Theoretical Studies of Organometallic Compounds. XIX. Complexes of

Organometallics 1996, 14, 5325-5336

5325

Complexes of Transition Metals in High and Low Oxidation States with Side-On-Bondedmligandsl Ulrich Pidun and Gernot Frenking* Fachbereich Chemie, Philipps- Universitat Marburg, Hans-Meerwein-Strasse, 0-35032 Marburg, Germany Received August 7, 1995@ The equilibrium geometries of the transition metal compounds WC4L and W(C0)bL (L = HCCH, C2H4, C02, CSZ,CH20) have been calculated at the HF and MP2 levels of theory by using a relativistic effective core potential for tungsten and valence shell basis sets of DZ+P quality. The W-L bond dissociation energies are predicted at CCSD(T). The calculations show that the inclusion of correlation energy is essential for the accurate description of transition metal donor-acceptor complexes. The theoretically determined geometries at the MP2 level are in very good agreement with experimental results. The calculated = 10.2 kcal mol-l) is in accord with the experimental estimate (C0)5W-C02 bond strength (DO (DO= 8.2 f 1.0 kcal mol-l). The calculations predict that the metal-ligand bond strength has the order for W(C0kL of L = C2H4 > HCCH > CH2O > CS2 > C02. For the WCLL complexes, the order is L = HCCH > CHpO > CZ& > CS2 > C02. The different sequences of W-L bond strengths are explained by the nature of the metal-ligand interactions in the two sets of compounds. The W-L bonds of WC4L are covalent bonds, while the W(C0)bL complexes have donor-acceptor bonds. Ethylene is a better donor than acetylene and formaldehyde and, therefore, forms a stronger bond in the W(C0)bL complexes. The WC4L compounds with L = HCCH, C2&, CH2O should be considered as metallacycles. Since the carbon atoms of the metallacyclopropenes are approximately sp2hybridized, while the carbon atoms of the metallacyclopropanes are sp3 hybridized, the former W-L bonds are stronger than the latter. In general, the W-L bond dissociation energies are clearly smaller for the WC4L complexes than for W(COkL, although the metal-ligand bonds are much shorter in the former compounds. This puzzling result can easily be explained by the high excitation energy that is necessary to promote the ligand into the triplet state prior to formation of the strong covalent bond. The classification of the WC4L compounds as covalently bonded molecules and the W(C0)sL complexes as donor-acceptor species is supported by the examination of the electronic structure using topological analysis of the electron density distribution, covalent bond orders, and charge decomposition analysis (CDA). The CDA method is found to be particularly useful for the analysis of the metal-ligand interactions. For the W(C0)sL complexes, reasonable values are calculated for the L W donation, the W L back-donation, and the W L repulsive polarization by using closed-shell fragments W(C0)s and L. The CDA results for WC4L using closed-shell fragments WC14 and L are physically unreasonable, which indicates that the Dewar-Chatt-Duncanson model for these compounds is not appropriate.

-

-

1. Introduction The bonding in transition metal complexes with sideon-coordinated n-ligands is usually described by the Dewar-Chatt-Duncanson m ~ d e l . ~The a model considers the bonding to arise from components* In the first part, donation from a filled n-orbital of the ligand into a suitably directed vacant metal orbital forms the electron pair donor bond. This bond has a-symmetry with regard t o the metalligand axis. The bonding is reinforced by the second component, which derives from the msymmetrical overlap of a filled metal d-orbital with the vacant antibonding n*-orbital of the ligand (Figure 1). Due to the @

Abstract published in Advance ACS Abstracts, October 15, 1995.

(1)Theoretical Studies of Organometallic Compounds. 17. Part 16: Jones, V.; Frenking, G.; Reetz, M. T. Organometallics 1995,14, 5316.

(2)Dewar, M. J. S. Bull. SOC.Chim. Fr. 1961, 18,C71. (3) Chatt, J.; Duncanson, L. A. J . Chem. SOC.1953,2939.

-

flexible interplay of these two components, the DewarChatt-Duncanson (DCD) model allows one to explain a number of experimentalobservations; in the theory convincinglyinterprets the orientation of the n-ligand with respect to the remaining and the observed lengthening ofthe intr&gand bond length. However, the DCD model is not suited to quantifythe distortion of the ligand in the complex or to predict details of the different bonding properties of different ligands. Also, the model does not distinguish between a n-bonded complex and a metallacycle as an alternative description of the theemmemberedcyclic structure. The latter bonding to be more for high-valent transition metal complexes. The Dewar-Chatt-Duncanson model has been supported by a number of semiempirical molecular orbital studies in the framework of extended Huckel the01-y.~ The aims of the present work are to investigate the

0276-733319512314-5325$09.00/00 1995 American Chemical Society

5326 Organometallics, Vol. 14, No. 11, 1995 u I ,

H --

H

H

Figure 1. Dewar-Chatt-Duncanson model of the bonding between a transition metal and a side-on-bonded nligand. Note that the ligand n-orbital has a-symmetry in the complex. bonding in transition metal complexes with side-oncoordinated n-ligands by using modern ab initio quantum chemical methods and to check the validity of the DCD model. We focus particularly on the different bonding situations of complexes where the transition metal is in a high or low oxidation state. For that purpose, calculations on two series of compounds were performed: WC4L and W(CO)aL, with L = HCCH, C2H4, C02, CS2, CHzO. The investigation thus considers the cases of a metal in a formally high and in a formally low oxidation state and a number of different n-bonded ligands. The geometries of the complexes were optimized at the Hartree-Fock and MP2 levels of theory, using basis sets of double-zeta polarization quality and effective core potentials for the heavier elements. The bond dissociation energies were calculated at the HF, MP2, and CCSD(T)levels. To gain insight into the metal-ligand bonding, we analyzed the wave function by using modern methods for the investigation of the electronic structure, including the topological analysis of the electron density of Bader,5,6the covalent bond orders of Cioslowski and M i ~ o n and , ~ the charge decomposition analysis of Dapprich and Frenking.8,9 The compounds investigated are of interest, not only from a theoretical point of view. Transition metal complexes of acetylene and ethylene play a dominant role as intermediates in a number of homogeneously catalyzed organometallic processes, such as alkene metathesis, oligomerization and polymerization of alkynes and alkenes, hydration and oxidation of olefins, and the hydroformylation process. A deeper understanding of the nature of the bonding in the reactive intermediates of these processes should lead to better insight into the mechanisms of the catalytic reactions. This knowledge might help to improve the present catalysts and to extend the processes to a wider range of systems. Transition metal complexes of C02 are a

+

(4) (a) Hoffmann, R.; Chen, M. M. L.; Thorn, D. L. Inorg. Chem.

1977,16,503.(b) Schilling, B.E. R.; Hoffmann, R. J.Am. Chem. SOC. 1979,101, 3456. (c) Mingos, D. M. P. In Comprehensive Orgunometullic Chemistry; Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.; Pergamon Press: New York, 1982. (5) Bader, R. F. W. Atoms in Molecules; Clarendon Press: Oxford, i wn.

(6)Bader, R. F. W. Chem. Reu. 1991,91,893. (7)Cioslowski, J.;Mixon, S. T. J . Am. Chem. SOC.1991,113,4142. ( 8 ) Dapprich, S.;Frenking, G. J . Phys. Chem. 1995,99,9352. (9) Dapprich, S.;Frenking, G. CDA 2.1.;Philipps-Universitat Marburg, Marburg, Germany.

