94
Organometallics 1986, 5, 94-98
distance is considerably longer, at 3.155 (10) A as would be expected for a Lewis acid-base interaction. The chemistry of the Cp*,Th(PR& complexes and other actinide analogues, in combination with a number of late-transition-metal fragments, is currently under investigation in our laboratory. Preliminary results, including the preparation of the above-mentioned nickel complex, (19)In this paper the periodic group notation is in accord with recent actions bv W A C and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111-3 and 13.)
indicate that a large number of bimetallic complexes can be successfullv " DreDared. _ Acknowledgment. This work was performed under the auspices of the u.s. Department of Energy and, in part, under the auspices of the Division of Chemical Energy Sciences, Office of Basic Energy Sciences, u.s. Department of Energy. Registry No. 1, 93943-04-5; 2, 98720-31-1; 3, 98720-32-2; Cp*ThClz,67506-88-1;LiPPh2,4541-02-0; LiPEt2, 19093-80-2; Lipcyzy 19966-81-5*
Supplementary Material Available: Tables of anisotropic parameters as
as
and
Of Observed
stn~cturefactors and isotropic tk"-nl Parameters (45 Pages). Ordering information is given on any current masthead page.
q2-Vinyl and q2-Ketenyl Ligands as Analogues of Four-Electron Alkyne Ligands Douglas C. Brower, Kurt R. Birdwhistell, and Joseph L. Templeton" W. R. Kenan, Jr. Laboratory, Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514 Received April 29, 1985
The donor and acceptor properties of [a2-HC=CH2]- and [$-HC=CO]- ligands resemble those of $-RC=CR ligands. In monomeric complexes requiring four-electron donation from an alkyne unit, the role of the rl electron pair of the alkyne can be fulfilled by related filled orbitals in [HC=CH2]- and [HC=CO]- (the charge provides a closed shell for the atoms in the free ligand). The two anionic Cz-based and r* functions of fragments also have empty n-acceptor orbitals which behave analogously to the T,,* alkynes and alkenes, respectively. Extended Huckel molecular orbital calculations suggest that in coordination spheres containing r-acids the orientation of a2-ketenylligands is determined by frontier orbital interactions, whereas the orientation of $-vinyl ligands is dictated largely by steric effects.
Introduction Comparing the electronic structures of new or unsymmetrical ligands with simpler or more thoroughly studied cases often aids in rationalizing molecular structures and predicting chemical reactivity;' this is the essence of Hoffmann's isolobal anal~gy.~ Metallacyclopropene complexes, 1 (also called $-vinyl derivatives or cyclic alkylidenes, see Scheme I), and metallacyclopropenone complexes, 2 (also called v2-ketenyl derivatives), are two examples of metal-stabilized organic fragments which have been prepared by metal-mediated ligand reactions. Conversion of metal-bound alkynes to q2-vinylligands has been accomplished by nucleophilic attack a t an alkyne carbon with hydride reagent^,^ thiols or thiolates? nitrogen5 and phosphorus6 reagents, and isonitriles.' $-Ketenyl ligands
Scheme I
a
b
0
2
Table I. Extended Hiickel Parameters exponents orbital
H
c2s
c2, 02,
(1)(a) Pettit, R.;Sugahara, H.; Wristers, J.; Merk, W. Discuss. Faraday SOC.1969,47,71.(b) Mango, F.D.; Schachtachneider, J. H. J. Am. Chem. SOC.1971,93, 1123. (c) KostiE, N. M.; Fenske, R. F. Organometallics 1982,I , 974. (d) Albright, T. A. Tetrahedron 1982,38,1339. (2)(a) Hoffmann, R.Angew. Chem., Znt. Ed. Engl. 1982,21,711.(b) Stone, F.G. A. Angew. Chem., Int. Ed. Engl. 1984,23,89and references therein. (3)(a) Green, M.;Norman, N. C.; Orpen, A. G. J. Am. Chem. SOC. 1981,103,1267. (b) Allen, S.R.; Green, M.; Orpen, A. G.; Williams, I. D. J. Chem. SOC.,Chem. Commun. 1982,826. (c) Allen, S.R.; Beevor, R. G.; Green, M.; Norman, N. C.; Orpen, A. G.; Williams, I. D. J . Chem. SOC.,Dalton Trans. 1985,435. (4)(a) Davidson, J. L.; Shiralian, M.; ManojloviE-Muir, L.; Muir, K. W. J. Chem. SOC.,Dalton Trans. 1984,2167. (b) Carlton, L.;Davidson, J. L.; Miller, J. C.; Muir, K. W.J. Chem. SOC.,Chem. Commun. 1984,Il. (c) Davidson, J. L. J. Chem. SOC.,Chem. Commun. 1979,597. (5)(a) Davidson, J. L.; Murray, I. E. P.; Preston, P. N.; Russo, M. V. J. Chem. SOC.,Dalton Trans. 1983,1783. (b) Davidson, J. L.; Murray, I. E. P.; Preston, P. N.; Russo, M. V.; ManojloviE-Muir, L.; Muir, K. W. J . Chem. SOC.,Chem. Commun. 1981,1059.
