Organometallics 1996, 14, 5308-5315
5308
SizHz and CSiH2 Isomers as Ligands in High-Valent Transition Metal Complexes' Ralf Stegmann and Gernot Frenking* Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse, 0-35032 Marburg, Germany Received April 14, 1995@
Quantum mechanical ab initio calculations using relativistic effective core potentials for tungsten are reported for isomers of WF4(SizHz) and WF4(CSiH2). The geometries are optimized at the Hartree-Fock level, and the metal-ligand bond energies are calculated at MP2 and CCSD(T) using valence basis sets of DZP quality. Four energy minimum structures are predicted for WF4(Si2H2). The energetically lowest lying isomer is the disilaacetylene complex C4. Structure C4 is 10.1 kcaymol lower in energy than Cla,which has the most stable doubly bridged Si2Hz form as a side-on bonded ligand. Two other isomeric forms Clb and C2 are clearly higher in energy. Three energy minimum structures are predicted for the WF'd(CSiH2) complex. The global energy minimum is the silavinylidene complex C7, which has a very short and strong W-C double bond. The silaacetylene complex C9 is 9.9 kcaVmol higher in energy. The third isomer is the silylidene complex CS,which is 33.2 kcaVmol less stable than C7. The electronic structure of the complexes is analyzed using the NBO partitioning scheme and the topological analysis of the electron density distribution. The results show that the polarization of the W-Si bonds can be quite different among different molecules and that it is very different from related complexes with W-C bonds. The W-Si bonds are more polarized toward the tungsten atom.
Introduction The stunning results of theoretical2and experimental3 research of the silicon analogues of acetylene have demonstrated that standard models of chemical structures are not good guidelines for predicting the most stable isomers of SizHz and CSiHz compound^.^ At the same time the power of modern quantum mechanical ab initio methods was once more documented by the correct predictionzaof the structures of the SizHz global energy minimum form 1 and the next higher-lying isomer 2, which was later confirmed by experimental s t ~ d i e s .The ~ calculated geometries of 1 and 2 (Chart 1) are in excellent agreement with experimental r e ~ u l t s . ~ Two ~ B other low-lying singlet isomers of SizHz (3 and 4), which have not been observed yet, are calculated within 20 kcaymol of the global energy minimum form 1. The linear form HSiSiH (4a) is not a minimum on the potential energy surface.2 Theory has recently also made accurate predictions about the structures and relative energies of CSiHz isomers in the singlet state.5 Three energy minimum forms were calculated at high levels of theory (CCSD(T)/ @Abstractpublished in Advance ACS Abstracts, October 1, 1995. (1) Theoretical Studies of Organometallic Compounds. 15. Part 14: Antes, I.; Frenking, G. Organometallics 1996, 14, 4263. (2)(a) Grev, R. S.; Schaefer, H. F., 111. J. Chem. Phys. 1992, 97, 7990. (b) Colegrove, B. T.; Schaefer, H. F., 111. J.Phys. Chem. 1990, 94, 5593. (c) HUhn, M. M.; Amos, R. D.; Kobayashi, R.; Handy, N. C. J. Chem. Phys. 1993,98, 7107. (d) Curtiss, L. A,; Raghavachari, K.; Deutsch, P. W.; Pople, J. A. J. Chem. Phys. 1991, 95, 2433. (3) (a) Bogey, M.; Bolvin, H.; Cordonnier, M.; Demuynck, C.; Destombes, J. L.; Cslszdr, A. G. J. Chem. Phys. 1994,100,8614. (b) Cordonnier, M.; Bogey, M.; Demuynck, C.; Destombes, J. L. Ibid. 1992, 97, 7984. (c) Bogey, M.; Bolvin, H.; Demuynck, C.; Destombes, J. L. Phys. Reu. Lett. 1991, 66, 413. (4) Reviews: (a)Apeloig, Y. The chemistry offunctional groups: The chemistry of silicon compounds; Patai, S., Rappoport, Z., Eds.;Wiley: New York, 1989; Chapter 2. (b) Schaefer, H. F., 111. Acc. Chem. Res. 1982, 15, 283. (c) Grev, R. S. Adu. Organomet. Chem. 1993,33, 125.
Chart 1 Relevant SizH2 and CSiH2 Structures
1
2
\ Si -Si 4
H/
3
H-Si-
\H
'H
Si -H
4a
9
0
\
H
H
\ :si
H/
-c
Si -H
H-C-
7
9a
TZPP). The global energy minimum structure is the silylidene species 8, which is predicted to be 34 k c d mol lower in energy than trans-bent silaacetylene (9). The energetically high-lying vinylidene isomer 7 is calculated to be 84 kcaYmol less stable than 8.5 The linear form of HCSiH (9a) is a higher-order saddle point on the potential energy s ~ r f a c e . A ~ ,molecule ~ with the formula CSiHz, which is probably the most stable isomer (5) Stegmann, R.; Frenking, G. J. Comput. Chem., in press. (6) (a) Gordon, M. S.; Pople, J. A. J.Am. Chem. SOC.1981 103,2945. (b) Hoffiann, M. R.; Yoshioka, Y.; Schaefer, H. F., 111. J.Am. Chem. SOC.1983,105, 1084. (c) Hopkineon, A. C.; Lien, M. H.; Csizmadia, I. G. Chem. Phys. Lett. 1983,95,232. (d)Luke, B. T.;Pople, J. A.; KroghJespersen, M.-B.; Apeloig, Y.; Kami, M.; Chandrasekhar, J. J.Am. Chem. SOC.1986, 108, 270.