Pidun and Frenking

subject of current interest in the endeavor to fix the C02 in air and to use it as a chemical C1 source.l0 In this context, CSZ complexes are often used as model compounds, and it must still be explained why CS2 complexes are usually so much easier to prepare than their COZanalogues." Finally, n-complexes of formaldehyde are postulated as intermediates in homogeneously catalyzed Fischer-Tropsch processeg. In view of the decreasing resources of mineral oil, the mechanistical investigation of these traditional C1 processes becomes more and more important. There are only a few experimental studies concerned with the complexes investigated by us. W(C0)dHCCH) and W(C0)5(C2H4)were synthesized by Stolz, Dobson, and Sheline through ultraviolet irradiation of W(CO)s in the presence of acetylene and ethylene, respectively.12 The compounds are rather unstable, and their isolation was difficult because of a tendency toward reversal of the reaction once irradiation was stopped. The third experimentally observed complex is W(CO)5(C02). It was prepared in 1985 by Almond, Downs, and Perutz13 through photolysis of W(CO)6in the presence of COz and was characterized by IR, Raman, and U V spectroscopies in solid argon matrices at 20 K. From the changes in the signals of the W(COk fragment compared with W(CO)6 and W(C0)5(NCCHd, the authors inferred an TJ, end-on coordination of the COZmolecule. In 1993, Zheng, Wang, Lin, She, and Fu observed W(CO)5(CO2) in the gas phase by time-resolved infrared spectroscopy.14 They found the complex to be in equilibrium with its precursor W ( c 0 ) ~at room temperature and estimated the bond energy for binding of C02 to W(CO)5 to be 8.2 f 1.0 kcal mol-l. For the remaining complexes investigated in this work, no experimental results are available. However, a considerable number of related compounds are known, which allows an indirect comparison at least. A whole series of WC4L complexes with substituted acetylenes [m~nophenylacetylene,~~ diphenylacetylene,16 bis(trimethylsilyl)acetylene171as ligand L has been prepared by Dehnicke and co-workers. The crystal structure analyses showed C-C bond lengths on the order of 1.34 A and relatively short W-C bonds of about 2.00 A. Most of the crystal structure analyses of transition metal C02 complexes found in the literature show the C 0 2 ligand in the side-on +coordination. Prominent examples are Nb(l15-C5H4Me)z(CHzSiMe3)(C02),18 Mo(y5-C5H5)2(C02),19 Typical C-0 and tr~ns,mer-Mo(CN-i-PrXPMe3)3(CO2~~~ bond lengths are on the order of 1.26-1.30 A for the (10)(a) Braunstein, P.; Matt, D.; Nobel, D. Chem. Rev. 1988,88, 747. (b) Behr, A.Angew. Chem. 1988,100,681;Angew. Chem., Int. Ed. Engl. 1988,27,661. (11)(a) Ibers, J. A. Chem. Soc. Rev. 1982,11, 57. (b) Butler, I. S.; Fenster, A. E. J . Orgunomet. Chem. 1974,66, 161. (12)Stolz, I. W.; Dobson, G. R.; Sheline, R. K. Inorg. Chem. 1963, 2,1264. (13)Almond, M. J.;Downs, A. J.; Perutz, R. N. I n o g . Chem. 1985, 24,275. (14)Zheng, Y.;Wong, W.; Lin, J.; She, Y.; Fu, K. Chem. Phys. Lett. 1993,202,148. (15)Paula, I. Dissertation, Universitat Marburg, Marburg, Germany, 1990. (16)Hey, E.; Weller, F.; Dehnicke, K. Nuturwissenschaften 1983, 70, 41. (17)Hey, E.; Weller, F.; Dehnicke, K. 2.Anorg. Chem. 1984,514, 18. (18)Bristow, G. S.;Hitchcock, P. B.; Lappert, M. F. J . Chem. SOC., Chem. Commun. 1981,1145. (19)Gambarotta, S.;Floriani, C.; Chiesi-Villa, A.; Guastini, C. J . Am. Chem. SOC.1985,107,2985.

Side-On-Bonded n-Ligand Complexes

Organometallics, Vol. 14, No. 11, 1995 5327

coordinated bond and on the order of 1.20-1.22 8, for result: while the Ni(0) complex shows the expected sidethe non-coordinated bond. The 0-C-0 angle usually on coordination of the COSligand, in the Cu(1) complex amounts to 132-135". Transition metal complexes with the most stable structure has COZ in an end-onCOZend-on coordinated via an oxygen lone pair have coordinated mode. The authors explained their results not yet been confirmed by X-ray crystal structure on the basis of an energy decomposition analysis by the analysis, but this coordination is postulated for W(COI5more favorable electrostatic interaction between posi(Cod on the basis of spectroscopic data.13J4 The situtively charged Cu+ and the oxygen lone pair of COZ. ation for CSZis rather similar to that of COz: all crystal To our knowledge, there are no further ab initio structure analyses of transition metal complexes of CSZ studies on transition metal complexes with side-onfound in the literature show the ligand in the side-on bonded n-ligands. Here we present a systematic invesn-bonded coordination. Important examples are Pttigation of the two series of compounds WC4L and ( P P ~ ~ ) z ( C SNb(y5-C5H5)Z(~-C3H5XCS2),2' Z),~~ CO(T~'-C~H~)- W(C0kL (L= HCCH, CZ&, COz, CSz, CHzO), including ( P M ~ ~ ) ( C Sand Z ) , V(y5-CsH5)z(CS~),24 ~~ For these comgeometry optimizations, bond energy calculations, and pounds, typical C-S bond lengths are on the order of analyses of the electronic structure. It is the first 1.67-1.72 8,for the coordinated bond and 1.54-1.628, theoretical study of these molecules at a correlated level, for the non-coordinated bond; S-C-S bond angles lie and we will show that the inclusion of correlation effects between 134" and 141". In spite of their supposedly is essential for a proper description of these compounds. high importance as intermediates in the reduction of carbon monoxide in the Fischer-Tropsch process, there 2. Computational Details are only a small number of transition metal complexes The geometries of the compounds have been optimized a t of formaldehyde that could be characterized experimenthe Hartree-Fock (HF) and MP2 (M~ller-Plesset perturbation tally. All compounds found in the literature have the theory terminated at second order)33levels of theory. The CHzO ligand y2-coordinated. They can be understood symmetries were restricted to CzUfor the HCCH and C2H4 to be the product of the addition of a carbene-type metal complexes and to C, for the C02, CS2, and CH2O complexes. fragment t o the C-0 double bond of the formaldehyde: The characterization of the stationary points was effected by F~(CO)Z(P(OM~)~)Z(CHZO),~~ OS(CO)Z(PP~~)Z(CHZO),~~ evaluating the second energy derivative matrix at the HarV(y5-C5H5)z(CH~0),27 and M O ( ~ ~ - C ~ H ~ ) Z ( Care H Z O ) tree-Fock ~~ level of theory. Bond dissociation energies have prominent examples. been calculated at the HF and MP2 levels. For the WC14L A number of theoretical studies of transition metal complexes, improved bond energies were predicted at the complexes with side-on-bonded n-ligands have been CCSD(T) level (coupled-cluster method34 with single and double excitations and a perturbative estimate of the triple found in the literature. Schwerdtfeger and co-workers excitation^^^) by using the MP2-optimized structures. CCSDinvestigated the anionic complex [WC15(HCCH)I- at the (T) calculations were technically not accessible for the W(CO)5L Hartree-Fock and MS-X, levels.29 Ziegler and Rauk complexes (single file size limit), but an estimate of the CCSDstudied ethylene complexes of Cu+, Ag+, Au+, Pto,and (T) bond dissociation energies by using isostructural reactions53 Ptz+a t the Hartree-Fock-Slater level.30 By using the is given. The harmonic vibrational frequencies and zero-point transition state method to analyze the bonding properenergies were calculated a t the H F level; all frequencies are ties, they found support for the Dewar-Chatt-Dununscaled. canson model. Kitaura, Sakaki, and Morokuma invesFor the heavier elements W, C1, and S, effective core tigated Ni(PH3)dHCCH) and N ~ ( P H ~ ) z ( C Zin H ~a) ~ ~ potentials were used. The quasirelativistic pseudopotential comparative Hartree-Fock study. The acetylene comfor tungsten was developed by Hay and Wadt (HW3).36The plex was calculated t o be more stable than the ethylene 5s, 5p, and 5d, and 6s electrons are treated explicitly. The corresponding valence basis set is of double-zeta quality with complex, but according to an energy decomposition a contraction (441/2111/21).36 The pseudopotentials for the analysis, the nature of the bond is very similar in these elements C1 and S have been developed by Stoll, Preuss, and two complexes. The same authors also studied the co-workers and are of the MEFIT type.37 The valence basis bonding between carbon dioxide and transition metals.32 sets are contracted as (31/31/1). Thus, they are also of doubleHartree-Fock calculations of the model complexes Nizeta quality, with an additional d-polarization function (PH3)2(COz)and [Cu(PH3)z(COz)I+yielded a remarkable (exponent: 0.65). For the lighter elements C, 0, and H, the (20)Alvarez, R.; Carmona, E.; Marin, J. H.; Poveda, M. L.; GutiBrrez-Puebla, E.; Monge, A. J . Am. Chem. SOC. 1986,108,2286. (21)Baird, M.; Hartwell, G.; Mason, R.; Rae, A. I. M.; Wilkinson, G. Chem. Commun. 1967,92. (22)Drew, M. G. B.; Pu, L. S. Acta Crystallogr. 1977,833,1207. (23)Werner, H.; Leonhard, K.; Burschka, C . J . Organomet. Chem. 1978,160,291. (24)Fachinetti, G.; Floriani, C.; Chiesi-Villa, A,; Guastini, C. J . Chem. SOC.,Dalton Trans. 1979,1612. (25)Brown, K.L.;Clark, G. R.; Headford, C. E. L.; Marsden, K.; Raper, W. R. J . Am. Chem. SOC.1979,101, 503. (26)(a) Berke, H.; Huttner, G.; Weiter, G.; Zsolnai, L. J.Organomet. Chem. 1961,219,353. (b) Berke, H.; Bankhardt, W.; Huttner, G.; v. Seyerl, J.; Zsolnai, L. Chem. Ber. 1981,114,2754. (27)Gambarotta, S.;Floriani, C.; Chiesi-Villa, A.; Guastini, C. J . Am. Chem. SOC. 1982,104,2019. (28)Gambarotta, S.;Floriani, C.; Chiesi-Villa, A,; Guastini, C. J . Am. Chem. SOC. 1985,107,2985. (29)Nielson, A.J.;Bayd, P. D. W.; Clark, G. R.; Hunt, T. A.; Melson, J. B.; Rickard, C. E. F.; Schwerdtfeger, P. Polyhedron 1992,11,1419. (30)Ziegler, T.; Rauk, A.Inorg. Chem. 1979,18,1558. (31)Kitaura, K.;Sakaki, S.; Morokuma, K. Inorg. Chem. 1981,20, 2292. (32)Sakaki, S.;Kitaura, K.; Morokuma, K. Inorg. Chem. 1982,21, 760.