b
1
OZP
W5d w 6 ~ W 6 ~
s;
H(i,i),eV -13.6 -21.4 -11.4 -32.3 -14.8 -10.37 -8.26 -5.17
.i-2
1.3 1.625 1.625 2.275
2.275 4.982 (0.6685) 2.341 2.309
2.068 (0.5424)
have been prepared by coupling reactions between carbyne and carbonyl ligandsa and by rearrangement of 11'-ketenyl ligands after loss of another ligand.g We now show, (6)Davidson, J. L.;Wilson, W. F.;ManojloviE-Muir, L.; Muir, K. W. J. Organomet. Chem. 1983,254,C6. (7) (a) Davidson, J. L.; Vasapollo, G.; ManojloviE-Muir, L.; Muir, K. W. J. Chem. SOC.,Chem. Commun. 1982, 1025. (b) Morrow, J. R.; Tonker, T. L.; Templeton, J. L., manuscript in preparation. (8) (a) Kreissl, F. R.; Sieber, W. J.; Alt, H. G. Chem. Ber. 1984,117, 2527. (b) Fischer, E. 0.;Filippou, A. C.; Alt, H. G.; Ackermann, K. J. Organomet. Chem. 1983,254,C21. (c)Birdwhistell, K. R.; Tonker, T. L.; Templeton, J. L. J. A m . Chem. SOC.1985,107,4474.
0276-7333/86/2305-0094$01.50/0 0 1986 American Chemical Society
Organometallics, Vol. 5,
Analogues of F o u r - E l e c t r o n A l k y n e Ligands
ligand alkyne alkene vinyl ketenyl
carbonyl all
Table 11. Bond Distances linkage dist, 8,
c-c w-c c-c w-c c-c w-e, w-c, c-c c-0 w-e, w-c, c-0 w-c C-H W-H
1.324 2.008 1.374 2.247 1.432 1.950 2.300 1.300 1.262 1.997 2.175 1.180 1.910 1.090 1.800
No. 1, 1986 95
ref 13 13 14 13 3a 3a 3a 8c 8c 8c 8c
X
X
b
'b
8C
8C 14 a
Collman, J. P.; Hegedus, J,. S. "Principles and Applications of Organotransition Metal Chemistry"; University Science Books: Mill Valley, CA, 1980; Chapter 3.
Figure 1. Analogies between the 7~ systems of free acetylene, vinyl anion, ketenyl anion, and ethylene (cross indicates no analogy).
