Organometallics 1984, 3, 1563-1573
1563
Metathesis of Acetylenes by (Fluoroalkoxy)tungstenacyclobutadiene Complexes and the A Higher Order Crystal Structure of W(C3Et3)[0CH(CF,),],. Mechanism for Acetylene Metathesis John H. Freudenberger,” Richard R. Schrock,” laMelvyn Rowen Churchill, Arnold L. Rheingold,lc and Joseph W. Zillerlb
Ib
Departments of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, and State University of New York at Buffalo, Buffalo, New York 14214 Received February 14, 1984
The reaction of 3 equiv of LiOR (R = CH(CF3)Z)or LiOR’ (R’ = CMe(CF3)2)with W(CCMe3)(dme)C13 yields W(CCMe3)(OR)3(dme) and W(CCMe&(OR’),(dme), respectively (dme = 172-dimethoxyethane).Each reacts with disubstituted acetylenes (R”C=CR”) to give tungstenacylobutadiene complexes W(C3R”3)(0R)3 and W(C,R”,)(OR’),. Both tungstenacycles are catalysts for the metathesis of disubstituted acetylenes. W(C,Et&(OR’), metathesizes acetylenes extremely rapidly by a dissociative mechanism, i.e., via formation of putative W(CEt)(OR’),. W(C3Et3)(OR),,on the other hand, slowly metathesizes acetylenes by an associative mechanism in relatively noncoordinating solvents (pentane, benzene, or toluene; AH* = 14.4 (6) kcal mol-’; A S = -22.8 (2) eu). In diethyl ether or in the presence of dme the rate of metathesis is much higher due (it is postulated) to the solvent assisted dissociation of an acetylene from the WC3 ring to give W(CEt)(OR),(ether), (3c = 1 or 2) or W(CEt)(OR),(dme)intermediates. W(C3Et3)[OCH(CF,),], crystallizes in the triclinic space group Pi [No. 21 with a = 9.949 (3) A, b = 16.419 (4) A, c = 18.485 (4) A, (Y = 112.81 ( 2 ) O , 0 = 93.53 (6)O, y = 98.60 (2)O,and 2 = 4. Diffraction data were refined to RF = 8.00% and RwF = 7.78% for 511 parameters refiied against all 6439 unique data and RF = 5.04% and RwF = 5.55% for those 4540 data with lFol > 3@‘,I). Two independent molecules define the crystallographic asymmetric unit. Each is a trigonal bipyramid containing a planar WC3ring system in the equatorial plane. The two molecules differ primarily in the orientation of the P-ethyl group. In one molecule the W-C, bond lengths are 1.902 (16) and 1.864 (14) A with W.-C,.= 2.093 (14) A and C,-C, = 1.429 (18) and 1.437 (21) A. Axial and equatorial alkoxide ligands show similar W-0-C angles of 129.4 (9), 133.6 (8), and 138.6 (l0)O and W-O distances of 1.982 (ll), 1.962 (12), and 1.932 (10) A, consistent with a poor 7-electron-donatingability by the hexafluoroisopropoxide ligand.
Introduction In the preceding paper2 we explore the fine balance between a tungstenacyclobutadiene complex and an alkylidyne complex formed by loss of an acetylene from the WC3 ring. We proposed that steric bulk of the phenoxide ligands, steric bulk of the metallacycle’s a substituents, and donation of 7-electron density to the metal by the equatorial phenoxide ligand all lead to destabilization of the metallacyclic ring, i.e., ejection of an acetylene. Our search for new examples of tungstenacyclobutadiene complexes and our interest in further understanding the factors that determine whether a system will catalyze acetylene metathesis or not led us to study fluoroalkoxidecomplexes. In this paper we report the preparation of tungsten alkylidyne and tungstenacyclobutadiene complexes containing hexafluoroisopropoxide and hexafluoro-tert-butoxide ligands and their reactions with simple disubstituted acetylenes. We will show that both electronic and steric effects subtly control the metathesis reaction and, furthermore, that a mechanism significantly different from that shown to be operative in the phenoxide system2obtains in the hexafluoroisopropoxide system.
