| Olefin complexes (silox)3M(ole) (silox = tBu3SiO; M = Nb (1-ole), Ta (2-ole) rearranged to alkylidene isomers, (silox)3M(alk) (M = Nb ( 1=alk), Ta (2=alk). Mechanistic studies suggested the rearrangement proceeded via a pathway utilizing (silox)2RM(kappa 2-O,C-OSitBu2CMe2CH2) (M = Nb (4-R), Ta (6-R) as an intermediary. Equilibration experiments and calculations permitted interpretation of all relative ground and transition state energies.; Mechanistic studies of olefin exchange with 1-ole suggested a dissociative process, and the rates of dissociation were measured. The rates of olefin association to (silox)3Ta (2) were measured. The data, in conjunction with calculations suggested that olefin dissociation proceeded through an asymmetric transition state. A rationale regarding the differences in the relative rates of reactivity between 1-ole and 2-ole was proposed on the basis of orbital symmetry principles and surface (singlet/triplet) crossing events.; The rate of oxidative addition of H2 to 2 to generate (silox)3TaH2 (2-H2) was determined. Analysis of an equilibrium mixture of (silox)3NbPMe 3 (1-PMe3) + H2 (1-3 atm) < => "(silox)3NbH2" (1-H 2) + PMe3 revealed two isomers of 1-H2 . 2-D EXSY studies revealed exchange between 1-H2 and H2, and magnetization transfer studies suggested were used to assess the rates of exchanges. 1-D EXSY studies suggested phosphine exchange with 1-PMe3 was dissociative. Calculations aided in assigning geometries to 1-H2.; Exposure of 1-PMe3 + H2 (1-3 atm) <=> 1-H2 + PMe3 to light caused P-C bond cleavage. The distribution of products, (silox) 3Nb=CH2 (1=CH2), (silox)3Nb=PH (1=PH), (silox)3Nb=PMe (1=PMe), (silox) 3Nb-H (1-H), and CH4, was dependent on reaction conditions. 1=PH was synthesized via addition of PH3 to 1-PMe3, and subsequent deprotonation yielded [(silox) 3NbP]Li (1≡PLi), whose methylation gave 1=PMe . Addition of HPMe2 and H2PMe to 1-PMe 3 generated (silox)3HNbPMe2 (1-H(PMe 2)) and (silox)3HNbPHMe (1-H(PHMe)) respectively, and both were found to degrade quickly. (silox)3HTaPMe2 (2-H(PMe2)) and (silox)3HTaPHMe ( 2-H(PHMe)) were synthesized and found to degrade slowly. Thermolysis of 2-H(PHMe) produced (silox)3Ta=PMe (2=PMe ), while 2-H(PMe2) generated 2=PMe, (silox)3Ta=CH2 (2=CH2), 2-H2, (silox)3Ta(eta-CH2PMe) ( 3), (silox)3Ta=C(H)PHMe (4), and (silox) 3Ta(PMe2)2 (5). A mechanism based on sequential PC or CH oxidative addition, and 1,2-elimination events, was proposed. The limiting step was a slow hydrogenation of 1=CH2 to 1-H2 and CH4. |