Computational Details The extended Huckel method was employed in this study.'OJ1 The parameters we used are summarized in Table I.12 Bond lengths are tabulated in Table 11. Metal-carbon distances for the alkene and alkyne calculations were derived from the X-ray diffraction structure (MA = maleic anhydride; of W(~2-MA)(92-PhCzH)(detc)z Ph = phenyl; detc = diethyl dithi~carbamate).~~ The olefin geometry was matched to a neutron diffraction structure of Zeise's salt (i.e., the angle between the normals to the H-C-H planes was 32').14 The H-C-C angle in the alkyne ligand was set at 135', similar to the angle between the ipso carbon of the phenyl ring and the distal alkyne carbon in the tungsten complex cited above. The bond distances and geometry of the ketenyl ligand were taken from the molecular structure of W(CO)(dppe)(detc)(C,C$-OC=CCH,Ph) (dppe = 1,2-bis(diphenylphosphino)ethane).% The 0-C-C angle was 154', and the C-C-H angle was 135'. The geometry and distances were idealized for the alkylidene fragment.3a The C,-C,-H angle was 130°, both C,-C,-H angles were 120°, and the H-C,-H angle was 114'. In model complexes, all cis hydride angles were fixed at 90°, and each Cz fragment was bound so that the z axis bisected the C-C bond. When a carbon monoxide ligand was used, it replaced the hydride bound along the x axis. Results and Discussion Molecular Orbitals of Cz-BasedFragments. Examination of planar vinyl and linear ketenyl fragments readily reveals frontier orbitals which, apart from degeneracies, are reminiscent of an alkyne. This is not surprising, since (9) (a) Uedelhoven, W.; Eberl, K.; Kreissl, F. R. Chem. Ber. 1979,112, 3376. (b) Kreissl, F. R.; Friedrich, P.; Huttner, G. Angew. Chem.,Int. Ed. Engl. 1977,16, 102. (10)Hoffmann, R.J. Chem. Phys. 1963,39,1397. (11)We wish to thank Professor Hoffmann for providing us with copies of programs ICONS and FMO. (12)KubEek, P.; Hoffmann, R. J.Am. Chem. SOC.1981,103,4320. (13)Morrow, J. R.;Tonker, T. L.; Templeton,J. L. J.Am. Chem. SOC., in press. (14)Love, R. A.; Koetzle, T. F.; Williams, G. J. B.; Andrews, L. C.; Bau, R. Inorg. Chem. 1975,14, 2653.
6 -
-71
through extended Huckel calculations, that these Cz-based ligands [HC=CHz]- and [HC=CO]-, resemble acetylene in their interactions with a single metal center. This analogy has also been recently elaborated for $-vinyl ligands by Allen et al.3c
"a
-
12
"a
-
"d
It
-
4
Figure 2. Orbital energy diagram for distorted ligands readyfor-bonding. (Left-hand labels for acetylene and ethylene follow conventional notations. Right-hand labels, in parentheses, use t h e same system as Figure 1 and Table 111.)
Table 111. Distribution of Density ( % ) in Frontier Orbitals of the Fragments "d -
ligand alkyne ketenyl vinyl
C, 50 59 92
C, 50 24 0
"a
U
C, 50 67 54
C, 50 20 45
C, 44 26 39
C, 44 58 55
6 -
C,
C,
50 38
50 56
the T systems of these C2moieties have much in common. The basic idea is illustrated in Figure 1 where the oribtal correlation among acetylene, vinyl anion, and ketenyl anion is set forth with ethylene included for reference purposes. The labels refer to the manner in which the orbitals would interact with a metal center when bound through both carbon atoms. The frontier orbitals change in both shape and energy as the substituents on these Cz linkages are bent back in preparation for coordination to a transition metal, and though the extent of orbital restructuring depends on the degree of distortion, the essential analogy is retained (Figure 2 and Table 111). Note that well-known C3 organic rings result from combining these bent Cz fragments with [CHI+: cyclopropenium ion (i), cyclopropene (ii), and cyclopropenone (iii). The methyne cation-with two electrons, one u orbital, and two T
96 Organometallics, Vol. 5, No. 1, 1986
Brower et al.
Scheme I1
'1
alkyne compki 1
I
H
Scheme I11
\
Figure 3. Energy level diagram for [H5W(q2-H2C=CHJ3-and
"\H
H
H '
[H5W(q2-HCsCH)]3-.