Results Preparation of W(CCMe3)[OCH(CF,),],(dme) and W (C,Et,) [OCH( CF3)2]3. Treating W(CCMe,) (dme)C13, (1) (a) Massachusetts Institute of Technology. (b) State University of New York at Buffalo. (c) University of Delaware. (2) Churchill, M. R.; Ziller, J. W.; Freudenberger, J. H.; Schrock, R R., preceding paper in this issue.
with 3 equiv of lithium hexafluoroisopropoxide in ether yields a bright yellow, crystalline complex of composition W(CCMe3)[OCH(CF3)2]3(dme) (1) (dme = 1,2-dimethoxyethane). Below 0 “C the ‘HNMR spectrum of 1 is consistent with the mer octahedral structure shown in eq 1. At 25 OC the signals for coordinated dme consist of
+
+
I
Me0’i”OR
three broad resonances that do not change upon adding dme to the sample, and the signals for the added dme remain sharp, consistent with an intramolecular exchange of the two ends of the dme ligand. The most likely intermediate is the fuc isomer. Although dissociation of one end of the dme ligand to give the TBP complex shown in eq 1 is reasonable-TBP monoadducts of W(CCMe3)q1 process (OCMe3), are k n 0 ~ n -this ~5 or any related alone does not exchange the two ends of the dme ligand. At 100 “C coordinated dme does rapidly exchange with free dme. In view of the highly electrophilicnature of the metal center the intermolecular exchange process most likely consists of attack by dme on a five-coordinate species (e.g., that shown in eq 1) to give intermediate fuc-W-
-
(3) Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. A.; Rocklage, s. M.; Pedersen, S. F. Organometallics 1982, 1, 1645. -(4) Listemann, M. L., unpublished results. (5)Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. Inorg. Chem. 1983,22, 2903.
0276-733318412303-1563$01.50/0 0 1984 American Chemical Society
Freudenberger et al.
1564 Organometallics, Vol. 3, No. 10, 1984 (CCMe,)(OR),(dme),rather than by complete loss of dme to give intermediate W(CCMe,)(OR),. The fact that 1 sublimes cleanly at 60 "C (0.1 pm) with dme intact is good evidence that W(CCMe,)(OR,) is not readily formed. These qualitative results suggest that at least one metal coordination site in 1 is likely to be made available at some rapid rate on the chemical time scale at 25 "C and that an acetylene ligand could in principle compete with the dimethoxyethane ligand for that site. One concern about the six-coordinate structure proposed in eq 1is the fact that only one 19FNMR signal is observed at 0 "C. We suspected this was accidental. In a bis(pyridine) adduct, prepared straightforwardly from 1 and pyridine, the pyridine ligands are inequivalent at 25 O C , the hexafluoroisopropoxides'methyne proton resonances appear as two overlapping septets of ratio 2:1, and the 19F spectrum (84.26 MHz) shows two signals in a ratio of 2:l separated by only 0.9 ppm. Under ideal conditions three 19Fpeaks actually should be observed in a ratio of 1:l:l since the CF, groups in the two mutually trans hexafluoroisopropoxide ligands should be diastereotopic. Therefore we feel our suspicion was correct; in spite of the large chemical shift differences generally observed in 19F NMR spectra, peaks for the different CF3 groups in hexafluoroisopropoxide ligand(s) are often accidentally coincident. It should be noted that the reaction between W(CCMe3)(dme)C13and 3 equiv of LiOCHMe, produces a dme-free species with the formulation W(CCMe3)(OCHMe2)3.6 NMR studies at low temperature suggest that this complex is dimeric, probably with a structure analogous to that of [W(CMe)(OCMe3)3]2.5Since the hexafluoroisopropoxideligand is a much poorer base than the isopropoxide ligand and for this reason also less likely to bridge between metals, the fact that dimethoxyethane is retained in 1 is not altogether surprising. W(CCMe,)[OCH(CF,),J,(dme)(1) reacts readily with 3-hexyne to produce a mixture of two metallacyclobutadiene complexes (eq 2). The relative amount of each W(CCMe3)(OR)3(dme)