0 1995 American Chemical Society
SizH2 and CSiHz Isomers as Ligands
Organometallics, Vol. 14, No. 11, 1995 5309
complexes containing Si2H2 and CSiHz isomers as 8, has recently been observed in gas-phase experiligands. To the best of our knowledge, no experimental m e n t ~ .There ~ are no other experimental studies of evidence for silaacetylene or silavinylidene complexes CSiH2 species known to US. or substituted analogues has been reported yet. There Unstable molecules or energetically high-lying isois also no theoretical study of the molecules known to meric isomers can be studied in the gas phase or in lowus. We were interested in the changes of the structures temperature matrices. Another way to stabilize tranand the relative energies of the SizHz and CSiH2 isomers sient species is by complexing them as ligands in when they are complexed as ligands. We have chosen transition metal complexes. The metal-ligand interacw F 4 as a complex fragment, because previous studies tions may be sufficiently strong to make it possible to have shown that the calculated geometries of acetylene isolate the complex even under room temperature. Of and vinylidene complexes m4(C2H2), WC4(CzHz),MOF4course, the resulting transition metal complex is only (C2H2), and MoCl4(C2H2) are in excellent agreement formally a stabilized form of the ligand. The metalwith experimental results.16 Although these complexes ligand interactions yield a new bond, and the electronic are usually dimers with two metal-halogen bridges, the structure of the ligand may change considerably upon structure of the metal-CnH2 unit seems to be little complexation. Nevertheless, the geometrical form of the disturbed by the dimerization. Also the corresponding ligand is frequently retained and the structure of the monomeric molecules WFs(C2H2)- and WCldC2Hz)ligand resembles in many cases the isolated species. have metal-CnH2 geometries very similar t o those of Examples of transient species stabilized in transition the neutral dimers.16 It seems therefore reasonable t o metal complexes are carbenes. Except for the Archose wF2(Si2H2)and WFd(CSiH2) as model compounds duengo-type species,8isolated carbenes are only intert o study the geometries and energies of silaacetylene mediates in chemical reactions. While classical carand silavinylidene complexes. We present also an benes can only be studied in the gas phase or in lowanalysis of the metal-ligand interactions using the temperature matrices, transition metal carbene NBO partitioning scheme17and the topological analysis complexes in high and low oxidation states have become a very important class of organometallic c o m p o u n d ~ . ~ J ~ of the electron density distribution.18 Another class of very reactive compounds which can be stabilized as ligands in transition metal complexes Methods are silenes and diselenes. Much progress has been The geometries of t h e molecules have been optimized a t the made in recent years in the synthesis and characterizaHartree-Fock (HF) level of theory using a relativistic effective tion of silenell and disilene12 complexes. Theoretical core potential (ECP) for tungsten, which replaces 60 core studies have been very helpful in the understanding of e 1 e ~ t r o n s . l The ~ outermost of 5s a n d 5p electrons a r e treated the bonding situation in these compounds.13 The theoexplicitely, and they are not included in the core. The (55/5/ retical studies of silene and disilene complexes have 3) valence basis set for t h e 5sz5p66sz5d4electrons of tungsten is contracted to [441/2111/21]. This valence basis set has been a posteriori; Le., the theoretical studies were double-5;plus p polarization functions quality. An all-electron performed after the complexes were synthesized and the basis set of 6-31G(d,p) quality has been used for C, Si, F, a n d experimental geometries were known. A priori calculaH. The d-type polarization functions consist of a set of five tions of molecular structures are common for molecules spherical primitives. This basis set combination is our standof first- and second-row elements but not yet for transiard basis set II.15 The stationary points on t h e potential tion metal compounds. It is still common saga that energy surface were characterized by calculating the vibraaccurate quantum mechanical calculations of heavytional frequencies at HF/II using numerical second derivatives atom molecules are not feasable. Systematic theoretical of the energy. Improved total energies have been calculated studies using density functional methods14and effective at t h e MP2 level (Mgller-Plesset perturbation theory termicore potentials ( E C P P have shown that this is not true. nated at second order)z0 and using coupled cluster theoryz1 with singles and doubles and a noniterative estimate of the Electronically saturated transition metal compounds triples CCSD(T)zzin conjunction with the standard basis set can be calculated with an accuracy comparable to that III.15 The latter basis set is basis set I1 with a n additional set for main group compounds. of f-type polarization functions a t tungsten (exponent af= In this paper we predict theoretically the equilibrium 0.823).23 The correlation energy was calculated with t h e structures and the relative energies of transition metal (7) Srinivas, R.; Sulzle, D.; Schwarz, H. J . Am. Chem. SOC.1991, 113,52. (8)Arduengo, A. J.; Harlow, R. L.; Kline, M. J . Am. Chem. SOC. 1991, 113,361. (9) Dotz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, U.; Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie: Weinheim, Germany, 1983. (10) Nugent, W. A,; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley: New York, 1988. (11)(a) Campion, B. K.; Heyn, R. H.; Tilley, T. D. J . Am. Chem. SOC.1988,110, 7558. (b) Campion, B. K.; Heyn, R. H.; Tilley, T. D. J . Am. Chem. SOC. 1990,112,4079. (12) (a) Pham, E. R ; West, R. J . Am. Chem. SOC.1989,111, 7667. (b) Pham, E. K.; West, R. Organometallics 1990,9, 1517. (c) Berry, D. H.; Chey, J.; Zipin, H. S.; Carroll, P. J. Polyhedron l991,10,1189. (13) (a) Anderson, A. B.; Shiller, P.; Zarate, E. A.; Tessier-Youngs, C. A,; Youngs, W. J. Organometallics 1989,8,2320. (b) Sakaki, S.; Ieki, M. Inorg. Chem. 1991,30,4218. (14) Ziegler, T. Chem. Reu. 1991,91,651. (15)Frenking, G.; Antes, I.; Bohme, M.; Dapprich, S.; Ehlers, A. W., Jonas, V.; Neuhaus, A,; Otto, M.; Stegmann, R.; Veldkamp, A.; Vyboishchikov, S. F. Reviews i n Computational Chemistry: Lipkowitz, K. B., Boyd, D. B., Eds.; VCH: New York, 1995; Vol. 7, in press.