2c
2a
orbitals-is suggestive of MLSd4fragments, which can also present two electrons, one u and two d a orbitals to incoming ligands. Y
H
For [H2C==CH]-and [O=C==CH]-, the orbitals labeled in Figure 2 correlate with the alland all*orbitals of HCICH and a and ?T* of H2C=CH2. These orbitals account for u donation to the metal and a acceptance from the metal as found in the Dewar-Chatt-Duncanson description of metal-olefin bonding.15J6 Note that +vinyl or ql-ketenyl ligands use the orbital labeled a d for forming their lone metal-carbon u bond in the plane of the HCzX fragment (see 3 and 4). u and a,
H
k
n 3
4
Numerous alkyne complexes are known in which the filled a,. orbital provides substantial electron density to a vacant metal d a orbital." This occupied alkyne orbital has vinyl and ketenyl anion counterparts (Figure 2) which set these Cz moieties apart from olefins since the olefins have no analogous filled orbital. The presence of an additional pair of electrons for metal complexation can be (15) (a) Dewar, M. J. S. Bull. SOC. Chim. Fr. 1951,18,C71. (b) Chatt, J.; Duncanson, L. A. J. Chem. SOC. 1953,2939. (16)(a) Albright, T. A.; Hoffmann, R.; Thibeault, J. C.; Thorn, D. L. J . Am. Chem. SOC. 1979, 101,3801. (b) Tatsumi, K.; Hoffmann, R.; Templeton, J. L. Inorg. Chem. 1982,21, 466. (17) (a) King, R. B. Inorg. Chem. 1968, 7, 1044. (b) Ward, B. C.; Templeton, J. L. J. Am. Chem. SOC. 1980,102, 1532. (c) Alt, H. G.; Hayen, H. I. Angew. Chem. Suppl. 1983, 1364. (d) Theopold, K. H.; Holmes, S. J.; Schrock, R. R. Angew. Chem. Suppl. 1983, 1409. (e) Cotton, F.A.; Hall, W. T. J.Am. Chem. SOC. 1979,201,5094.(0 Ricard, L.;Weiss, R.; Newton, W. E.; Chen, G. J.-J.; McDonald, J. W. J. Am. Chem. SOC. 1978,100,1318.(8) Davidson, J. L.; Green, M.; Stone, F. G. A.; Welch, A. J. J. Chem. SOC.,Dalton Trans. 1976,738. (h) Winston, P.B.; Burgmayer, S. J. N.; Templeton, J. L. Organometallics 1983,2,167.
i
Figure 4. Energy level diagram for [H5W(q2-H2C=CH)I4-and [H5W(q2-HC=C0)]".
visualized for the q2-vinylcase by imagining deprotonation of a metal-ethylene fragment (Scheme 11). The metallacyclopropane valence bond picture then leads to a carbanion which can be rehybridized to sp2 while the MCH2-C geometry is retained. Feeding the C, p electrons to the metal completes the picture. Structural results reveal that the C, substituents are located above and below the MC2 plane as e ~ p e c t e d . ~ ~ The isolobal analogy between an $-ketenyl ligand and an alkyne ligand is emphasized in valence bond representation 2c (Scheme 111). The synthetic relationship between the two confirms their kinship. As with cyclopropenone itself,le the oxygen atom is nucleophilic and prot~nation'~ or a1kylation2Oreadily converts q2-ketenyl ligands to hydroxyalkynes or alkoxyalkynes. Valence bond pictures l b and 2b (Scheme I) do not require the metal to donate a pair of electrons to a vacant ligand orbital. We expect back donation into a, to be sufficiently important, however, that $-vinyl and q2ketenyl complexes will be isolated only when a metal (18) Potts, K. T.; Baum, J. S. Chem. Reu. 1974,74,189. (19)(a) Jeffery, J. C.; Laurie, J. C. V.; Moore, I.; Stone, F. G. A. J . Organomet. Chem. 1983,258,C37. (b) Reference 8c. (20) (a) Kreissl, F. R.; Sieber, W.; Wolfgruber,, M. Angew. Chem., Int. Ed. Engl. 1983,22,493. (b) Reference 8c.