3- HEXYNE
(RO)3wb
(RO)&V@
(2)
PENTANE
R = CH(CF,),
2
3
spectrum of the residue in C6D6is consistent with a mixture of many, if not all, possible metallacycles. (The spectrum is too complicated for exact analysis.) We propose that ether is a good enough donor to "displace" acetylene from the WC3 ring to form small amounts of complexes of the type W(CR)[OCH(CF,),],(ether),( x = 1 or 2), which then react with any available acetylene to give (ultimately) all possible metallacycles. A result that supports this proposal is shown in eq 3 (see Experimental
E'
/
(3) R = CH(CFd2
Section for details). Therefore it is not surprising that 2 is not stable in ether. After 1 h a mixture containing -20% 3 is obtained. The remainder consists largely of 2 and a small amount of an unidentified tert-butyl-containing material. Hexafluoro- tert -butoxide Complexes. W (CCMe,)[OCMe(CF3)2]3(dme)( 5 ) can be prepared by treating W(CCMe3)(dme)C13with 3 equiv of lithium hexafluorotert-butoxide in ether. In c&D,5 appears to have the same structure as 1. A significant difference, however, is that the ends of the dme exchange more readily intramolecularly (the coalescence temperature for the intramolecular exchange process in 5 is -5 "C compared to -45 "C in 1) and coordinated dme exchanges more readily with free dme (exchange is rapid at 60 "C in 5 vs. -100 "C in 1). One could argue that increased steric crowding by the hexafluoro-tert-butoxideligands favors formation of a five-coordinate species containing +dme and that both exchange processes are for that reason faster. In spite of sterically induced greater lability of one end of the dme ligand in 5 , it, like 1, sublimes with the dme ligand intact at 60 "C and 0.1 pm. The reaction of 5 with pyridine also gives a bis(pyridine) adduct in which the two pyridine ligands are inequivalent and two types of alkoxide ligands are observed (cf. eq 3). If 2-8 equiv of 2-butyne are added to a solution of 5 in pentane, W(CMe)[OCMe(CF,),J,(dme)can be isolated in good yield. If, however, 20-30 equiv are used, W(C3Me3)[OCMe(CF,),], crystallizes from solution. This result can be explained in terms of the equilibrium in eq 4. Surprisingly, a similar reaction between 5 and 3-hexyne
metallacycle depends upon the quantity of 3-hexyne added. The triethyl metallacycle 3 can be prepared in pure form by selectively crystallizing it from a pentane solution containing a large excess of 3-hexyne (see Experimental Section). The a-tert-butyl metallacycle 2 cannot be obtained in pure form by this method; some 3 is formed even when only 1 equiv of 3-hexyne is added to 1. However, 2 can be prepared by reacting W[C(CMe,)C(Et)C(Et)]Cl: with 3 equiv of lithium hexafluoroisopropoxidein toluene. In a reaction analogous to that shown in eq 2, W(C3Pr3)[OCH(CF3),], (4) can be prepared from 1 and excess 4-octyne. It is important to note that the C,CH2CH,CH3 signal in 4 is a normal 1:2:1 triplet, not a second-order as a repattern as it was in W(C3Pr3)[0-2,6-C6H3(i-Pr)z]3 sult of restricted rotation of the C,-propyl group.2 No mixed (propyl/ethyl) metallacycles are formed when 3 and 4 are combined in benzene. We conclude that in benzene 3 and 4 do not readily lose 3-hexyne or 4-octyne, respectively. However, when 3 and 4 are dissolved in ether and then all solvent is removed in vacuo, an 'H NMR
failed to yield the analogous triethyl metallacycle 7 even when 40 equiv of 3-hexyne were used only W(CEt)[OCMe(CF,),],(dme) could be isolated. We propose that for steric reasons alone 3-hexyne simply cannot compete as successfully with dme as 2-butyne can. W(C3Et3)[OCMe(CF,),], can be prepared by treating 6 with an excess of 3-hexyne in pentane (eq 5, OR = OCMe(CF3)2).