frozen-core approximation. Unless otherwise noted, energies are discussed a t the CCSD(T)/III level using HFAI optimized geometries. The geometries have been optimized using the program T u r b o m ~ l e .The ~ ~ energy calculations a t the correlated levels were calculated using t h e program ACES ILZ5 For the
(16) Stegmann, R.; Neuhaus, A.; Frenking, G. J . Am. Chem. SOC. 1993,115,11930. (17)Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988,88, 899. (18)Bader, R. F. W. Atoms i n Molecules: A Quantum Theory; Oxford University Press: Oxford, U.K., 1990. (19)Hay, P. J.; Wadt, W. R. J . Chem. Phys. 1985,82,299. (20)(a) M~ller,C.; Plesset, M. S. Phys. Reu. 1934,46, 618. (b) Binkley, J. S.; Pople, J . A. Intern. J . Quantum Chem. 1975,9S, 229. (21) Cizek, J. J . Chem. Phys. 1966,45,4256. (22) (a)Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. Chem. Phys. Lett. 1989,157,479. (b) Bartlett, R. J.;Watts, J. D.; Kucharski, S. A,; Noga, J. Ibid. 1990,165, 513. (23) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth, A,; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993,208,111.
Stegmann and Frenking
5310 Organometallics, Vol. 14,No. 11, 1995 F3
Fs
1 5
Si I
C6
5 517
Cla
Clb
@
Fz I
Si
5
1
2
1
4
s11
F3 F3
c2
c3
5
4
3
SI 2
c4
51 1
4a
c5
6
12
Cl
5
a
Figure 1. Optimized geometries (HFLI)of WF4(Si2HZ)complexes Cla-C4, W F 4 C2H2)complexes C5 and C6, Si2H2 isomers 1-4a7 and C2H2 isomers 5 and 6. The calculated bond distances are given in , and bond angles, in deg. The geometry of 9 was optimized at MP2/II.
calculation of the electron density distribution p(r), the gradient vector field vp(r), and its associated Laplacian v2p(r) the programs EXTREME, GRID, and GRDVEC were used.26 The NBO analysis17was carried out with the subroutine available in Gaussian 92.27 WF4(Si2H2) Complexes Figure 1shows the optimized structures and the most (24) (a) Haser, M.; Ahlrichs, R. J . Comput. Chem. 1989, 10, 104. (b)Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C . Chem. Phys. Lett. 1989, 162, 165. (c) Horn, H.; Weiss, H.; Haser, M.; Ehrig, M.; Ahlrichs, R. J. Comput. Chem. 1991,12,1058. (d) Haser, M.; Almlof, J.; Feyereisen, M. W. Theor. Chim. Acta 1991, 79, 115. (25)ACES 11, an ab initio program system written by J. F. Stanton, J. Gauss, J. D. Watts, W. J. Lauderdale, and R. J. Bartlett, University of Florida, Gainesville, FL, 1991. (26) Biegler-Konig, F. W.; Bader, R. F. W.; Ting-Hua, T. J . Comput. Chem. 1982,3, 317. (27) Gaussian 92, Revision C. Frisch, M. J.; Trucks, G. W.; HeadGordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1992.
important bond lengths of the WF4(Si2H2) complexes Cla-C4 predicted at HF/II. Previous theoretical studies have shown that the geometries of transition metal complexes in high oxidation states optimized at HF/II are in very good agreement with experimental values.15J6,2*The HF/II optimized structure of the acetylene complex WFXHCCH) ((35) is also shown in Figure 1. The predicted W-C bond length (1.994 A) is in very good agreement with experimental values of W(V1)alkyne complexes, which are typically 1.98-2.04 The theoretical geometries of the Si2H2 structures 1-4a optimized at HF/II are also shown in Figure 1.A comparison of the calculated Si-Si bond lengths with previously reported2bCISD/TZ2P values shows that the HF/II values are slightly (-0.02-0.03 A) shorter than H1.16p29
(28) (a)Jonas, V.; Frenking, G.; Reetz, M. T. Organometallics 1993, 12, 2111. (b) Veldkamp, A.; Frenking, G. J . Am. Chem. SOC.1994, 116,4937. (c) Neuhaus, A.; Veldkamp, A.; Frenking, G. Inorg. Chem. 1994,33, 5278. (29) (a) Kersting, M.; ELKohli, A.; Muller, U.; Dehnicke, K. Chem. Ber. 1989,122,279. (b) Pauls, I.; Dehnicke, K.; Fenske, D. Chem. Ber. 1989,122,481. (c) Pauls, I. Dissertation, Universitat Marburg, 1990.