Organometallics, Vol. 5, No. 1, 1986 97
Analogues of Four-Electron Alkyne Ligands electron pair is available. Structural and 13CNMR data support the assignment of carbenoid character to C,, as Davidson, Muir, and co-workers have recently pointed These two metal electrons merge with the four ligand-based electrons to fill three bonding combinations which result from the interaction of the C2 ligand with a metal fragment (see next section). Model Octahedral Complexes with H C W H , [HC=CH2]-, and [HC=CO]- Ligands. The primary bonding features of these C2-basedligands in octahedral d4metal complexes can be illustrated by attaching them to the hypothetical [H5WI3-fragment. Coordination of ethylene to the [H5Wl3-moiety is presented (Figure 3) as a reference point. The [HSW(q2-HC=CH)l3-diagram, also presented in Figure 3, sets the course which is roughly followed by the molecular orbitals of the metallacyclopropene and [v2-HC=CO]- complexes (Figure 4). The filled IJ orbital interacts strongly with the dpdominated metal acceptor orbital in each case. The vacant aa orbital overlaps, and hence stabilizes, the metal d,, orbital in the coordinate system shown. Donation from a d destabilizes dyzand completes the analogy between q2-vinyls, q2-ketenyls, and four-electron-donor alkyne ligands. Although there are four orbitals present in the free alkyne a system, the a L * orbital is not a crucial component of transition metal-alkyne bonding. It is of 6 symmetry relative to the metal-C2 linkage, and it interacts only weakly with d,,, so three orbitals are adequate for constructing effective isolobal ligand relationships with alkynes.21 The calculated energies of the three metal d a orbitals, the set of true octahedra, provide a crude probe of the simultaneous a-donor and a-acceptor strength of these C2 derivatives toward a metal center. A d4 configuration, common for alkyne c~mplexes,~' reflects the divergence of the d a levels as bonding, nonbonding and antibonding. When the incoming C2ligand approaches along the z axis, the d,, orbital, virtually unchanged in the environment of five a-innocent hydride ligands, is the HOMOl for these model d4 compounds. It is only slightly perturbed by C2 &type orbital combinations. The HOMOl - LUMO gap between d, and d,, reflects the C2a-donor strength while the HOMdl - HOMO2 difference between d,, and d,, depends on the a-acidity of the C2 ligand. Some experimental data relevant to the HOMO1-LUMO energy var(M = Mo, W) iation for M(C0)(~~-alkyne)(S~CNR~)~ compounds as a function of the alkyne ligand substituents has been provided by electronic spectra and electrochemical measurements.22 Our calculations indicate that a d destabilization of d,, produces HOMOl (d,) - LUMO (d,,) separations of 1.33, 1.10, and 0.91 eV for [HC=CH2]-, HC=CH, and [HC= COJ-, respectively. The superior strength of the vinyl anion as a a donor is in accord with the substantially higher energy of a d as seen in Figure 2. The energies of the a d orbitals of acetylene and the ketenyl fragment are much closer to one another, with the ketenyl a d orbital slightly higher in energy (Figure 2). The lesser a-donor ability of the ketenyl fragment reflects poorer overlap of a d with the metal and competing delocalization of the C, lone pair into the C=O system. These substantial HOMOl - LUMO gaps, all approximately 1 eV, are invariably increased by replacing H- by a a-acid ligand which stabilizes the d,, HOMOl orbital. In contrast, the absence ~
(21) Templeton, J. L.; Winston, P. B.; Ward, B. C. J.Am. Chem. SOC. 1981,103, 7713.
(22) Templeton, J. L.; Herrick, R. S.; Morrow, J. R. Organometallics 1984, 3, 535.
1
1d
0.8 0.6
eV 0.4
0.2
I
0
45
90
135
180
225
270
315
360
0 , degrees
Figure 5. Rotational profiles for [H4(C0)W(v2-HC=CO)l3-, [H,(CO)W(T~-HC=CH)]~-, and [H4(C0)W(v2-H,C=CH)]*. The zero of energy is arbitrary.