(6)Pedersen, S. F. Ph.D. Thesis, Massacusetta Institute of Technology, 1983. (7) Schrock, R. R.; Pedersen, S. F.; Churchill, M. R.; Ziller, J. W., following paper in this tissue.
Likewise 6 and 7 yield complexes of the type W(CR)[OCMe(CF3)2]3(THF)2 upon dissolving them in THF and removing all volatile componentsin vacuo. Peculiarly, both
t2-butyne-dme W ( CMel (OR13(dme) R=
CMe(CF3)2
4-
(R0)jW.O
- 2-butyne + dme
7 6
(4)
(5) R
CMe(CFd2
As expected, addition of 1equiv of dme to 7 in C6D6yields W(CEt)[OCMe(CF3),J3(dme) quantitatively (by 'H NMR).
Organometallics, Vol. 3, No. 10, 1984 1565
Tungstenacyclobutadiene Complexes Table I. Kinetic Data for the Reaction between W( C Et )[ OCH(CF ),I and 3-Hexyne-d ,” -4
314.2 307.7 304.7 295.0 295.0 295.0 278.0
0.366 i 0.366 f 0.366 i 0.366 f 0.630 i 0.982 -f 0.366 t
3% 3% 3% 3% 3% 3% 3%
24.7 (10) 12.9 ( 5 ) 11.2 ( 4 ) 4.12 (12) 6.75 (18) 10.4 ( 3 ) 1.07 ( 4 )
t
68 ( 3 ) 35.2 (17) 30.6 (14) 11.3 ( 5 ) 10.7 ( 4 ) 10.6 ( 4 ) 2.92 (14)
Table 11. Interatomic Distances ( A ) for W(C,Et,)[OCWCF,),l, 3a 3b
1 31
Distances about 1.982 (11) 1.932 (10) 1.962 (12) 1.902 (16) 2.093 (14) 1.864 ( 1 4 )
the Tungstem Atoms W(2)-0(4) 1.970 (11) W(2)-0(5) 1.934 (10) W(2)-0(6) 1.969 (11) W( 2)-C( 13) 1.885 (15) W(2)-C(14) 2.100 (15) W(2)-C(15) 1.860 (17)
B) Distances within C,Et, Ligands 1.429 (18) C(13)-C(14) C(1)-C(2) 1.437 (21) C(14)-C(15) c(2)-c(3) c(i)-c(iA) 1.499 ( 2 5 j c ( i 3 j ~ j i 3 A ) C(lA)-C(lB) 1.491 (26) C(13A)-C(13B) C(2)-C(2A) 1.548 (22) C ( 1 4 ) 4 ( 1 4 A ) C(2A)-C(2B) 1.575 (30) C(14A)-C(14B) C(3)-C(3A) 1.509 (21) C ( 1 5 ) 4 ( 1 5 A ) n , n * \
n,nn\
L(aAJ-b(aISJ
r r n
, n r \
1.441 (34)
n
8
-
r
A
\
n / , r n \
L(lOAJ-b(lOD)
1
32
1
1
33
34
&.
