS-3 FGeSe
1200, 536, 465, 360, 137, 1084i 684, 437, 180
HGeSe
1980, 742, 426
rotational constants (MHz)
dipole moment (Debye)
q(Ge)
q(Se)
q(F)
q(H)
relative energies (kcal/mol)a
A 18538.54 B 1886.94 C 1712.62
2.854
0.7921
-0.3378
-0.4265
-0.02782
0.0
A 10511.15 B 1921.69 C 1624.66
3.217
A 10517.64 B 1898.12 C 1607.93
1.878
0.6795
-0.2383
-0.4804
0.03921
44.11 38.91 -10.43 -1.627
0.6923
-0.2782
-0.4919
0.07777
-9.550
A 11399.37 B 1965.49 C 1676.43
3.388
0.4127
0.05246
-0.3216
-0.1436
10.87 55.14 37.90
A 11674.58 B 1903.81 C 1636.88
3.148
0.3453
0.05076
-0.3069
-0.08918
57.43 41.31
A 22708.15 B 1827.16 C 1691.09 A 292114.92 B 2739.28 C 2713.83
1.601
0.7060
-0.2829
2.824
0.3958
-0.3247
-0.4230
s
s -0.07110
66.86 69.95b 112.2c
a At the QCISD(T)/6-311++G(3df,3pd)//MP2(fc)/6-311++G(d,p) level of theory, see the text. b The relative energy of two radicals (i.e., FGeSe and H) with respect to HFGedSe. c The relative energy of two radicals (i.e., HGeSe and F) with respect to HFGedSe.
place, a considerable, additional energy (91 kcal/mol) is required to surmount the barrier for cis-HCSeH f H2 + CSe. Thus, in the case of H2CdSe, it appears that H2 and CSe are directly formed by the molecular dissociation. (2) HFGedSe Decomposition Reactions. In the case of HFGedSe, there are five kinds of reaction paths, i.e., (A) 1,1hydrogen elimination, (B) 1,2-hydrogen shift, (C) 1,2-fluorine shift, (D) formation of FGeSe and H radicals, and (E) formation of HGeSe and F radicals. The fully optimized geometries of the equilibrium structures and transition states are presented in Figure 3. The calculated vibrational frequencies as well as the dipole moments, rotational constants, net atomic charges, and relative energies of the HFGedSe and its derivatives are collected in Table 2. The corresponding reaction energy profiles for the HFGedSe decomposition reactions are shown in Figure 4. While there are no experimental values available for the HFGedSe structural parameters to compare with the calculated values, we still believe that both structures and energies of the HFGedSe species are also well described at the QCISD(T) level of theory. As one can see in Figures 3 and 4, HFGeSe-A-TS is the transition structure for the 1,1-HF elimination of HFGedSe leading to HF + GeSe (reaction A), and is nonplanar. HFGeSeB-TS and HFGeSe-C-TS are the transition structures for the 1,2-hydrogen migration (reaction B) and the 1,2-fluorine migration (reaction C), respectively. Some of them are calculated to be planar. As Figure 4 shows, it is apparent that reaction path A is predicted to be slightly endothermic (+1.2 kcal/mol at QCISD(T)) and to proceed with a large barrier of 44 kcal/ mol. For reaction pathways B and C, our QCISD(T) results predict that the activation energies are at least > 39 kcal/mol. Likewise, the energies required for the two radical dissociations
Figure 4. Energy level diagram for the unimolecular decomposition reactions of HFGedSe. The relative energies are taken from the MP2 level as given in Table 2. The MP2-optimized structures of the stationary points see Figure 3. The quantities in brackets are at the QCISD(T) level of theory (see the text).