Si.&
and CSiHz Isomers as Ligands
Organometallics, Vol. 14, No. 11, 1995 5311
Table 1. Calculated Total Energies Etot (au) Relative Energies E,, (kcal mol-'), Zero-Point Vibrational Enemies ZPE (kcal mol-'), and Number of Imaginary Frequencies i molecule WFlSiHzSi (side-on) WF4SiH2Si (end-on) WFdSiHSiH WF4Si2H2 WF4HSiSiH
WF~HCCH WF4CCHz WF4CSiHZ WF4SiCH2 WF4HCSiH SiHzSi (butterfly) HSiHSi SiSiH2 HSiSiH (trans) HSiSiH (linear) HCCH CCHz CSiH2 SiCH2 HCSiH (trans) HCSiH a
no. Cla Clb c2 c3 c4 c5 C6 c7
cs c9 1 2 3 4 4a 5 6 7 8 9 9a
Et&.
HF/II E1 ,
-1043.91959 -1043.893 84 -1043.913 46 -1043.902 16 -1043.92298 -541.885 50 -541.86533 -792.904 57 -792.86095 -792.881 23 -578.889 75 -578.868 92 -578.883 61 -578.851 36 -578.825 15 -76.821 70 -76.767 02 -327.723 63 -327.858 67
2.1 18.3 6.0 13.1 0.0 0.0 12.7 0.0 27.4 14.6 0.0 13.1 3.9 24.1 40.5 0.0 34.3 84.7 0.0
-327.769 36
56.0
MP2AII
i
ZPE
0 0 0 1 0 0 0 0 0 0 0 0 0 0 2 0 0 0 0 0" 2
17.9 16.5 16.4 17.6 18.2 26.6 26.0 20.6 22.5 22.1 10.2
9.0 10.1 8.5 9.1 18.4 16.3 11.0 14.9 12.W 12.9
CCSD(T)/III
-1045.164 -1045.108 -1045.140 -1045.148 -1045.201 -543.239 -543.213 -794.228 -794.135 -794.206
70 47 59 50 89 13 71 23 67 73
23.3 58.5 38.5 33.5 0.0 0.0 16.0 0.0 58.1 13.5
-1045.224 -1045.179 n.c.b -1045.220 -1045.240 -543.270 -543.254 -794.255 -794.202 -794.239
-579.061 -579.045 -579.042 -579.036 -579.002 -77.076 -76.997 -327.912 -328.069 -328.009 -328.003
52 94 86 88 61 74 33 67 16
0.0 9.8 11.7 15.5 37.0 0.0 49.8 98.2 0.0 37.2 41.4
-579.103 -579.087 -579.088 -579.080 -579.041 -77.104 -77.036 -327.961 -328.106 -328.045 -328.033
80" 13
43 30
10.1 (9.8) 38.5 (36.8)
26 60 36 60 08 25 35
12.8 (12.2) 0.0 (0.0) 0.0 (0.0) 9.9 (9.3) 0.0 (0.0) 33.2 (35.1) 9.9 (11.4)
39 38 37 80 76 44 62 87 32 27'
0.0 (0.0) 10.0 (8.8) 9.4 (9.3) 14.2 (12.5) 38.7 (37.6) 0.0 (0.0) 42.6 (40.5) 90.6 (86.7) 0.0 (0.0) 38.3 (36.2) 45.5 (43.5)
88
Values in parentheses include ZPE corrections. Not calculated for technical reasons (single file size > 2GB). Optimized a t MP2AI.
the bond lengths predicted a t the correlated level. These differences are probably not very important, because the relative stabilities of the SizHz isomers calculated a t CCSD(T)/III using the HFAI optimized geometries are nearly the same as those which are calculated at CISD/TZ2P//CISD/TZ2P.2bThe calculated energies of 1-4a and the complexes Cla-C4 are shown in Table 1. The doubly-bridged SizHz global energy minimum 1 forms with WF4 the side-on WF4(SizHz) complex C l a and the end-on complex C l b . Both structures are minima on the potential energy surface a t HFAI ( i = 0). The end-on isomer C l b is clearly higher in energy than C l a . Isomer C l a is calculated at CCSD(T)/IIIto be 28.4 kcdmol more stable than Clb. The Si-Si bond of C l a is significantly lon er (2.432 A) than in the separated ligand 1 (2.180 if), while it is even slightly shorter in C l b (2.145 A) than in 1 (Figure 1). This is because different orbitals of the SizHz ligands are involved in the formation of the tungsten-silicon bonds of C l a and Clb. The nature of the metal-ligand bonds in the complexes is discussed below. Geometry optimizations of a side-on bonded WF4(SizHz)complex of next higher lying SizHz isomer 2 did not lead to an energy minimum structure. A geometry optimization of end-on bonded 2 gave the molecule C2 shown in Figure 1. However, the structure of the SizHz ligand of C2 does not correspond to 2. Rather, it has a twisted form which indicates a disylilene structure. Such a structure is an energy minimum form of SizH2 ~ ~higher , ~ ~ levels of only at a low level of t h e ~ r y . At theory the C2-symmetric twisted form collapses to the dibridged global minimum 1. It is not uncommon that species which are not a minimum on the energy hypersurface may become stable molecules upon complexation by a transition metal. A recent example is cyclopropyne. (30) (a) Lischka, H.; Ktihler, H. J.Am. Chem. SOC.1983,105,6646. (b) Binkley, J. S. J. Am. Chem. Soc. 1984, 106, 603. (c) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y.; Kami, M.; Chandrasekhar, J.; Schleyer, P.v.R. J. Am. Chem. SOC.1986, 108, 270.