1
35
1
36
104
Figure 1. Arrhenius plot for the reaction of W(C3Et3)[0CH(CF&I3 with 3-hexyne-dlp
1.348 (27) 1.492 (23) 1.567 ( 2 0 j 1.505 (43) 1.572 (25) 1.594 (36) 1.502 (30) nnn
1 0 0 ,
1 . J Z W [JOJ
(C) 0-C Distances within OCH(CF,), Ligands 1.385 (20) 1.403 (23) 0(4)-C(16) 0(1)-C(4) 1.428 (20) 1.399 (18) 0(5)-C(19) 0(2)-C(7) 1.415 (19) 0(3)-C(10) 1.391 (22) 0(6)-C(22) (D) C-C Distances within OCH(CF,), Ligands 1.520 (35) C(4)-C(5) 1.507 ( 2 9 ) C ( 1 6 ) 4 ( 1 7 ) C(4 )-C(6) 1.461 (26) C ( 1 6 ) 4 ( 1 8 ) 1.469 (26) 1.514 (35) 1.516 (22) C(19)-C(20) C(7)-C(8) 1.511 (24) C(19)-C(21) 1.427 (37) C(7 )-C(9) 1.527 (30) C(lO)-C(ll) 1.529 (23) C(22)-C(23) C(lO)-C(12) 1.448 (31) C ( 2 2 ) 4 ( 2 4 ) 1.510 (22) C(5)-F(5A) C(5)-F(5B) C(5)-F(5C) C(6)-F(6A) C(6)-F(6B) C(6)-F(6C) C(8)-F(8A) C(8)-F(8B) C(S)-F(8C) C(9)-F(9A) c ( 9 j-F(9Bj C( 9)-F( 9C) C( 11)-F( 11A) C( 11)-F( 11B) C ( l l ) - F ( 11C) C( 12)-F( 12A) C( 12)-F( 12B) C(12)-F(12C)
( E ) C-F 1.292 (23) 1.327 (22) 1.268 (29) 1.387 (35) 1.319 (36) 1.291 (25) 1.262 (31) 1.315 (24) 1.328 (26) 1.262 (22) 1.320 1.298 1.291 1.346 1.293 1.299 1.307 1.319
Distances C(17)-F(17A) C(17)-F(17B) C(17)-F(17C) C( 18)-F(18A) C(18)-F(18B) C(18)-F(18C) C(20)-F(20A) C(20)-F(20B) C(20)-F(20C) Cf21kFf21A) c ( 2 1j-F(21Bj C(21 )-F( 21C) C( 23)-F(23A) C( 23)-F( 23B) C(23)-F(23C) C(24)-F(24A) C(24)-F(24B) C(24)-F(24C)
1.260 (36) 1.322 (27) 1.274 (24) 1.349 (31) 1.259 (42) 1.261 (36) 1.326 (38) 1.331 (29) 1.321 (38) 1.393 (27) 1.394 (35 j 1.227 (35) 1.279 (26) 1.290 (23) 1.267 (19) 1.311 (24) 1.312 (26) 1.280 (32)
6 and 7 appear to decompose in diethyl ether. (Note that 3 and 4 do not.) Perhaps ether is simply too poor a ligand
in these more crowded molecules, and “W(CR)[OCMe(CF3I2l3”decomposes. Mixing 6 and 7 in C6D6at 25 “cimmediately yields a mixture whose ‘H NMR is consistent with the presence of all six possible metallacycles. This result contrasts markedly with that obtained for the analogous hexafluoroisopropoxide complexes; a mixture of 3 and 4 is
Figure 2. Labeling of atoms in W(C3Et..&OCH(CF3)2]3-molecule 3a.
Figure 3. Labeling of atoms in W(C3EtJ[OCH(CF3)2]3-molecule 3b.
stable in C6D6and ether is required for the scrambling process. The most reasonable explanation, one which is consistent with other results discussed below, is that 6 and 7 lose 3-hexyne and Codyne in CJI, to give small amounts of the putative alkylidyne complexes, whereas 3 and 4 do not in the absence of donor solvents. Metathesis of Alkynes. Both 6 and 7, or any of the hexafluoro-tert-butoxidealkylidyne complexes, will catalyze the metathesis of 20 equiv of 3-heptyne to equilibrium in less than 5 min in pentane or ether. (The rates are qualitatively the same in the two solvents.) We attempted
Freudenberger et al.
1566 Organometallics, Vol. 3, No. 10, 1984
Figure 4. Stereoscopic view of molecule 3a.