are both sizable. Namely, 70 and 112 kcal/mol for reactions D and E, respectively. On the other hand, HFGedSe is calculated to be an average of 10 kcal/mol less stable than trans- and cisFGeSeH (1,2-H shift isomers). All these computational results strongly imply that HFGedSe should be kinetically stable despite its thermodynamic instability. In particular, the dipole moment of HFGedSe is quite large (2.85 D at MP2/ 6-311++G(d,p)). This means that HFGedSe should be experimentally observable in the gas phase. It should be noted that the activation energies of reactions A and B are comparable (44 and 39 kcal/mol for reactions A and B, respectively, at
Thermodynamic and Kinetic Stability of HXGedSe QCISD(T)). Accordingly, the two reaction pathways are likely to be competitive at high temperatures. For reaction path C (i.e., the 1,2-F shift), trans- and cis-HGeSeF possess the highest energy of all the minima on the HFGedSe surface at both MP2 and QCISD(T) computational levels. The average energy difference between HGeSeF and HFGedSe is 46 (MP2) and 40 (QCISD(T)) kcal/mol. Additionally, QCISD(T) calculations estimate that the activation barrier for reaction pathway C is at least >55 kcal/mol. Such a substantial barrier for the isomerization of HGeSeF can be attributed to repulsions between the GedSe π electrons and the F electron lone pairs. In any event, our theoretical findings suggest that the HGeSeF isomers are unlikely to be observed experimentally. Finally, as can be seen in Figure 4, there is a large energy difference (50 kcal/mol at QCISD(T))) favoring the FGeSeH species over the HGeSeF isomers. The reasons for the large difference in stability between FGeSeH and HGeSeF may be attributed to two factors as follows. First, FGeSeH can be stabilized by the polarity of the F-Ge bond, increasing its ionic character and making the germanium more positive. For instance, from Tables 1 and 2, it is apparent that the double bond in fluorine-substituted germaneselone (HFGe0.792+dSe0.338-) is more strongly polarized than that in parent germaneselone (H2Ge0.523+dSe0.386-). Second, the fluorine lone pair, which possesses a strong π donor character as shown in 1, can greatly enhance the stability of FGeSeH isomers relative to HGeSeF isomers. In fact, these two factors of electron withdrawal and π-donation are readily compatible. Electronegative substituents withdraw electron density from the germanium, making it more positively charged. This increased positive charge makes the germanium a better π-acceptor, and π-donation from the substituents (i.e., nπ f pπ delocalization) is thus enhanced,14,15 which can greatly stabilize the divalent FGeSeH species.
IV. Concluding Remarks From our survey of the mechanisms of the HXGedSe unimolecular reactions, three important conclusions reached from this study are the following. (1) According to our previous study for the H2CSe species,14 both HGeSeH and HCSeH are calculated to be more stable in the trans form than in the cis form. In either case, the energy difference in favor of the trans form is rather small (2.0 kcal/ mol for HGeSeH and 0.21 kcal/mol for HCSeH). As already discussed, the trans-to-cis isomerizations of the divalent species prefer rotation to inversion. The rotational barriers calculated for HGeSeH and HCSeH are 14 and 82 kcal/mol, respectively. It is noteworthy that the latter value is quite sizable and 3.5 times as large as the former. The large barrier for the isomerization of HCSeH was explained by us14 in terms of π-bonding between C and Se. We noted that the electronic polarization of C-sSe+, as expected from the electronegativity difference between carbon and selenium atoms, leads to an increase in the π-overlap and the strength the CdSe π-bond. The view that the CSe bond contains some double-bond character comes from the fact that its bond length increases by ∼0.2 Å at the rotational transition state in which the CH and SeH bonds are nearly orthogonal. On the other hand, there is appreciable change in the GeSe bond length on rotation about