Singlet cyclopropyne is a transition state,31but it forms stable complexes such as W F ~ ( C - C ~ H ~ ) . ~ ~ The structure of C2 is interesting because it is an example of a silylene complex, the silicon analogue of a carbene complex. Transition metal silylene complexes have been discussed for a long but only recently have the first stable silylene complexes been isolated.34 There is no experimental geometry of a tungsten silylene complex known t o us. The calculated W-Si distance of C2 (2.348 A) is slightly shorter than experimental values for typical W-Si single bonds. The W-Si bond of a W(W) trimethylsilyl complex has been reported with a bond length m-si = 2.388 A.35a The W-Si bond length of a W(0) complex is longer (2.53-2.56 A recent study of transition metal carbene and carbyne complexes in high and low oxidation states has shown that the calculated metal-carbon distances are in excellent agreement with experimental values.36 The same can be assumed for the calculated and silylene complexes. "he complex C2 has a second silylene center. It is conceivable that a second WF4 fragment can bind to silicon and that a bidsilylene) complex is formed. The geometry optimization of the end-on complex of disilavinylidene (3) with WF4 gave the structure C3 shown in Figure 1. However, C3 is not a minimum a t HFAI on the potential energy surface ( i = 1). Also (31) (a) Fitzgerald, G.; Schaefer, H. F., 111. Isr. J. Chem. 1983, 23, 93. (b) Saxe, P.; Schaefer, H. F., 111. J. Am. Chem. SOC.1980, 102, 3239. (32) Stegmann, R.; Frenking, G. Manuscript in preparation. (33) (a)Tilley, T. D. Comments Inorg. Chem. l g S O , l O , 37. (b)Zybill, C. Top. Curr. Chem. 1992, 160, 1. (34) (a) Zybill, C.; Muller, G. Angew. Chem., Int. Ed. EngE. 1987, 26,669. (b) Straus, D. A.; Tilley, T. D.; Rheingold, A. L.; Geib, S. J. J. Am. Chem. SOC.1987,109, 5872. (c) Straus, D. A,; Grumbine, S. D.; Tilley, T. D. J.Am. Chem. SOC.1990,112, 7801. (d) Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheinhold, A. L. J.Am. Chem. SOC.1993, 115,7884. (e) Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheinhold, A. L. J.Am. Chem. SOC.1994,116, 5495. (35) (a) Barron, A. R.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J. Chem. SOC.,Dalton Trans. 1987, 837. (b) Schmitzer, S.; Weis, U.; Kab, H.; Buchner, W.; Malisch, W.; Polzer, T.; Posset, U.; Kiefer, W. Inorg. Chem. 1993,32, 303. (36)Vyboishchikov, S. F.; Frenking, G. Manuscript in preparation.
5312 Organometallics, Vol.'14,No.11, 1995
Stegmann and Frenking
Table 2. Calculated Reaction Energies (kcal molF1)at CCSD(T)/III/HF/IIh 1 1 1
c4
4
-28.2 -56.5 -52.3 -3.8
c7
7 8 9
79.9 (78.5) -43.9 (-43.3) 17.7 (16.6)
C8
I 5
(a) IRC step 25
Cla Clb c2
c9
(b) IRC step 50
(-27.7) (-54.6) (-50.3) (-5.3)
-60.8 (-58.8) -89.2 (-85.8) -86.2 (-82.7) -36.5 (-36.5) 42.2 (42.3) -76.6 (-74.5) -15.0 (-14.6)
a Values in parentheses include ZPE corrections. MP2/III/HF/ I1 for C2.
F3 6 16
(c) IRC Step 75
(d) IRC step 100
c7
C8
c9
7
Fs F
c10 (e) bent structure
( f ) quasi-linear svucture
Figure 2. Calculated structures at HF/II along the intrinsic reaction coordinate (IRC)leading from the transition state C3 to the silyne complex ClO: (a) after 25 IRC steps; (b) after 50 IRC steps; (c) after 75 IRC steps; (d) after 100 IRC steps; (e) bent structure ClOa (minimum at HF/ 11); (f) HF/II optimized structure C10.