Figure 5. Stereoscopic view of molecule 3b. to determine the order of the reaction by measuring the rate of incorporation of 3-hexyne-dlointo 6. Unfortunately, the reaction is too fast (even at -50 "C) to measure by routine NMR methods. In spite of the lack of kinetic proof we feel comfortable in proposing that the rate-limiting step for acetylene metathesis by 6 or 7 in noncoordinating solvents consists of loss of an acetylene from the WC3 ring to give putative alkylidyne complex W(CR)[OCMe(CF,)2]3. In the presence of dimethoxyethane, tetrahydrofuran, or other donor solvents that may compete with alkynes for metal coordination sites, the reaction could be considerably more complex mechanistically; the result might depend upon the relative concentration of the solvent vs. free acetylenes, and the solvent's donor ability. At least we can say that in noncoordinating hydrocarbons the hexafluoro-tert-butoxide tungstenacylobutadiene complexes catalyze metathesis in a manner analogous to that we were able to document fully in the previous paper for 2,6-diisopropylphenoxidederivatives. What is important is how these results in noncoordinating hydrocarbons differ with those employing 1, 3, or 4 as the metathesis catalyst in noncoordinating hydrocarbons. W(CCMe,)[OCH(CF,),],(dme)(1) will catalyze the metathesis of 3-heptyne in ether (tllz 10 min for 20 equiv of 3-heptyne). Approximately the same rate is observed when metallacycle 3 is used as the catalyst. If the reaction is run in pentane by using the same concentrations of 3 and 3-heptyne, the rate drops dramatically (tllz 21 h). Adding 1 equiv of dme to the pentane yields an intermediate rate of metathesis (tljz = 4.5 h). It should be noted that in all cases a significant amount of polymer is observed after a day at room temperature. The main
point, however, is the large qualitative difference in rate of reaction in ether and in pentane vs. similar rates for 6 and 7 in the two solvents. The reaction between 3 and 3-hexyne-dloin toluene-d8 was monitored by following the disappearance of the C,CH2CH3 signal in the lH NMR spectrum. From 10 to 40 equiv of 3-hexyne-dlo were employed in each experiment. During each reaction a small amount of what is probably acetylene polymer was formed. The rate of incorporation of 3-hexyne-dlowas first order in tungsten. At the highest concentrations of 3 - h e ~ y n e - we d ~ followed ~ the reaction for 3 half-lives. Surprisingly, the rate was first order in 3-hexyne-dlo(Table I). A variable-temperature study yielded AH* = +14.4 (6) kcal mol-l and A S = -22.8 (20) cal mol-l K-l (see Figure l),consistent with an associative mechanism. Although we will defer proposing any details of this bimolecular reaction until the Discussion, we do want to point out one potential problem that we touched upon in the previous paper, Le., whether 3 reacts with 3-hexyne to give only W[C(CH2CH3)C(CDzCD,)C(CD2CD3)][OCH(CF,),], or whether some W[C(CD2CD3)C(CH2CH3)C(CD2CD3)] [OCH(CF,),J, forms. In this case we cannot tell, but the answer is not necessary for our purposes here. What is important is that acetylenes are metathesized by 3 in toluene-d, in a relatively slow, bimolecular reaction. X-ray Crystal Structure of W(C3Et3)[0CH(CF3)2]3. Crystals of W(C3Et3)[OCH(CF3)2]3 (3) are composed of discrete monomeric molecular units separated by normal van der Waals' distances; there are no abnormally short intermolecular contacts. The crystallographic asymmetric unit consists of two complete, approximately trigonal-bi-
-
Organometallics, Vol. 3, No. 10, 1984 1567
Tungstenacyclobutadiene Complexes
/
- " a132 1439 36 @ 5
c7
to*
Figure 6. Molecule 3a, projected onto its WC3 plane.
9:: (&?