all-electron 6-31G(d) standard basis sets38 were used. The basis sets and pseudopotentials (denoted as basis set 11)have proven to be very appropriate for the description of transition (33)(a) Mgller, C.; Plesset, M. S. Phys. Rev. 1934,46, 618. (b) Binkley, J. S.; Pople, J. A. Int. J . Quantum Chem. 1975,9, 229. (34)Cizek, J. J. Chem. Phys. 1966,45,4256. (35)(a) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Int. J . Quantum Chem. 1978,14,545. (b) Bartlett, R. J.; Purvis, G. D. Int. J . Quantum Chem. 1978,14,561. ( c ) Purvis, G. D.; Bartlett, R. J. J . Chem. Phys. 1962,76,1910.(d) Raghavachari, K.;Trucks, G. W.; Pople, J. A.; Head-Gordon, M. Chem. Phys. Lett. 1989,157,479. (e) Bartlett, R.J.; Watts, J. D.; Kucharski, S.A,; Noga, J. Chem. Phys. Lett. 1990,165,513. (36)Hay, P. J.;Wadt, W. R. J . Chem. Phys. 1985,82,299. (37)(a) Dolg, M.;Wedig, U.;Stoll, H.; Preuss, H. J . Chem. Phys. 1987,86,866. (b) Andrae, D.; Haussemann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990,77,123. ( c ) Bergner, A.;Dolg, M.; Kuchle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993,80 (61,1431. (38)(a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J . Chem. Phys. 1971, 54,724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J . Chem. Phys. 1972,56,2257. ( c ) Francl, M. M.; Pietro, W. J.;Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; Defrees, D. J.; Pople, J. A. J . Chem. Phys. 1982, 77,3654.

5328 Organometallics, Vol. 14, No. 11, 1995

Pidun and Frenking

n

lb

2.30)

2b

6s

6b

Figure 2. Optimized geometries at MP2AI of the WC4L complexes. Values at HFAI are given in parentheses. metal complexes.39 The calculations have been carried out basis set W c1, s C, 0,H I1

[ECP] 441/2111/21

[ECPI 31/31/1

6-31G(d)

using the program packages GAUSSIAN 92,40 TURBOMOLE,41and ACES II.42 For the topological analysis of the electron density, the programs SADDLE, GRID, CONTOUR, GRDVEC, and VECAIM43were used, and for the calculation of the covalent bond orders, the programs MAKAOMP2 and BONDERMP244were used. The charge decomposition analysis was carried out with the program CDA

3. Results and Discussion In this section, we will first present the results of the geometry optimizations and bond energy calculations (39) Frenking, G.; Antes, I.; Boehme, M.; Dapprich, S.; Ehlers, A. W.; Jonas, V.; Neuhaus, A.; Otto, M.; Stegmann, R.; Veldkamp, A,; Vyboishchikov, S. F. Reviews in Computational Chemistry; VCH Publishers: New York, in press, Vol. 7. (40)Frisch, M. J.; Trucks, G. W.; Head-Gordon, 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.; Gonzales, C . ; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J . A. GAUSSIAN 92, Revision C : Gaussian Inc.: Pittsburgh, PA,- 1992. (41) TURBOMOLE: (a) Haser, M.; Ahlrichs, R. J. Conput. Chem. 1989.80,104. (b)Ahlrichs. R.:Bar. M.: HBser. M.: Horn. H.: Kolmel. M. C:Chem.Phys. Lett. 1989,i62,165. '(c) Horn, H:; Weiss, H.;Haser; M.; Ehring, M.; Ahlrichs, R. J . Comput. Chem. 1991,12, 1058. (d) Haser, M.; Almlof, J.; Feyereisen, M. W. Theor. Chim. Acta 1991,79, 115. (e) Schafer, A.; Horn, H.; Ahlrichs, R. J . Chem. Phys. 1992,97, 2571. (f) Haser, M.; Ahlrichs, R.; Baron, H. P.; Weiss, P.; Horn, H. Theor. Chim. Acta 1992,83,455. (42)ACES 11, an ab initio program written by J. F. Stanton, J. Gauss, J. D. Watts, W. J. Lauderdale, and R. J. Bartlett, University of Florida, Gainesville, FL, 1991. (43) Biegler-Konig, F. W.; Bader, R. F. W.; Ting-Hua, T. J. Comput. Chem. 1982,3,317. (44)Cioslowski, J. BONDER Florida State University, Tallahassee, FL, 1990.

for the WC4L and W(C0)sL complexes. Subsequently, the most important conclusions from the electronic structure analyses are described, and in the end the comparison of the WCLL and W(C0kL complexes is summarized. 3.1. Structures and Bond Dissociation Energies. 3.1.1. W C U Complexes. Figure 2 shows the optimized structures (MP2AI) of the complexes WC14(HCCH) ( l a eclipsed, l b staggered), WCl.dCzH4) (2a eclipsed, 2b staggered), WC14(COz) (3a eclipsed, 3b staggered), WCL(CSz)(4a eclipsed, 4b staggered), and WClK!HzO) (Sa eclipsed, 5b staggered) and of WCL (6a singlet, 6b triplet). The results of the Hartree-Fock optimizations are given in parentheses. The corresponding total energies, relative energies of the two conformers, and zero-point energies (HFflI) are given in Table 1. With the exception of the acetylene complex, the eclipsed conformations are more stable than the staggered ones a t all theoretical levels. For the HF structures the more stable conformer was verified as a true minimum on the potential energy surface by only positive eigenvalues of the Hessian matrix. In the same way, the other conformer could be characterized as a transition state of first order (one negative eigenvalue of the Hessian matrix). The only exception is the WC14(COz) complex, which dissociates during optimization at the Hartree-Fock level so that no HF frequencies could be determined. The calculated structures of the acetylene complexes l a and l b are in very good agreement with experimental data for W c 4 complexes of substituted acetylenes. The theoretical W-C and C-C bond lengths of the more stable conformer l b at the MP2 level (2.001 and 1.336 A) are identical to the experimental values (2.00 and

Organometallics, Vol. 14,No.11, 1995 5329

Side-On-Bondedn-Ligand Complexes

Table 1. Calculated Total Energies Etot (au), Relative Energies of the Two Conformations (kcal mol-'), Zero-Point Energies ZPE (kcal mol-'), and Number of Imaginary F'reauencies i of the WCLL Complexes ___

