see Eq. (a)] were studied. The change in the thermodynamic stability in going from R = H to R = F (6/7 -+ 8/9) was as expected. While 7 (R = H) is 13.5 kcalmol-' less stable than 6, complex 9 (R = F ) is 17.6 kcalmol-' more stable than 8. These trends will be of use in designing appropriate ligands so that even the Zr complexes with C-C coupled products can be obtained. The relative energies at the B3LYP/LANL2DZ level show that electron correlation does not affect the trend of isomerization.
An explanation for the difference in reactivity between the Ti and Zr complexes was sought from the molecular The Walsh diagrams for the process 2 ---t 3 were essentially the same for the two metals. The difference in the ionic radii between Ti and Zr suggests another explanation. The larger size of Zr leads to longer Zr-acetylene distances, which in turn results in a longer C3-C4 bond (3.081 A) in 6. In the isostructural Ti complex 4 the corresponding bond length is 2.714 A, which is further along the path to C-C bond formation. We found that fluoride substitution also helps in reducing the C3-C4 distance;
it was calculated to be 2.898 A in 8.
Due to the enormous sizes of4-9, locating the corresponding transition states and calculating the vibrational frequency are beyond our computing facilities. However, we have carried out DFT calculations at the B3LYP/LANL2DZ level on the simplified models 10-13 (Cz,,). The relative energies (Table 1) indicate similar trends between the Ti and Zr complexes. Comparison of two sets of structures, 4-7 and 10-13, reveals that replacement of Cp by H has a larger effect on the model Ti complexes than on the Zr complexes. The transition structures for conversion of 10 to 11 (TS1) and 12 to 13 (TS2) have been characterized fully with one imaginary frequency each. The relative energies of the transition structures indicate that C-C coupling is considerably more favorable for Ti (10) than for Zr (12). We expect similar trends for 4 and 6.
In conclusion, the unusual C-C coupling observed in dimeric bis(~5-cyclopentadienyI)phenylethynyltitanium complexes and lacking in the corresponding zirconium complexes is a consequence of thermodynamic energy differences. Highly electronwithdrawing substituents on the ethynyl bridging group might help in making structure 7 more competitive to 6. Model studies indicate that Ti is preferred over Zr for the C-C coupled product 3 when R = H. When R = F, the C-C coupled structure is more favorable for Zr complexes than the uncoupled structure.