Figure 7. Molecule 3b, projected onto its WC3 plane. pyramidal molecules (designated 3a and 3b). Labeling of atoms in these two molecules is shown in Figures 2 and 3; stereoscopic view of the molecules are provided by Figures 4 and 5; a projection onto the WC3 plane in each molecule is shown in Figures 6 and 7. Interatomic distances and angles are collected in Tables 11 and 111, respectively. Note that the two crystallographically independent molecules are virtually identical, the principal difference being the conformation of the ethyl group on the @-carbonatom of the WC3 systems. In 3a both the ethyl group's C(2B) and the alkoxide's C(7) atoms are above the equatorial coordination plane. In 3b, the ethyl group on the @-carbonatom points down (C(14B) is below the equatorial coordination plane) while C(19) of the equatorial alkoxide ligand is above this plane. Let us first consider the tungstenacyclobutadiene ring systems in 3a and 3b. In each of the two crystallographically independent molecules the WC3 systems are planar within the limits of experimental error (see planes A and D of Table IV); root-mean-square deviations from planarity are 0.010 A for atoms of the WC3 system in 3a and 0.005 A for atoms of the WC3 system in 3b. The W-C, distances for 3a are W(l)-C(l) = 1.902 (16) A and W(l)-C(3) = 1.864 (14) A; corresponding distances in 3b are W(2)-C(13) = 1.885 (15) A and W(2)-C(15) = 1.860 (17) A. The average W-C, distance is 1.878 [20] The W.-C(@) distances in the two molecules are W(l)-.C(2) = 2.093 (14) A and ( 8 ) Ed's of average values, shown in square brackets, are calculated by using the scatter formula, viz.
Here, xi is the ith of N equivalent measurements and y, is the mean of these N measurements.
WCI,
system
W(OR), (R
i
system
2,6-dtisopropylphenyi )
1224
W(OCH(CF,),), system
Figure 8. A comparison of bond lengths and angles in several tungstenacyclobutadiene complexes. W(2).-C(14) = 2.100 (15) A. Carbon-carbon distances within the WC3 systems appear to be equivalent in 3a (C(l)-C(2) = 1.429 (18) A and C(2)-C(3) = 1.437 (21) A) and inequivalent in 3b (C(13)-C(14) = 1.348 (27) A and C(14)-C(15) = 1.492 (23) A); however, the central atom is common to the two measurements in each case and errors in this atom's coordinates can affect the difference in the two C-C distances by an unusual amount. Therefore we attribute no great significance to the discrepancy in the C-C distances in 3b. The more accurately known W-C distances suggest that 3-hexyne is not about to be lost from these WC3 ring systems. Other notable features of the WC3 systems include the internal angles at the @-carbonatoms [C(l)-C(2)-C(3) = 122.4 (13)' and C(13)-C(14)-C(15) = 121.4 (14)O], the C,-W-C, angles [C(l)-W(l)-C(3) = 83.6 (6)O and C(13)-W(2)-C(15) = 82.8 (7)O], and the large external angles at the a-carbon atoms [W (l)-C( 1)-C( 1A) = 154.5 (12)O, W(l)-C(3)-C(3A) = 157.1 (13)' W(2)-C(13)-C(13A) = 148.8 (14)O, and W(2)-C(15)-C(15A) = 158.8 (13)OI. The axial alkoxide ligands are all essentially equivalent [W(l)-O(l) = 1.982 (11) A, W(1)-0(3) = 1.962 (12) A, W(2)-0(4) = 1.970 (11) A, W(2)-0(6) = 1.969 (11) A] and have W-0-C angles close to 130' [viz., W(l)-O(l)-C(4) = 129.4 (9)", W(1)-0(3)-C(lO) = 133.6 (8)O, W(2)-O(4)C(16) = 130.9 ( l O ) O , and W(2)-0(6)-C(22) = 130.8 (ll)"]. The equatorial alkoxide ligands are associated with only slightly shorter tungsten-oxygen distances [W(1)-0(2) = 1.932 (10) A and W(2)-0(5) = 1.934 (10) A] and slightly more obtuse W-O-C angles [W(1)-0(2)