HF/II//HF/II

molecule la lb 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b

E+"+ -203.185 -203.188 -204.370 -204.363 -184.122 -184.121 -240.184 -240.180 -126.335 -126.394

45 42 39 40 97 06 79 78 44 04

Em1 0.0 -1.9

0.0 +4.3

0.0 +1.2

0.0 +2.5

0.0 -36.8

MPP/IU/MP2iII

ZPE

i

23.7 23.8 40.0 39.8

1 0 0 1

8.6 8.5 24.2 24.1 3.1 3.0

0 1 0 1 0 0

1.34 A).l5-l7 It should be noted that the W-C bond lengths of la and lb calculated at the HF and MP2 levels are very similar, while the C-C distances clearly are different. This is because the C-C bond length of free acetylene is calculated to be much shorter at HFI 6-31CXd) (1.185 A) than a t MP2/6-31G(d) (1.218 A). At both theoretical levels HF/II and MP2/II it is predicted that the C-C bond length of the complex 1 is significantly longer than that in free acetylene. The calculated C-C bond length a t M P N I of lb is exactly the same as that in free ethylene (1.336 A), which suggests that l b should be considered as a metallacyclopropene. In the case of WC14(C2H4),the eclipsed conformer 2a is more stable than the staggered one (2b),and the consideration of correlation effects is more important than for 1: on going from the HF to the MP2 level, the W-C bond length decreases by about 0.11 A, while the C-C bond is lengthened by 0.07 A to a value of 1.459 A. Thus, due to the neglect of correlation effects, the Hartree-Fock method severely underestimates the bonding interactions between the ethylene ligand and the W c 4 fragment, and it is only at the MP2 level that the C-C bond in the complex approaches the order of a typical C-C single bond (1.54 A), according to a description of WC14(C2H4) as a metallacyclopropane. For the WCl4(CO2) complex, the differences between the results a t HF/II and MP2/II are even more dramatic: a t the Hartree-Fock level, no stable adduct between WC4 and C02 could be found. The complex dissociates during the optimization. The MP2 optimizations yield two conformations with significantly different structural features. The eclipsed conformer 3a was calculated t o be 5 kcal mol-' (8kcal mol-' at CCSD(T)/ 11)more stable than the staggered one (3b). In 3a the C02 ligand occupies an axial position in the trigonalbipyramidal coordination sphere of the tungsten, and the W-C and W-0 bonds are rather long (2.086 and 2.050 A, respectively), while the coordinated C-0 bond length remains quite short (1.335 compared to 1.204 for the non-coordinated bond). These calculated structural parameters compare favorably with the experimental data available:18-20typical C - 0 distances of 1.26-1.30 A for the coordinated C-0 bond and 1.201.22 A for the exocyclic C-0 bond and a 0-C-0 angle of 132-135" are reproduced reasonably well. Surprisingly, in the energetically less stable staggered conformer 3b,the interaction between the C02 ligand and the metal seems t o be stronger. The W-C and W-0 bonds are considerably shorter (2.046 and 1.962 A), and the coordinated C - 0 bond is lengthened to 1.415 A, which is on the order of magnitude of a typical C - 0

E+.+ -204.253 -204.256 -205.427 -205.422 -315.206 -315.197 -185.342 -185.340 -241.329 -241.323 -127.069 -127.105

03 94 90 56 09 77 39 33 75 61 26 27

CCSD(T)/IU/MPP/II

E.,I 0.0

E+,+

-2.5

0.0

-204.314 75 -205.504 40

+3.4 0.0 +5.2 0.0

-315.252 72 -315.239 77 -185.410 37

f1.3 0.0

-241.381 99

+3.9 0.0 -22.6

-127.132 12 -127.165 68

E..I

0.0 +8.1

0.0 -21.1

single bond (1.43 A). These structural results indicate that there might be a fundamental difference in the nature of the bonding in the two conformations of WC4( C o d While the eclipsed conformer seems to correspond to a typical donor-acceptor complex, which can be described by the Dewar-Chatt-Duncanson model, the staggered conformer approaches a metallacycle with polar covalent bonds. In the case of WCldCS2), the two conformations calculated at the HF and MP2 levels of theory are very similar; the eclipsed conformation 4a is more stable by 1.2 and 1.3 kcal mol-l, respectively. The W-C distance is significantly shorter (0.07-0.09 A) and the coordinated C-S bond is longer (0.09-0.11 A) at the MP2 level. The calculated MP2 energy minimum structure 4a compares well (C-S bond lengths of 1.749 and 1.596 A; S-C-S angle of 138.4") with crystal structure analysis data of experimentally found CSZc o m p l e x e ~ ~ ~ - ~ ~ (C-S bond lengths of 1.67-1.72 and 1.54-1.62 A; S-C-S angles between 134 and 141"). The optimized geometries of 3 and 4 can also be interpreted as snapshots of the formation of oxo-carbonyl and sulfidothiocarbonyl complexes, respectively. In fact, side-ony2-bonded CS2 complexes are known as intermediates in the formation of thiocarbonyl c ~ m p l e x e s . ~ ~ The MP2 geometry optimizations of WC14(CHzO) yielded the structures Sa and Sb. Again, the structural differences between the two conformations are not very significant, and the eclipsed conformation Sa is slightly more stable than Sb. The most striking feature of the MP2-optimized conformations is the very long C-0 bond in the complex (1.461 A for Sa), which is even longer than a typical C-0 single bond (1.43 A). On the other hand, the W-0 bond is very short (1.874 A for Sa) and should be compared with W-0 double bonds in WOc4 (1.684 A)46and WOF4 (1.666 A).47 Figure 2 clearly indicates that the tungsten atom and the methylene group of Sa and Sb lie almost in one plane (dihedral angle: 19.5"). Thus, the theoretically determined geometry of WC14(CHzO) shows some indications for a transformation of the +formaldehyde complex into an oxo-carbene complex. Such a reaction has actually been observed experimentally: the formation of the oxo-carbene complex W(O)(=C5H&lz(PMePh2)2 by reacting WC12(PMePhz)d with cyclopentanone was Sa may be reported by Bryan and M a ~ e r Structure .~~ ~

~

~

(45) Broadhurst, P.V.Polyhedron 1985, 4 , 1801. (46) Robiette, A. G.;Hedberg, K.; Hedberg, L. J.Mol. Struct. 1977, 37,105. (47) Iijima, K.;Shibata, S. Chem. Lett. 1972,1033. (48) Bryan, J. C.;Mayer, J. M. J.A m . Chem. SOC.1987,109,7213.


Table 2. Calculated Bond Dissociation Energies De (kcal mol-') for the WCL& Complexes Relative to Singlet WCl, (S) and Triplet WCL (TP HF/II//HF/II MPS/II//MPP/II CCSD(T)/II//MP2/II molecule S T S T S T lb 22.4 -14.4 74.7 52.1 57.7 (55.4) 36.6(34.4) 2a 2.3 -34.5 45.0 22.4 33.2 (30.6) 12.1(9.6) 3.8 -17.7 18.8 -3.8 3a 4a -34.3 -71.1 36.5 13.9 17.9 (16.9) -3.2(-4.1) 5a -10.3 -47.1 58.2 35.6 39.8(37.0) 18.7(16.0) a

ZPE-corrected values DO(kcal mol-') are given in parentheses.

considered as a snapshot along the reaction coordinate of the formation of the oxo-carbene complex. Experimental studies of formaldehyde complexes have shown that the CH20 ligand is always y2-coordinated to the metal and that the metal-oxygen distance is 0.03-0.15 8, shorter than the metal-carbon d i ~ t a n c e . ~How~-~~ ever, the calculated C-0 distance Of 5a is much longer than the observed values for low-valent transition metal formaldehyde c ~ m p l e x e s . ~ ~ - ~ ~ The theoretically determined bond dissociation energies for the MP2-optimized energy minimum structures of the WC4L complexes are given in Table 2. The bond energies were calculated with respect to the singlet (6a) and triplet forms (6b) of WC4. The MP2-optimized geometries of the Wc4 fragment are displayed in Figure 2. In the triplet case, the calculation yielded a tetrahedral (Td)geometry, while the singlet structure shows a tetragonal distortion toward D2d symmetry, as expected for a d2-transition metal (Jahn-Teller effect). The triplet state of WC4 is lower in energy than the singlet state. Since we are mainly concerned with the relative bond strengths of the ligands L in the WCLL complexes, the calculated absolute bond energies are less important. As mentioned earlier, no stable HF structure could be found for the WCL(C02)complex, and consequently no ZPE correction can be provided. The calculations for the C14W-L bond strength show the order HCCH > CH20 > C2H4 > CS2 > C02. This trend is predicted at all levels of theory. The calculated bond energies at MP2/II are much higher than those at HF/II. The predicted bond dissociation energies at CCSD(T)/II are somewhat lower than those at MP2AI. Previous theoretical studies have shown that bond energies calculated at CCSD(T)/II are in good agreement with experimental results, while the HF/II values are much too low and the MP2/II values are too high.39,49 No experimental data for the C14W-L bond energies are available, but the predicted trend is in agreement with experimental observations. Numerous acetylene complexes of WC14are known, which are usually found as dimers in the solid state.15-17 Olefin complexes of transition metals are a well-characterized class of compounds, but there is no olefin complex of WCL known to us. The acetylene complex l b is calculated to be significantly more stable than the ethylene complex 2a (Table 2). This can be explained if both complexes are considered as metallacyclic compounds. The carbon atoms of l b are approximately sp2 hybridized, while the carbon atoms of 2a are sp3 hybridized. The former carbon atoms should have stronger bonds to the metal than the latter carbon atoms. Yet the calculations indicate that olefin complexes such as 2a (49) Ehlers, A. W.; Frenking, G. J.Am. Chem. SOC.1994,116,1514.