of a "d orbital on ethylene produces a corresponding gap of only 0.15 eV in the [H5W(q2-H2C=CH2)I3reference complex. The HOMO2 (dz,) - HOMOl (d,) energy differences of 1.36, 1.31, and 0.72 eV for HCGCH, [HC=CHz]-, and [HC=CO]- indicate significant back-donation from metal to ligand in each case. The strong aa interactions of the alkyne and vinyl ligands result from the proximity in energy of the acceptor orbitals to the d a levels of the metal and from the hybridization of a, which directs the carbon atomic orbitals involved toward the metal. Rotational Properties of HC=tH, [HC=CH2]-, and [HC=CO]- Ligands. Because the d a orbitals in the [H5WI3-fragment are degenerate, the energy of octahedral complexes will be nearly independent of the rotational orientation of the sixth ligand. Linear combinations of d,, and dyzwill fulfii the roles of metal a-acceptor and a-donor orbitals for every rotamer. Placing a cylindrically symmetrical a-acid in the xy plane will break the degeneracy of d,, and d,, and lead to rotational preferences, as seen for alkyne ligands in d4 cis-L4M(CO)(q2-alkyne)complexes.23 Related analyses for d2 metal complexes of group 6 with cylindrically symmetrical a donors have been Wpresented (e.g., (~-C~H~)Mo(O)(SR)(v~-alkyne),~~ (0)(q2-alkyne)(S2CNR2)2)25, and W(S)(v2-alkyne)(S2CNR2)226). I t is observed in W(CO)(dppe)(detc)(C,C-v2-OC= CCH2Ph) and in other structurally characterized monocarbonyl ketenyl c ~ m p l e x e s that * ~ ~the ~ ~C2 moiety is oriented with C, proximal to CO and the C,-C, bond lies parallel to the M-carbonyl axis, i.e., 8 = 0". The C2 rotational energy profile for ~~S-[H~(CO)W(~~-HC=CO)]~indeed has a global minimum at 6 = 0" (Figure 5). The contour is reminiscentof the one obtained for the q2-alkyne case, but the local minimum at 6 = 180" for the ketenyl ligand, with C, proximal to CO, lies more than 10 kcal mol-l, above the ground-state orientation. The destabilization at 6 = 180" results from cooperative steric and electronic effects. Congestion is induced by placing C, next (23) Rotational barriers for the alkyne ligand in a number of such complexes have been determined by NMR spectroscopy: (a) Allen, S. R.; Baker, P. K.; Barnes, S. G.; Green, M.; Trollope, L.; ManojloviE-Muir, L.; Muir, K. W. J. Chem. SOC.,Dalton Trans. 1981,873. (b) Reference 17b. (c) Winston, P. B.; Burgmayer, S. J. N.; Templeton, J. L. Organometallics, accepted for publication. (24) Howard, J. A. K.; Stansfield, R. F. D.; Woodward, P. J. Chem. SOC.,Dalton Trans. 1976, 246. (25) (a) Templeton, J. L.; Ward, B. C.; Chen, G. J.-J.; McDonald, J. W.; Newton, W. E. Inorg. Chem. 1981, 20, 1248. (b) Newton, W. E.; McDonald, J. W.; Corbin, J. L.; Ricard, L.; Weiss, R. Inorg. Chem. 1980,
19. 1997. (26) Morrow, J. R.; Tonker, T. L.; Templeton, J. L. Organometallics 1985, 4, 745.
98 Organometallics, Vol. 5, No. 1, 1986
Brower et al.
to the carbonyl (the distance between C, and Cco is small, 2.30 A); moreover, the total energy of the system drops when when the carbonyl ligand is bent away from the v2-ketenylmoiety, although the d a orbitals are destabilized by this distortion. When 0 = O', the CB-Cco distance is 2.41 A and the total energy increases when the carbonyl is bent away, consistent with partial destruction of the three-center two-electron bond formed between d,,, CO P*, and ketenyl a, orbitals. In addition to stabilizing d,, and d,,, the carbonyl ligand in [H4(CO)WI2-causes the metal to hybridize these orbitals by mixing metal p and pz character into the d functions. Hoffmann and Kuiacek recently utilized this phenomenon in rationalizing the geometries of distorted-octahedral d4 complexes of Mo(I1) and W(1I).l2 For the metal fragment treated here, d,, is shaped so that more orbital density is located toward the carbonyl than away from it (see iv below). The overlap of the a, orbital of the ketenyl fragment with this filled d orbital equals 0.205 when 0 = 0" and 0.193 when 0 = 180' (see v). The reason for this difference lies in the asymmetry of xa, which has twice as much orbital density on C, as on C, (see v), leading to superior overlap when C, lies over the enlarged lobe of dxz.