geometry optimizations of a W F 4 complex with side-on bonded 3 did not give an energy minimum structure. We searched for the stationary points to which the transition state C3 is connected to. This was done by calculating the intrinsic reaction coordinate (IRC)37 starting from C3. The results are very interesting. Figure 2 shows the structures along the reaction coordinate calculated after 25, 50, 75, and 100 steps using default values for the IRC calculations as used in Gaussian 92.27 The reaction is a 1,3 fluorine rearrangement from tungsten to the terminal silicon atom, leading to the tungsten silyne complex C10 (Figure 2). The W-Si bond of C10, which is formally a tungstensilicon triple bond, is very short (2.158 A). The W-SiSi angle is nearly linear (177.5'). To the best of our knowledge there are no silyne complexes known. The structures and bond energies of transition metal silene and silyne complexes are subjects of a present study by us. The structure C10 is 20.0 kcaVmol (MP2AII) lower in energy than C4. There is also a bent tungsten silyne comples ClOa predicted as an energy minimum structure at HF/II (Figure 2). This may be an artifact of the HF calculations. The bent form ClOa is 25.0 kcaVmol (MP20II) higher in enery than C10. The energetically lowest lying W F 4 ( Si2H2) complex calculated in our study is the structure C4 (Figure 1). This is remarkable, because the correspondingstructure (37) (a)Fukui, K. Acc. Chem. Res. 1981,14,363. (b) Gonzalez, C.; Schlegel, H. B. J . Chem. Phys. 1991,95,5853.
Si 1
9
3: J
8
9a
Figure 3. Optimized geometries (HFDI) of WF4(CSiH2)
complexes C7-C9 and CSiH2 isomers 7-9a. The calculated bond distances are given in A,and bond angles, in deg. of the Si2H2 ligand (4) is the least stable isomer investigated here. The complex C4 is predicted to be 10.1 kcaVmo1 lower in energy than C l a (Table 1). The geometry of the Si2H2 ligand of C4 is very similar to a classical acetylene complex (see structure C5 in Figure 1). The structure and bonding properties of C5 have been reported in a previous paper.16 The isolated Si2H2 ligand with the connectivity HSiSiH has a trans-bent geometry as shown in Figure 1(structure 4). The SiSi bond of the complex C4 is slightly shorter (2.061 A) than in 4 (2.084 A). Also the bending angle Si-Si-H of C4 is larger (147.5') than in 4 (127.4'). The W-Si bonds of C4 are clearly shorter (2.460 A)than those of Cla. This indicates the electronicallyless stable nature of 4 compare with 1, which is the reason why the silicon atoms of the former isomer have shorter and stronger W-Si bonds in C4 than the latter isomer in the complex Cla.
Organometallics, Vol. 14, No. 11, 1995 5313
S i a 2 and CSiH2 Isomers as Ligands
Table 3. Results of the NBO Analysisa complex
q(W)
Cla
2.14
0.53
q(Si)
Clb
2.30
c2
2.21
0.40 0.44 (term) 0.30 0.56 (term) 0.32
%6s(W)
% 6p(W)
% 5d(W)
%s(C/Si)
% p(C/Si)
BO
64.6 64.6 28.2
12.5 12.5 22.5
3.8 3.8 1.4
83.6 83.6 76.1
7.5 7.5 46.1
90.7 90.7 53.8
0.73 0.73 0.91
47.4 65.7 50.7 50.7 36.0 36.0 26.8 60.6 30.4 34.1 29.4 85.6 46.2 32.6 27.7
22.8 10.2 13.5 13.5 12.6 12.6 21.0
17.0 20.0 3.4 3.4 0.3 0.3 0.1 0.6 0.0 25.7 1.2 33.6 2.1 0.3 1.6
60.2 69.9 83.2 83.2 87.1 87.1 79.0 99.4 70.4 74.3 57.1 66.4 81.8 87.0 98.4
24.4 10.8 28.1 28.1 24.7 24.7 50.3 0.0 46.9 0.0 48.5 0.0 33.3 25.8 0.0
74.4 87.5 70.7 70.7 75.1 75.1 49.7 99.8 53.1 99.9 51.4 96.7 66.0 74.1 99.7
1.42
ow-Si n W-Si
2.12
c5
2.83
-0.42
C6
2.75
c7
2.80
1.51
-0.45 -0.27 (term) -1.37
C8
2.25
1.01
-1.21
2.52
%W
ow-Si7 ow-Si
c4
c9
bond oW-Si6
q(C)
0.98
-1.13
uW-Si6 o W-Si7 u W-C2 u W-C3 ow-C n W-C ow-C RW-C ow-Si nW-Si ow-Si OW-C nW-C
a Partial atomic charges q and analysis of the W-Si"-C index in NAO basis BO.
-
+
WF,(Si2H2)
+ CCH, 6
+ Si,H,
+hER
(1)
+
+hER
(2)
c5
WF4(CCH2) Si2H2 C6
0.0 16.1 12.7 0.0
2.06 1.16 1.01 1.21
bonds. Polarity given by % W, hybridization of W and C/Si, Wiberg bond
We tried to find other minima for complexes of W F 4 with Si2H2. None was found. Although we cannot exclude that other structures may exist, we believe that the structures Cla-C4 are probably the most stable WF4(Si2H2) isomers. In order to estimate the F4W-SizH2 bond strengths in Cla-C4 we calculated the energies of the isostructural reactions 1 and 2. WF4(Si2H2) HCCH 5 WF,(HCCH)
0.0 29.6 0.0 41.7
1.13 1.13 0.97 0.97 1.67
The calculated reaction energies of reactions 1 and 2 for the energy minimum complexes Cla-C4 are shown in Table 2. All calculated reaction energies are negative. This means that the respective SizH2 isomer is more weakly bound t o WF4 than acetylene in C5 and vinylidene in C6. The WF4-SizH2 bond strength of the most stable SizHz complex C4, however, is nearly as large as the tungsten-acetylene bonding in C5. The reaction energy is only slightly exothermic by -3.8 kcaymol (-5.3 kcaymol after ZPE corrections, Table 2).