Pidun and Frenking

might be stable enough to be synthesized under appropriate conditions. The calculated metal-ligand bond energies also show that CS2 is much more strongly bound than C02. This is in agreement with the considerably larger number of CS2 complexes than C02 complexes, which are usually rather unstable. 3.1.2. W(C0)sL Complexes. The optimized structures (MP2AI) of the complexes W(C0)5(HCCH) (7a eclipsed, 7b staggered), W(C0)5(C2H4)(8a eclipsed, 8b staggered), W(CO)5(C02) (9a eclipsed, 9b staggered), (loa eclipsed, lob staggered), W(C0kW(CO)~(CSZ) (CH20) ( l l a eclipsed, l l b staggered), and W(CO)5 (12) are shown in Figure 3, with the geometrical parameters of the HF optimizations given in parentheses. The corresponding total energies, relative energies of the two conformers, and zero-point energies (HF/II) are given in Table 3. For all complexes, the eclipsed conformations are slightly more stable than the staggered ones. For the HF structures, the eclipsed conformers could be verified as true minima on the potential energy surface by only positive eigenvalues of the Hessian matrix. There are significant differences between the HF- and MP2-optimized conformations of the acetylene complexes 7a and 7b. At the MP2 level, the tungstencarbonyl bonds clearly are shorter, while the carbonyl C-0 bonds are longer. The acetylene ligand is bonded more strongly at the correlated level: the W-C bond length decreases (by 0.12 A for 7a),the C-C bond length increases (by about 0.05 A), and the H-C-C angle becomes markedly smaller. Thus, the neglect of correlation effects leads to a severe underestimation of the bonding interactions between the w(co)5fragment and the acetylene ligand. In agreement with previous studies, the investigations clearly indicate that correlated methods are essential for a proper description of transition metal complexes in low oxidation states.39 The differences in the structures between W(c0)s(HCCH) (7a) and WCL(HCCH) (lb) are remarkable. In 7a the C-C distance is shorter than that in l b by about 0.07 A. The C-C bond length of 7a (1.262 A) is intermediate between the C-C distance in free acetylene (1.205 A) and that in free ethylene (1.330 A). The W-C bond length is significantly larger in 7a (by 0.33 A) and the H-C-C angle is expanded by about 10". It follows that the HCCH ligand keeps its acetylene character to a larger extend in the W ( c 0 ) complex ~ than in lb. While WC14(HCCH) is best described as a metallacyclopropene, W(CO)5(HCCH) seems to be a genuine donor-acceptor complex, with the acetylene ligand occupyingjust one coordination site a t tungsten. The examination of the electronic structure will show that these conclusions based on geometrical results can be confirmed by the bonding analysis. The situation for W(C0)5(C2H4)(8a and 8b) is similar to the case of W(CO)5(HCCH). Again, the structural differences between the two conformations are only small and the eclipsed conformer is slightly more stable. The consideration of correlation effects at the MP2 level leads to the typical changes: the W-carbonyl and W-ethylene distances decrease, while the C-0 and C-C bonds become longer. As compared to WC14(C2&), the C-C bond length in W(C0)5(C2H4)is shorter by 0.06 A, and with a value of 1.402 A it is more similar to a typical double bond (1.33 A) than to a typical single bond


Organometallics, Vol. 14,No.11, 1995 5331

Q

P

I166l1.114)

7a

8

7b

lo.

lob

8b

1I.

llb

9b

12

I 161 11 122)

9a

Figure 3. Optimized geometries at MPPIII of the W(C0)sL complexes. Values at HF/II are given in parentheses. Table 3. Calculated Total Energies Etot(au), Relative Energies of the T w o ConformationsE,1 (kcal mol-l), Zero-Point Energies ZPE (kcal mol-'), and Number of Imaginary Frequencies i of the W(CO)& Complexes HF/II//HF/II molecule 7a 7b 8a 8b 9a 9b 10a 10b lla llb 12

E,I ZPE -707.720 44 0.0 47.2 -707.717 14 +2.1 47.1 -708.939 94 0.0 64.3 -708.937 19 +1.7 64.4 0.0 36.3 -818.520 85 -818.520 85 fO.0 36.3 -688.723 97 0.0 32.9 -688.731 53 -4.7 32.9 -744.760 09 0.0 47.8 -744.75983 f0.2 47.7 -630.876 30 27.7 Etot

MP2/II//MP2/II Est 0 -709.759 73 1 -709.754 83 0 -710.987 94

0.0 +3.1

2

+3.6

i

-710.982 0 -820.758 0 -820.758 0 -690.889 1 -690.888 0 -746.845 1 -746.844 0 -632.623

28 96 97 11 22 95 53 96

Ere1

0.0 0.0 fO.0 0.0 +0.5 0.0 +0.9

(1.54 A). Accordingly, the W-C distance is longer by 0.17 A in the W(CO)5 complex. Thus, the C2H4 ligand still has some ethylene character in W(C0)5(C2Hd,and a description as a donor-acceptor complex seems appropriate. A compilation of experimentally determined W-C bond distances50 for hexacoordinated W(0) complexes with side-on-bonded substituted alkenes (L= CH2CHR) yields an average value of 2.386 A, which is in excellent agreement with the calculated W-C distance for 8a (2.372 A, Figure 3). As mentioned in the Introduction, W(CO)5(HCCH) and W(C0)5(C2H4) could be detected (50) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, 0.; Watson, D. G.; Taylor, R. J . Chem. Soc., Dalton Trans. 1989,S1.

experimentally by IR spectroscopy as unstable species.12 Stolz, Dobson, and Sheline observed that the decrease in the C - 0 stretching frequency of the trans-carbonyl group is significantly larger for W(CO)5(HCCH)than for W(C0)5(C2H4). This experimental result is in accord with our calculations: the trans W-C distance is shorter in the acetylene complex (2.020 compared to 2.026 A), while the corresponding C-0 distance is slightly longer in the case of acetylene (1.172 compared to 1.170 A). IR spectroscopic studies suggest for W(CO)5(CO2)an end-on coordination of the C02 ligand,I3J4but a crystal structure analysis could not be provided for this unstable species. Our calculations strongly support the experimental conjecture: as can be seen from Figure 3, the geometry optimizations a t MP2 as well as HF yielded structures with end-on-coordinated carbon dioxide (9a and 9b). At both theoretical levels, the bond lengths and relative energies of the two conformations are virtually the same, as can be anticipated from the locally a-symmetrical coordination of the ligand. The calculated W-0 distance is rather large (2.377 A), and the two C-0 bond lengths of the bonded C02 differ by only 0.017 A. A strong trans effect is predicted by the calculations: the W-C(O) bond trans to the CO2 ligand is more than 0.08 A shorter than the cis W-carbonyl bonds, and the trans C-0 bond is longer by about 0.01 A than the cis C-0 bonds. The strong trans effect can be traced back to the end-on-coordinated C 0 2 being a strong a-donor without n-acceptor properties. The geometry optimization of the eclipsed and staggered conformations of W(CO)5(CS2)at the HF and MP2

5332 Organometallics, Vol. 14, No. 11, 1995 Table 4. Calculated Bond Dissociation Energies De (kcal mol-') for the W(C0)sL ComplexeP molecule

HF/II//HF/II

MPP/II//MP2/II

CCSD(T)/II//MPS/II (estimated)

7a 8a 9a 10a lla

16.8 20.3 6.9 3.4 11.4

42.2 48.3 17.6 31.5 34.1

35.3 (34.3) 41.4 (39.1) 10.7(10.2) 24.6 (24.1) 27.2 (25.4)

a

ZPE-corrected values DO(kcal mol-') are given in parentheses.