v
IV
The shape of the LUMO in [H4(CO)WI2-,the d,z orbital, is also important in setting the geometric preference of the incoming ketenyl moiety. In the all-H metal fragment this orbital is aligned with the z axis and hybridized toward the vacant octahedral coordination site. Replacing H- with CO along the x axis tilts this orbital, through an admixture of d,,, away from the carbonyl (see vi). Since the a-donor orbital of the ketenyl ligand is heavily weighted toward C, (see Table I11 and vii), the 0 = Oo orientation allows the greatest interaction between u and d,2. The relevant overlap integral is 0.290 when 0 = 0' and 0.240 when 0 = 180'. n
z
L,
*Wo W
VI1
The d,, orbital is unaffected in shape and energy when CO replaces H-along the x axis, and therefore interactions of dyzwith Pd are equivalent a t 0 = 0' and 180'; indeed the dyzLUMO is only slightly stabilized (0.006 eV) when 0 = 0'. The d interaction with d,, is not significant. Overall the calculations suggest that rotation of the q2ketenyl ligand may be accessible on the NMR time scale with a barrier near 20 kcal mol-l, but since only the 0 =
0' isomer will ever by sufficiently populated for NMR observation, no rotational process will be observable in these systems. The rotational profile for the &vinyl case also has a global minimum a t 0 = 0" (Figure 51, but while the orientation at 0 = 180" is an energy minimum for the alkyne and ketenyl complexes, it is a maximum for [H4(CO)W(v2-HCCH2l3-with a trough near 0 = 120" and another maximum at 0 = 60". This rotational behavior results from competing steric and electronic effects. As in the ketenyl case, steric crowding is introduced by locating C, proximal to CO in the model complex: the carbon-carbon distance is extremely short, 2.19 A. The frontier orbitals of the vinyl fragment are similar to those of the ketenyl moiety, since x, has greater density on C, and u is weighted toward C,, but there is much less polarization (Table 111). Furthermore, there is a greater difference in metal-carbon distances in the model v2-vinyl complex compared to the ketenyl case (Table 11). These factors are reflected in the overlap integrals for the frontier orbitals. That x , overlap with d,, is better when 0 = 180' seems strange given the shapes of these orbitals, but C, is so close to the metal that it brings its share of a, into greater contact with d,, than does C,. The overlap between u and dZzis better when 0 = O", as in the ketenyl case, but the difference is smaller (0.269 a t 0' and 0.235 at 180'). This analysis shows that steric crowding and relative metal-carbon distances should be important in determining the geometries of real q2-vinylcomplexes. In fact, distances and geometries vary widely for $-vinyl complexes. Tungsten-CB distances vary by about 0.15 A for coma series of [ (a-C5H5)W(F3CC=CCF,)(q2-vinyl)L] plexes, depending on the nature of L (chloride or thiolate) and the identity of the nucleophile employed to generate the v2-vinylmoiety (phosphine or i ~ o n i t r i l e ) The . ~ ~ variation in W-C, is smaller, about 0.07 A, in this series. There is also a startling variety of orientations for the $-vinyl ligands. In (a-C5H5)Mo(F,CCrCCF3)(q2-F3CC=CCF3(PEt3))C16the v2-vinyl ligand is parallel to the alkyne, analogous to the geometry adopted by the alkyne ligands in the tungsten bis(a1kyne) precursor complex.27 In (PC5H5)W(F,CC=CCFJ($-F,CC=CCF,(CNBu))Cl, however, the v2-vinyl ligand is nearly perpendicular to the alkyne.7a Finally, the T2-vinyl moiety in (a-C5H5)W(C0)2(q2-F,CC=CCF3(COSMe) lies about 30" away from the best orientation for W-C, x bonding.4a These structural results are compatible with the soft energy surface, spanning less than 10 kcal mol-l, calculated for rotation of the $-vinyl ligand in our model system. Similar conclusions were drawn by Allen et al., who showed that $-vinyl rotation is facile in ( P - C ~ H ~ ) M O ( P ( O C H ~ ) ~ ) ~ ( ~ * vinyl) complexes.3c
Acknowledgment. This work was generously supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society. We wish to thank Dr. D. M. P. Mingos for helpful discussions of this work. Registry No. W($-MA)(v2-PhCzH)(detc),, 98735-53-6; W-
(CO)(dppe)(detc)(c,c-vz-OC=CCH2Ph), 96454-70-5. (27) Davidson, J. L.; Green, M.; Stone, F.G. A,; Welch, A. J. J. Chem. Soc., Dalton Trans. 1976, 738.