WFdCSiHd Complexes Figure 3 shows the optimized geometries of the W F 4 (CSiH2)complexes C7-C9 and the correspondingCSiHz isomers 7-9. The calculated energies are shown in Table 1. The energetically lowest-lying complex is the silavinylidene structure C7 (Figure 3). The complex C7 is 33.2 kcaymol lower in energy than the silylidene complex C8 (Table 1). The energy ordering of the CSiHz complexes C7 and C8 is a dramatic reversal of the stabilities of the separate CSiH2 ligands. The silavinylidene isomer 7 is 90.6 kcal/mol (!) higher in energy than 8 (Table 1). The carbon atom of 7 is much more reactive than the silicon atom of 8. The electronically unsatu-
rated state of the carbon atom of 8 is the reason for the very short W-C bond of C7 (1.775 A). The W-C bond of C7 is significantly shorter than the W-C bond of the vinylidene complex C6 (1.839 A), which is shown in Figure 1. It is interesting to note that the C-Si bond of the free CSiHz ligand 7 is longer (1.754 A) than in the complex (1.702 A). The same holds true for the Si-C bond of SiCH2. The Si-C bond length of the complex C8 is shorter (1.680 A) than that of the free ligand 8 (1.700 A). The silylidene complexe C8 is a minimum on the potential energy surface, unlike the disilavinylidene complex C3 which is a transition state. The CSiHz ligand with the connectivity HCSiH gives the complex C9 (Figure 3). This complex is 9.9 kcall mol higher in energy than C7 (Table 1). The HCSiH ligand has a cis-bent geometry in C9. The separated HCSiH ligand 9 has a trans-bent geometry (Figure 3). Structure 9 can only be calculated at a correlated level of theory; at the Hartree-Fock level it is not a minimum on the potential energy surface.38 Geometry optimization of 9 at CCSD(T)/TZBP gives a shallow minimum with an energy barrier of 5.1 kcal/mol for rearrangement to 7.5 The silaacetylene form 9 is 52.3 kcaymol more stable than 7, but the complex C9 is 9.9 kcaymol less stable than C7. It should be noted that the W-Si bond of C9 is clearly shorter (2.391 A) than the W-Si bond at C4, while the W-C bond is slightly longer (2.026 A) than the W-C bond of C5 (1.994 A). We calculated the energies of the isostructural reactions 3 and 4 for the different complexes C7-C9. The results are shown in Table 2.
-
+
WF4(CSiH2) HCCH 5 WF,(HCCH) c5 WF,(CSiH,)
+ CSiH,
+hER (3)
+ CSiH,
+hER
+ CCH, 6
WF,(CCHZ) C6
(4)
The CSiHz ligand is much stronger bound in C7 than acetylene or vinylidene in C5 and C6, respectively. The calculations predict that the W-C bond of the sila-
Stegmann and Frenking
5314 Organometallics, Vol. 14, No. 11, 1995
Table 4. Results of the Topological Analysis of the Electron Density Distribution: Values of the W-C and W-Si Bonds” complex bond Qb v2@b Hb n Cla Clb c2 c4 c5
W-Si6 W-Si7 W-Si W-Si W-Si6 W-SiT W-Cz W-C3
C6 c7
w-c w-c
c9
W-Si W-Si
cs
w-c
0.38 0.38 0.56 0.74 0.66 0.66 1.05 1.05 1.32
-0.42 -0.42 1.53 -2.13 -2.41 -2.41
-0.31 -0.31 -0.20 -0.40 -0.33 -0.33
1.55 1.55 1.19 1.25 1.34 1.34
1.77 1.77 8.63
-0.56 -0.56 -0.85
1.07 1.07 0.97
1.55 0.63 0.64 1.06
4.72 1.09 -1.49 1.22
-1.19 -0.26 -0.31 -0.56
0.97 1.18 1.29 1.07
Charge density at the bond critical point @b (e/A3);Laplacian at the bond critical point @Qb (e/A5); energy density at the bond critical point Hb (hartree/A3); location of the bond critical point n given by the distance from the tungsten atom (A).
vinylidene complex C7 is 42.2 kcal/mol stronger than the W-C bond of the vinylidene complex C6 (Table 2). This is in agreement with the very short tungstencarbon bond of C7. The silylidene complex C8, however, has a significantly weaker metal ligand (W-Si) bond than the acetylene and vinylidene complexes C5 and C6. The silaacetylene ligand of the complex C9 is rather strongly bound; it is 17.7 kcdmol stronger bound that the acetylene ligand in C5 (Table 2).