Pidun and Frenking W(COk fragment (12) is shown in Figure 3; it has a quadratic-pyramidal geometry (C4" symmetry). The bond dissociation energies were determined a t the Hartree-Fock and MP2 levels by using the respective optimized structures. CCSD(T)bond energies were not feasible for the W(C0)aLcomplexes for technical reasons (single file size limit), but the CCSD(T)bond energies could be estimated by calculating the reaction energy of the isostructural reaction:53 W(CO),L

levels led to stationary points on the potentialpergy surface that are quite different (Figure 3,lOa and lob). At the HF level the eclipsed conformation loa, with side-on-coordinated CS2, is found as energy minimum structure.51 At the MP2 level, the energetically favored form 10a has an end-on-coordinated CS2. The W-S distance of 10a is rather large (2.616 A), the two C-S bond lengths differ by only 0.036 A, and the CS2 ligand is approximately linear (174.6'). A significant trans effect is observed for loa: the W-CO trans bond is much shorter (1.986 A) than the W-CO cis bond (2.062 A>. The MP2-optimized staggered conformer 10b is higher in energy by only 0.5 kcal mol-'. Its structure lies between a side-on and an end-on coordination of the CSz ligand: on the one hand, the W-C distance (2.662 A) is longer than the W-S distance (2.623 A), the two C-S bond lengths are rather similar (difference only 0.035 A), and a marked trans effect occurs. On the other hand, the CS2 molecule is significantly bent (160.7') and the W-C distance is too short to exclude an interaction. On the whole, the results at the MP2 level suggest that an end-on coordination of the CS2 ligand is likely for W(CO)B(CS~), as was postulated on the basis of IR spectroscopic studies for the related complex Fe(C0)4(CSd.52 The geometry optimizations of W(C0)5(CH20) ( l l a and l l b ) yielded structures with side-on-coordinated formaldehyde. The HF and MP2 geometries differ significantly; the W-C and W-0 distances particularly are much shorter at the MP2 level than a t the HF level (0.29-0.38 and 0.18-0.22 A, respectively). Figure 3 shows that the CH20 ligand occupies just one coordination site at tungsten. As compared to WCl4(CH20),the W-C(H20) and the W-O(CH2) bonds of l l a are 0.33 A longer, while the C-0 bond of the formaldehyde ligand is 0.16 A shorter. The small departure of the coordinated formaldehyde molecule from planarity (dihedral angle: 25.9") also supports the description of W(CO)5(CH20) as a typical donor-acceptor complex, rather than a metallacycle with polar covalent bonds. The calculated formaldehyde C-0 bond length (1.304 A) favorably compares with the measured C - 0 distances (1.32-1.35 A)of experimentally known transition metalformaldehyde c ~ m p l e x e s . ~ ~ - ~ ~ The calculated bond dissociation energies of the MP2 energy minimum structures of the W(C0)aL complexes are given in Table 4. The optimized geometry of the (51)Optimization of the staggered conformation 10b yielded a structure with end-on-coordinated CSz which is 4.7 kcal mol-' more stable than loa. However, the frequency analysis showed that lob is a transition state of first order with respect to the rotation of the CSz molecule out of the plane of symmetry. It can be anticipated that on the HF potential energy surface there exists another minimum of lower symmetry with end-on-coordinated CS2. (52) Baird, M. C.; Hartwell, G.; Wilkinson, G. J . Chem. SOC. A 1967, 2037.

+ co - W(CO), + L

(1)

The energy of reaction 1 gives the difference of the metal-ligand bond energy between L and CO. Since the reactant and the product are both octahedral complexes, the reaction energy predicted at the MP2 level is quite accurate. By adding the MP2 value for the reaction energy of reaction 1 to the CCSD(T)value = 45.7 for the first CO dissociation energy Of W(CO)6(DO kcal mol-l, experimental value = 46.0 f 2.0 kcal m ~ l - l ) one , ~ ~obtains an estimated CCSD(T) value for the (C0)sW-L dissociation energy. It has been shown that calculated energies of isostructural reactions yield rather accurate bond dissociation energies.53 Table 4 shows that the (CO)5W-L bond energies calculated at HF/II are very low and that they clearly are higher a t MP2AI. The estimated CCSD(T)/IIvalues are somewhat lower than the MP2/II results. The theoretical results give a relative order of bond strengths for the ligands L = C2H4 > HCCH > CH20 > CS2 > Cog. A comparison with the trend predicted for the WC4L complexes shows that ethylene becomes relatively more strongly bound in W(CO)5Lthan in WCl4L. This is readily explained by the higher donor strength of ethylene over acetylene, which appears t o be the dominant factor for the metal-ligand interactions in the donor-acceptor complexes W(CO)5L. Experimentally, the complexes W(C0)5(HCCH), W(CO)5(C2lH4), and W(CO)5(CO2) could be detected, although as rather unstable species.12-14 However, the calculated bond energies suggest that the corresponding CS2 and CH2O complexes should also be worthwhile as synthetic targets, a t least from a thermodynamic point of view. For W(CO)5(CO2) an experimental bond dissociation energy of 8.2 f 1.0 kcal mol-l has been determined on the basis of time-resolved IR spectroscopy.14 This value is in good agreement with our estimated CCSD(T) bond energy of 10.2 kcal mol-I. 3.2. Analysis of the Electronic Structure. To gain deeper insight into the nature of the metal-ligand bonds in the complexes, we analyzed the electronic structure of the molecules. To this end, we carried out a topological analysis of the electron density distribution@ and determined atomic charges and covalent bond orders according to the definition of Cioslowski and Mixon.7 In addition, we used the newly developed charge decomposition analysis (CDA)8,gto estimate the relative importance of charge donation and back-donation between the metal and the ligand. Figure 4 shows the contour line diagrams of the Laplacian distribution v2p(r)of the complexes lb, 2a, 3a, 4a, Sa, 7a, 8a, 9a, loa, and l l a in the respective (53) Dapprich, S.; Pidun, U.; Ehlers, A. W.; Frenking, G. Chem. Phys. Lett. 1996, 242, 521. (54) Ehlers, A. W. Dissertation, Universitat Marburg, Marburg, Germany, 1994.


lb

Organometallics, Vol. 14,No. 11, 1995 5333

7s

3s I

I

7

4a

/

5s

I

11s

Figure 4. Contour line diagrams of the Laplacian distribution v2p(r)at MP2/II of the complexes in their respective plane of symmetry. Dashed lines indicate charge depletion [v2p(r)> 01, and solid lines indicate charge concentration [v2p(r)< 01. The solid lines connecting the atomic nuclei are the bond paths, and the solid lines separating the atomic nuclei indicate the zero-flux surfaces in the plane. The crossing points of the bond paths and the zero-flux surfaces are the bond critical points Q,.

plane of symmetry of the molecule. For each ligand, the WC4L and W(CO)5L complexes in their lower

energy conformations are compared.55 The most striking feature of the contour line diagrams in Figure 4 is that for all complexes investigated with side-on-coordinated ligands a cyclic structure of the tungsten-ligand fragment is found, with two metal-ligand bonds and a ring critical point. Thus, even for the W(C0)sL complexes, whose structural properties indicate a donoracceptor type of bonding, cyclic structures are predicted for the WL moiety. However, the topological definition of a cyclic structure may be quite different from the chemical interpretation. For example, the W(H2) unit of the dihydrogen complex W(CO)5H2 is suggested by the topological analysis of the electron density distribution to have a cyclic structure.56 Other criteria such as a strong distortion of the Laplacian distribution of the ligand L should be used as a measure for the nature of the ML moiety. Indeed, there are significant differences in the Laplacian distributions between the WCl4L and W(CO)5L series of compounds. For each ligand L, the Laplacian distribution shows a markedly stronger distortion toward the metal in the WC4L complexes than in the W(C0)sL complexes. This indicates a higher degree of covalent W-L bonding for the WC4L complexes, as was already inferred fkom the structural data. In particular, the end-on-coordinated complex W(CO)5(CO2)(9a)has a Laplacian distribution typical for a pure closed-shell interaction. There is virtually no distortion of the C02 Laplacian distribution in 9a. Generally, the carbon atoms of the ligands respond more strongly to the perturbation caused by the metal than the oxygen atoms, in accordance with the smaller difference in electronegativity between W and C compared to W and 0. The carbon atom is softer than the oxygen atom. The visual impression of the Laplacian distributions shown in Figure 4 is supported by the calculated results of the topological analysis (Table 5). The energy density a t the bond critical point H b of the W-L bonds of the WC4L complexes clearly is more negative than those for the respective W(CO)5L complexes. It has been shown that a negative value for H b indicates covalent character for the bond, while H b 0 indicates closedshell interaction^.^^ Another indicator for a more covalent W-L bond in the WC4L complexes is the covalent bond order. The calculated BO values for the W-L bonds are much higher for the WCl4L complexes than for W(C0)sL (Table 5). Also, the difference of the intraligand bond order between the free ligand L (Table 5, values in parentheses) and the coordinated ligand L is much higher for the WC14L complexes than for W(C0)5L. In the case of the acetylene and ethylene complexes, the results of the electronic analyses fully support the conclusions from the geometry calculations. The tungsten-carbon bond orders are higher for WClXHCCH) (lb) than for WC14(C2H4) (2a)(1.09 compared to 0.91), in accordance with the markedly shorter W-C bond lengths for l b (2.001 A) than for 2a (2.103 A). This observation can easily be explained in the framework (55) Note that for computationalreasons the Laplacian distributions of the CS2 complexes 4a and 10a have been calculated without using effective core potentials for sulfur. (56)Dapprich, S.; Frenking, G. Angew. Chem. 1995, 107, 383; Angew. Chem., Int. Ed. Engl. 1995,34,354. (57) Cremer, D.; Kraka, E. Angew. Chem. 1984,96, 612; Angew. Chem., Int. Ed. Engl. 1984,23,627.