J. Phys. Chem. B, Vol. 105, No. 38, 2001 9243 the GeSe bond of HGeSeH (see Figure 1). Thus, it is not surprising that the rotational barrier for HGeSeH is calculated to be relatively small. (2) As Figure 4 shows, since the barriers for isomerization from trans-FGeSeH to HFGedSe and from cis-FGeSeH to HF + GeSe are significantly higher than the rotational barrier from the trans-FGeSeH to cis-FGeSeH, the FGeSeH species should be stable from both the kinetic and thermodynamic point of view. In other words, the FGeSeH species should have a lifetime long enough for it to be observed spectroscopically. Nevertheless, our QCISD(T) results predict that trans-FGeSeH is more stable than cis-FGeSeH by 0.88 kcal/mol. Additionally, the internal rotational transition state (i.e., FGeSeH-B-TS-2) lies 1.6 kcal/mol below HFGedSe. This indicates that the isomerization reaction (trans-FGeSeH f cis-FGeSeH) should proceed without activation. Thus, for the above two reasons (namely, a small energy separation between the FGeSeH isomers and barrier-free trans-cis isomerization), the assignment of FGeSeH singlet state structure is uncertain. (3) Comparisons with germaneselone as well as selenoformaldehyde reveal that selenium is much less reluctant to form double bonds with carbon than with germanium. Thus, selenoformaldehyde should be more stable and less reactive than germaneselone. Acknowledgment. We are grateful to the National Center for High-Performance Computing of Taiwan and the Computing Center at Tsing Hua University for generous amounts of computing time. We also thank the National Science Council of Taiwan for their financial support. We express our gratitude to anonymous referees for their valuable comments. References and Notes (1) For a recent review, see: (a) Raabe, G.; Michl, J. In The Chemistry of Organic Silicon Compounds; Patai, S.; Rappoport, Z. Eds., John Wiley: New York, 1989; Chapter 17, p 1015. (b) Barrau, J.; Esoudie´, J.; Satge´, J. Chem. ReV. 1990, 90, 283. (c) Grev, R. S. AdV. Organomet. Chem. 1991, 33, 125. (d) Esoudie´, J.; Couret, C.; Ranaivonjatovo, H.; Satge´, J. Coord. Chem. ReV. 1994, 94, 427. (e) Okazaki, R.; West, R. AdV. Organomet. Chem. 1996, 39, 31. (f) Klinkhammer, K. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2320. (g) Barrau, J.; Rina, G. Coord. Chem. ReV. 1998, 178, 593. (h) Power, P. P. J. Chem. Soc., Dalton Trans. 1998, 2939. (i) Tokitoh, N.; Matsumoto, T.; Okazaki, R. Bull. Chem. Soc. Jpn. 1999, 72, 1665. (j) Robinson, G. H. Acc. Chem. Res. 1999, 32, 773. (k) Power, P. P. Chem. ReV. 1999, 99, 3463. (l) Leigh, W. J. Pure Appl. Chem. 1999, 71, 453. (m) Tokitoh, N. Pure Appl. Chem. 1999, 71, 495. (n) Okazaki, R.; Tokitoh, N. Acc. Chem. Res. 2000, 33, 625. (2) (a) Withnall, R.; Andrews, L. J. Phys. Chem. 1990, 94, 2351. (b) Norman, N. C. Polyhedron 1993, 12, 2431. (c) Tokitoh, N.; Matsumoto, T.; Okazaki, R. Chem. Lett. 1995, 1087. (d) Lin, C.-L.; Su, M.-D.; Chu, S.-Y. Chem. Phys. 1999, 249, 145; and references therein. (3) (a) Veith, M.; Becker, S.; Huch, V. Angew. Chem., Int. Ed. Engl. 1989, 28, 1237. (b) Veith, M.; Detemple, A.; Huch, V. Chem. Ber. 1991, 124, 1135. (c) Tokitoh, N.; Matsumoto, T.; Manmara, K.; Okazaki, R. J. Am. Chem. Soc. 1993, 115, 8855. (d) Kuchta, C. M.; Parkin, G. J. Chem. Soc., Chem. Commun. 1994, 1351. (e) Khabashesku, V. N.; Boganov, S. E.; Zuev, P. S.; Nefedov, O. M. J. Organomet. Chem. 1991, 402, 161. (f) Lin, C.-L.; Su, M.-D.; Chu, S.-Y. J. Phys. Chem. A 2000, 104, 9250. (4) (a) Kuchta, C. M.; Parkin, G. J. Chem. Soc., Chem. Commun. 1994, 1351. (b) Matsumoto, T.; Tokitoh, N.; Okazaki, R. Angew. Chem., Int. Ed. Engl. 1994, 33, 2316. (c) Foley, S.; Bensimon, C.; Richeson, D. S. J. Am. Chem. Soc. 1997, 119, 10359. (d) Matsumoto, T.; Tokitoh, N.; Okazaki, R. J. Am. Chem. Soc. 1999, 121, 8811. (e) Nowek, A.; Sims, R.; Babinec, P.; Leszczynski, J. J. Phys. Chem. A 1998, 102, 2189. (5) (a) Tokitoh, N.; Matsumoto, T.; Okazaki, R. J. Am. Chem. Soc. 1997, 119, 2337. (b) Matsumoto, T.; Tokitoh, N.; Okazaki, R. Angew. Chem., Int. Ed. Engl. 1994, 33, 2316. (c) Tokitoh, N.; Matsumoto, T.; Manmara, K.; Okazaki, R. J. Am. Chem. Soc. 1993, 115, 8855. (d) Tokitoh, N.; Matsuhashi, Y.; Shibata, K.; Matsumoto, T.; Suzuki, H.; Saito, M.; Manmara, K.; Okazaki, R. Main Group Met. Chem. 1994, 17, 55. (e) Tokitoh, N.; Matsumoto, T.; Ichida, H.; Okazaki, R. Tetrahedron Lett. 1991, 32, 6877. (f) Tokitoh, N.; Saito, M.; Okazaki, R. J. Am. Chem. Soc. 1993, 115, 2065.
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