Electronic Structure Analysis In order t o investigate the metal-ligand bonding in the complexes we analyzed the electronic structure of Cla-C9 using the natural bond orbital (NBO) partitioning scheme17 and the topological analysis of the electron density distribution.18 The results of the NBO analysis are shown in Table 3. The results of the topological analysis are shown in Table 4. The contour line diagrams of the Laplacian distribution of Cla-C9 are shown in Figure 4. The NBO analysis shows that the W-Si bonds of C l a are polarized toward the tungsten end. A 64.6% amount of the W-Si bond orbital belongs to tungsten (Table 3). The hybridization at W has mainly d character (83.6%), while the W-Si bond a t Si has mainly p character (90.7%). The Wiberg bond order for the W-Si bond of C l a is only 0.73. The polarization of the W-Si bond toward tungsten is also obvious from the topological analysis. The ring critical point n is 1.55A away from the tungsten atom, which is more than half of the W-Si bond length (Table 4). This means that a larger part of the W-Si bond belongs to the tungsten atom than t o silicon. The W-Si bond of the end-on complex C l b is clearly different from C l a . The NBO analysis shows that the W-Si bond is strongly polarized toward the silicon atom; only 28.2%of the bond is at tungsten (Table 3). (38) One reviewer pointed out that the failure of the HF method to predict 9 as an energy minimum structure could indicate that the present approach is not sufficient for calculating the geometries of the complexes. This might be true for those isomers of WF+Hz) and WF4(CSiH2) which are shallow minima on the potential energy hypersurface. We think, however, that the structures discussed here are the most important isomers of the WFI complexes with the ligands SiZH2 and CSiH2.
I
c9
Figure 4. Contour line diagrams of the Laplacian distribution v2p(r)of the complexes Cla-CQ. Dashed lines indicate charge depletion (v2p(r)> 0), and solid lines indicate charge concentration (v2p(r)< 0). The solid lines connecting the atomic nuclei axe the bond paths, and the solid lines separating the atomic nuclei indicate the zeroflux surface in the plane. The crossing points of the bond paths and zero-flux surfaces are the bond critical points rb.
Organometallics, Vol. 14,No.11, 1995 5315
SizH2 and CSiHz Isomers as Ligands The hybridization at Si has 46.1% s character, which means that it is sp hybridized. The Laplacian distribution (Figure 4)and the position of the bond critical point n,which is closer to the Si end, demonstrate also that the W-Si bond of C l b is significantly different from the W-Si bonds of C l a . The bond order of the former is slightly higher (0.91)than the bond order of the latter bonds (0.73). The NBO analysis indicates a W-Si double bond for the silylene complex C2. Because the complex has C I symmetry, there are no clear u and n bonds. One bond is nearly unpolar, while the other is polarized toward tungsten (65.7% at W). The accumulated bond order for the double bond is 1.42.The NBOs for the two W-Si bonds of C4 are less polarized than those of C l a (Table 3). The NBO analysis indicates that the bonds are nearly unpolar (50.7% at W). The hybridization at Si is approximately sp3. The main difference compared to the W-C bonds of the related acetylene complex C5 is the polarization. The W-Si bonds are clearly less polarized than the W-C bonds. The W-C bonds of C5 have a 36% contribution from the tungsten atom; the W-Si bonds of C4 have 50.7%. The bonding situation of the WF4(CSiHd complexes is very interesting. The NBO method gives two metalligand bonds, a r-7 and a n bond for the silavinylidene complex C7 and the silylidene complex C8. The u and the n W-C bonds of C7 are polarized toward the carbon end (30.4and 34.1% a t W, respectively). A comparison with the vinylidene complex C6 shows that the n bond of the latter is much more polarized toward the tungsten end. This is an important result for the reactivity of the two complexes. An electrophilic attack upon the W-C n bond of C6 should occur at the tungsten end, while nucleophilic attack should be preferred at the carbon end. The opposite regioselectivity should be expected for the W-C bond of C7. The W-Si u bond of C8 has nearly the same polarity as the W-C u bond of C7 (29.4and 30.4%, Table 3). The n components of the two bonds are very different,
however. The W-Si n bond of C8 is strongly polarized toward tungsten (85.6% a t W), while the W-C n bond of C7 is polarized toward the carbon end (34.1% at W). The NBO analysis gives three metal-ligand bonds for the silaacetylene complex C9. The W-C bond has u and n components, which are both clearly polarized toward carbon (Table 3). This means that C9 can be considered as an alkylidene complex with an additional W-Si bond. The calculated charge distribution predicted by the NBO scheme indicates that the tungsten atom carries always a positive charge between +2 and f3. \
summary
The energetically lowest lying WF4(Si2H2) complex is the disilaacetylene complex C4. The compound C4 is 10.1 kcaVmol lower in energy than the isomeric form C l a , which has the most stable doubly bridged Si2H2 form as a ligand. Two other isomeric forms C l b and C2 are clearly higher in energy. Three energy minimum structures are predicted for the WF4(CSiH2) complex. The global energy minimum is the silavinylidene complex C7, which has a very short and strong W-C double bond. The silaacetylene complex C9 is 9.9 kcaVmol higher in energy. The third isomer is the silylidene complex C8, whic his 33.2kcdmol less stable than C7. The analysis of the electronic structure of the complexes shows that the polarization of the W-Si bonds is clearly different compared to related complexes with W-C bonds. The W-Si bonds are more polarized toward the tungsten atom.
Acknowledgment. This work has been supported by the Deutsche Forschungsgemeinschaft (Grant SFB 260-D19)and the Fonds der Chemischen Industrie. We acknowledge generous support and excellent service by the computer centers HRZ Marburg, HHLRZ Darmstadt, and HLRZ Julich. OM950273G