Pidun and Frenking


Table 5. Calculated Bond Orders BO, Energy Densities at the Bond Critical Point H b (hartreed As),Atomic Charges q(A) and q(B), and Charges of the Ligands g(L) in the Complexes" molecule bond (A-B)

lb 7a 2a Sa

3a

9a 4a

10a 5a

1la

a

w-c

BO

Hb

1.09 -0.486 1.64 (2.76) -2.605 0.40 -0.088 2.46(2.76) -3.273 -0.391 0.91 1.11 (1.89) -1.694 0.39 -0.094 1.61 (1.89) -2.078 0.80 -0.128 0.84 -0.412 0.96(1.35) -3.354 1.32(1.35) -4.548 0.24 -0.034 1.25(1.35) -4.785 1.39 (1.35) -4.940 1.16 -0.202 -0.409 0.92 1.30(2.32) -1.034 2.02 (2.32) -1.902 -0.049 -1.912 -1.949 -0.432 1.19 0.82 -0.405 0.89(1.45) -2.038 0.46 -0.027 0.46 -0.101 1.13(1.45) -3.664

q(L) -0.34

q(A)

q(B)

+2.34 -0.35 +1.76 -0.30 $2.28 -0.38 $1.73 -0.24 +2.35 f2.35 +1.40 +1.40 $1.73 +2.31 +2.31 +2.22 +2.22 -0.96 -0.96

-0.35 -0.35 -0.30 -0.30 -0.38 -0.38 -0.24 -0.24 -1.02 +1.40 -1.02 -1.11 -1.21 -1.21 -1.10 10.00 -0.96

$2.49 +2.49 -0.01 +1.77 +1.77 +0.57

-0.85 -0.64 -0.01 -0.85 -1.11 -0.40 +0.57 -1.11

-0.22

Table 6. MP2 Charge Decomposition Analysis of the Complexes in Their MP2 Geometries" molecule

7a Sa lla 9a 10a W(C0)6

d

b

r

0.297 0.225 0.241 0.126 0.322 0.315 0.265

0.165 0.148 0.163 -0.011 0.033 0.233 0.168

-0.391 -0.422 -0.350 -0.100 -0.275 -0.278 -0.569

-0.36

3a

-0.15

d , donation; b , back-donation; r , repulsive part.

-0.73

fO.OO -0.42

fO.OO f0.53

The bond orders of the free ligands a r e given in parentheses.

of a covalent bonding picture by the sp2 hybridization of the acetylene C-atom being more favorable than the sp3 hybridization of the ethylene carbon in the complex. The C-C bond orders in the WC4L complexes (1.64 for lb, 1.11for 2a) are significantly smaller in comparison with the free ligand, and they are on the order of a typical double bond (1.89 in ethylene) and a typical single bond (1.02 in ethane), respectively, as expected for a substituted metallacyclopropene or metallacyclopropane. In contrast, the C-C bond orders in the W(CO)5L complexes 7a and 8a are only slightly smaller than those in the free ligand, in accordance with the Dewar-Chatt-Duncanson description of the bonding: donation and back-donation weaken the C-C multiple bond, but they do not lead t o a significant reduction of the bond order. The W-C bond orders of 7a and 8a are rather low. As mentioned earlier, the very short W-0 and very long C-0 bonds of 5a provide some indications for a transformation of the v2-formaldehydecomplex into an oxo-carbene complex. This conclusion is supported by the electronic structure analysis. Table 5 shows a large bond order of 1.19 for the W-0 bond, which suggests some double-bond character. The C-0 bond order clearly is reduced in the complex, and the charge distribution in the coordinated ligand indicates an important change in the bonding situation: the formaldehyde ligand is negatively charged in the complex (-0.641, but the oxygen atom loses electronic charge upon coordination (-0.85 compared to -1.10 in free CHzO), while the carbon atom gains about 1 unit of negative charge and is virtually neutral in the complex. Apparently, the C - 0 bond is weakened to such an extent that the electron-attracting influence of the oxygen atom on the carbon has decreased markedly.

The calculated charge distribution shows that the ligand L always has a higher formal charge in WCLL than in W(CO)5L. It is noteworthy that the ethylene ligand is more negatively charged in 2a than the acetylene ligand in lb, while the opposite trend is found for 7a and 8a (Table 5). Taken together, the topological analyses of the electron density distribution indicate that the W-L bonds of the W(C0)sL complexes have more donor-acceptor character than the WC4L complexes. The electronic structure of the ligands L is much more distorted in the WCLL complexes than in W(C0)sL. Further insight into the bonding situation of the complexes is given by the results of the CDAaigcalculations. The CDA method considers the bonding in a complex in terms of (fragment) molecular orbital interactions between two closed-shell fragments. In the present case, the fragments are w(co)5and L for the W(C0)sL complexes and WC14 and L for the WC14L complexes. The mixing of the occupied orbitals of L and the unoccupied orbitals of W ( c 0 ) ~or W c 4 gives the electron donation d. The mixing of the unoccupied orbitals of L and the occupied orbitals of W(C0)5or wcl4 gives the back-donation b. The mixing of the occupied orbitals of the two fragments gives the repulsive polarization r. The CDA method can be used as a quantitative expression of the familiar Dewar-Chatt-Duncanson m ~ d e l . Previous ~,~ studies of M(CO)5L (M = Cr, Mo, W) and M(C0)3L complexes (M = Ni, Pd, Pt) with various ligands L have shown that the results of the CDA method are in agreement with qualitative bonding models of transition metal c o m p l e ~ e s . ~ ~ ~ ~ ~ Table 6 shows the CDA results for W(CO)5L. The amount of L M charge donation is always higher than the M L back-donation, even for W(CO)6. This should not be taken as evidence that the ligand metal charge donation is the dominant contribution to the bond energy. It is now generally agreed that the metal ligand back-donation from the filled metal d-orbitals into the n*-orbitals of CO (tzg in octahedral symmetry) contributes more t o the bond energy in Cr(C0)s than the OC Cr a-donation.60 There are also repulsive interactions between the occupied orbitals of the two fragments, which lead to negative values for the repulsive polarization r (Table 6). Negative values indicate that electronic charge is depleted from the overlapping

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(58)Ehlers, A. W.;Dapprich, S.; Vyboishchikov, S. F.; Frenking, G. Organometallics, in press. (59) Frenking, G.; Dapprich, S.; Ehlers, A. W.; Otto, M.; Vyboishchikov, S. F. In Metal-Ligand Interactions: Structure and Reactiuity; Russo, N., Salahub, D., Eds.; Proceedings of the NATO Advanced Study Institute, Cetraro, Italy, 1994,in press. (60)(a)Li, J.; Schreckenbach,G.; Ziegler, T. J . Am. Chem. SOC.1996, 117, 486. (b) Blomberg, M. R. A,; Siegbahn, P. E. M.; Lee, T. L.; Rendell, A. P.; Rice, J. E. J . Chem. Phys. 1991,95,5898.(c) Davidson, E. R.; Kunze, K. L.; Machado, F. B. C.; Chakravorty, S. J. Acc. Chem. Res. 1993,26,628 (and references cited therein).

Organometallics, Vol. 14, No. 11, 1995 5335

Side-On-BondedJc-Ligand Complexes area of the occupied orbitals. A detailed CDA study of W(C0)s has shown that most of the repulsive term r arises from the interactions between the occupied lone pair a-orbital of CO and the respective filled orbitals of tungsten.58 Consequently, the net effect of OC M donation upon the metal-ligand interactions is less stabilizing than the M CO back-donation, although the CDA shows a larger amount of OC M charge donation. However, this does not necessarily hold for other ligands L. For the C02 and CS2 complexes (9a and loa), the CDA shows that there is virtually only donation of electron density from the ligand to the metal and no back-donation M L, as can be expected for an end-on coordination of the ligand. Besides, the value of d is significantly larger for the CS2 complex than for the C02 complex (0.322 versus 0.126), in accordance with the higher bond dissociation energy for 10a than for 9a. The clearly larger donation d and repulsive polarization r of 10a than 9a is also in agreement with the Laplacian distribution of the complexes, which shows that the deformation of the electronic structure of the C02 ligand in the latter complex is much lower than that for CS2 in 10a (Figure 4). In W(C0)5(HCCH)(7a) and W(C0)5(C2H4)@a),significant values are found for the donation (0.297 and 0.225, respectively) as well as for the back-donation (0.165 and 0.148). In both cases the donation from the ligand t o the metal is markedly larger than the backdonation. This result seems t o be in opposition to the results of the topological analysis, which yielded negative charges for the ligands in the complexes. However, the repulsive term r also has an effect upon the charge distribution. The direction of this effect is not obvious from the calculated data. The data for W(CO)dCH20) ( l l a ) are intermediate between those of the acetylene and ethylene complexes. Apparently, the bonding situation is very similar in these three compounds. The comparison of the CDA results for the complexes 7a, 8a, and l l a with the results for W(C0)s clearly indicates that the carbonyl ligand is a better donor, as well as a better acceptor, than the ligands HCCH, C2H4, and CH20. Thus, the structurally observed small trans effect in these complexes (Figure 3) can be traced back to the poorer n-acceptor capacity of the ligands, rather than to their better a-donor ability. On the whole, the end-on-coordinated W(C0)sL complexes 9a and 10a are clearly distinguished from the side-on-coordinated complexes by the charge decomposition analysis. On the other hand, the results for the side-on-coordinated compounds 7a, 8a, and l l a are quite similar t o each other. The donation from the ligand to the metal is larger than the back-donation in each case. The CDA results for the WC4L complexes using closed-shell WC4 and L as interacting fragments are very interesting. The values for the donation and backdonation are nearly always negative, while the repulsive polarization has positive values. This would indicate that electronic charge is removed from the occupied/ unoccupied area and concentrated in the occupied/ occupied area. Obviously, this is a physically absurd result. The conclusion is that the electronic structure of WCl& cannot reasonably be explained by the DewarChatt-Duncanson model. Closed-shell fragments wcl4

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and L are not proper reference systems for the metalligand bonds in these complexes. This is in agreement with the information gained by the optimized geometries and the topological analysis of the electron density distribution. The bonding in WC4L should not be considered as donor-acceptor-type bonding. Rather, it is a normal covalent bond between open-shell fragments. WCld has a triplet ground state. The ligands L have a singlet ground state and a rather high excitation energy to the first open-shell electronic state. Because of the high excitation energy of the ligands L from the closedshell ground state t o the open-shell valence state, the net bonding of the complexes is rather low (Table 2). 3.3. Comparison of WCLL and W(C0)sL Complexes. The assessment of the interaction between a metal and its ligand as covalent or as of the donoracceptor type is an oflen-used method for the classification of transition metal compounds, which is very useful for an understanding of the reactivity of the molecules. However, this classification is not canonical, and it is necessary to formulate criteria for the presence of donor-acceptor or covalent bonds. These criteria might be structural in nature: in the case of a donor-acceptor interaction, the metal-ligand distance should be longer than in the covalent case, and the structure of the ligand should be changed less markedly compared to the nonbonded state. The theoretical investigation may provide some further electronic criteria: the characteristic values of the critical points in the gradient field of the electron density [Laplacian v2p(rt,),energy density H(~I,),and electron density p(rt,)l make possible the classification of the metal-ligand bond as a closed shell or as a shared interaction. In the same way, the comparison of the characteristic values of the critical points of the intraligand bonds with the values of the free ligand informs on the electronic changes within the ligand upon coordination. Covalent bond orders may also help for a classification: the bond orders in donoracceptor interactions can be expected to be significantly smaller than those in covalent bonds. The results of our calculations clearly indicate that the WC4L complexes should be classified as covalent, while the W(CO)5L complexes are of the donor-acceptor type. This follows from all criteria cited in this paper. However, we want to point out that for the donoracceptor complexes covalent contributions to the bonds are also found, as given by cyclic structures in the gradient field of the electron density (ring critical points) and by finite covalent metal-ligand bond orders. The CDA method appears t o be very useful to classify a transition metal compound as a donor-acceptor complex or as a covalent compound. The CDA results for a complex using properly chosen closed-shell fragments give positive (or nearly zero) values for the charge donation and back-donation and negative values for the repulsive polarization. The CDA results of a covalent compound show positive values for the repulsive polarization, which is a physically unreasonable result. A principal structural difference between WC4L and W(CO)5L complexes can be observed for the ligands C02 and CS2. In the WC4L complexes the ligands are sideon-n-coordinated, but for the W(CO)5L complexes an end-on coordination via an oxygen or sulfur lone pair is found. Presumably, this result can be traced back t o the different number of coordination sites available at

Pidun and Frenking


the two metal fragments. However, there might also be electronic reasons: the side-on coordination is better suited for the development of covalent bonds, while the end-on coordination is more favorable for a donation of electron density to the electron-deficient metal center. The different types of metal-ligand bonding in WCLL and W(C0)sL explain the puzzling result that the bond dissociation energies relative to the fragments in their electronic ground states are much lower for C4W-L than for (COkW-L (Tables 2 and 4; for L = acetylene the De values are similar), although the CLW-L bond lengths are much shorter than the (C0)sW-L distances (Figures 2 and 3). The covalent C4W-L bonds are formally formed from the triplet states of wc14 and L. The triplet states of L are much higher in energy than the singlet ground states. For example, the lowest lying (3B2)triplet state of acetylene, which has a cis-bent geometry, is calculated (CISDTPTZ2P//CISDPTZ2P)to be 82.6 kcal mol-' higher in energy than the ('Cp-) singlet ground state.61 By using this value and the calculated Cl4W-(HCCH) bond dissociation energy (34.4 kcal molp1, Table 2), the theoretically predicted tungstenethylene bond strength amounts to 117.0 kcal mol-l. This large value correlates with the short W-C bond length of l b (Figure 2). Thus, the metal-ligand bonds in the WC4L complexes clearly are stronger than those in the W(CO)aLcomplexes, but the corresponding bond dissociation energies relative to the fragments in their electronic ground states are rather low because the ligand L must first be excited into the triplet state in order to form the short covalent CLW-L bonds.

essential for the accurate description of transition metal donor-acceptor complexes. The calculations predict that the order of the metal-ligand bond strengths for W(C0)sL is L = CzH4 > HCCH > CHzO > CS2 > C02. For the WC4L complexes the order is L = HCCH > CH2O > CZH4 > CS2 > COz. The different orders of the bond strengths are explained by the nature of the metal-ligand interactions. The W-L bonds of the WC14L compounds are covalent bonds, and the molecules with L = HCCH, CzH4, CH20 should be considered as metallacycles. Since the carbon atoms of the metallacyclopropene are approximately sp2hybridized, while the carbon atoms of the metallacyclopropane are sp3hybridized, the former W-L bonds are stronger than the latter. The W-L bonds of the respective W(C0kL complexes are donor-acceptor bonds. Since ethylene is a better donor than acetylene, the (CO)sW-CzH4 bonds are stronger than the (C0)sW-HCCH bonds. For COz and CS2, the calculations predict a side-oncoordinated mode for the WC4L complexes and an endon-coordinated mode for the W(CO)5L complexes. The C-0 bond of the formaldehyde ligand is significantly lengthened in the WCL(CH20) complex, which can be considered as a snapshot along the formation of an oxocarbene complex. The analysis of the electronic structure supports the classification of the WC4L compounds as covalently bonded molecules and the W(C0)sLcomplexes as donoracceptor-type structures. In particular, the CDA results are qualitatively different for the two types of compounds.

4. Summary

Acknowledgment. This work has been supported by the Deutsche Forschungsgemeinschaft (SFB 260 and Graduiertenkolleg Metallorganische Chemie) and the Fonds der Chemischen Industrie. We acknowledge generous support and excellent service by the computer centers HRZ Marburg, HHLRZ Darmstadt, and HLRZ Julich. U.P. thanks the Studienstiftung des deutschen Volkes for a scholarship.

The equilibrium geometries and metal-ligand bond energies of the WC4L and W(C0)sLcomplexes predicted at the CCSD(T)/IIIIMPB/II level of theory are in very good agreement with available experimental data. The results show that the inclusion of correlation energy is (61)Yamaguchi, Y.; Vacek, G.; Schaefer, H. F., I11 Theor. Chim.Acta 1993,86, 97.

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Theoretical Studies of Organometallic Compounds. XIX. Complexes of

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