Mechanistic and Theoretical Analysis of the Oxidative Addition of H2 to Six-Coordinate Molybdenum and Tungsten Complexes M(PMe3)4X2 (M = Mo, W; X = F, Cl, B
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- 作者：Tony Hascall ; Daniel Rabinovich ; Vincent J. Murphy ; Michael D. Beachy ; Richard A. Friesner ; and Gerard Parkin
- 刊名：Journal of the American Chemical Society
- 出版年：1999
- 出版时间：December 15, 1999
- 年：1999
- 卷：121
- 期：49
- 页码：11402 - 11417
- 全文大小：347K
- 年卷期：v.121,no.49(December 15, 1999)
- ISSN：1520-5126
文摘
Experimental observations, together with a theoretical analysis, indicate that the energetics of theoxidative addition of H2 to the six-coordinate molybdenum and tungsten complexes trans-M(PMe3)4X2 (M =Mo, W; X = F, Cl, Br, I) depend very strongly on the nature of both the metal and the halogen. Specifically,the exothermicity of the reaction increases in the sequences Mo < W and I < Br < Cl < F. Of most interest,this halogen dependence provides a striking contrast to that reported for oxidative addition of H2 to the Vaskasystem, trans-Ir(PPh3)2(CO)X. A theoretical analysis suggests that the halide dependence for trans-M(PMe3)4X2is a result of both steric and electronic factors, the components of which serve to reinforce each other. Oxidativeaddition is thus favored sterically for the fluoride derivatives since the increased steric interactions upon formingthe eight-coordinate complexes M(PMe3)4H2X2 would be minimized for the smallest halogen. The electroniccomponent of the energetics is associated with the extent that 
-donation from X raises the energy of thedoubly occupied 3e*, -antibonding, dxz and dyz pair of orbitals in trans-M(PMe3)4X2. Consequently, with Fas the strongest -donor, trans-M(PMe3)4X2 is destabilized with respect to M(PMe3)4H2X2 by p-d interactionto the greatest extent for the fluoride complex, so that oxidative addition becomes most favored for this derivative.Equilibrium studies of the oxidative addition of H2 to trans-W(PMe3)4I2 have allowed the average W-H bonddissociation energy (BDE) in W(PMe3)4H2I2 to be determined [D(W-H) = 62.0(6) kcal mol-1]. Thecorresponding average W-D BDE [D(W-D) = 63.8(7) kcal mol-1] is substantially greater than the W-HBDE, to the extent that the oxidative addition reaction is characterized by an inverse equilibrium deuteriumisotope effect [KH/KD = 0.63(5) at 60 
deg.gif">C]. The inverse nature of the equilibrium isotope effect is associatedwith the large number (six) of isotope-sensitive vibrational modes in the product, compared to the singleisotope-sensitive vibrational mode in reactant H2. A mechanistic study reveals that the latter reaction proceedsvia initial dissociation of PMe3, followed by oxidative addition to five-coordinate [W(PMe3)3I2], rather thandirect oxidative addition to trans-W(PMe3)4I2. Conversely, reductive elimination of H2 does not occur directlyfrom W(PMe3)4H2I2 but rather by a sequence that involves dissociation of PMe3 and elimination from theseven-coordinate species [W(PMe3)3H2I2].
-donation from X raises the energy of thedoubly occupied 3e*, -antibonding, dxz and dyz pair of orbitals in trans-M(PMe3)4X2. Consequently, with Fas the strongest -donor, trans-M(PMe3)4X2 is destabilized with respect to M(PMe3)4H2X2 by p-d interactionto the greatest extent for the fluoride complex, so that oxidative addition becomes most favored for this derivative.Equilibrium studies of the oxidative addition of H2 to trans-W(PMe3)4I2 have allowed the average W-H bonddissociation energy (BDE) in W(PMe3)4H2I2 to be determined [D(W-H) = 62.0(6) kcal mol-1]. Thecorresponding average W-D BDE [D(W-D) = 63.8(7) kcal mol-1] is substantially greater than the W-HBDE, to the extent that the oxidative addition reaction is characterized by an inverse equilibrium deuteriumisotope effect [KH/KD = 0.63(5) at 60 deg.gif">C]. The inverse nature of the equilibrium isotope effect is associatedwith the large number (six) of isotope-sensitive vibrational modes in the product, compared to the singleisotope-sensitive vibrational mode in reactant H2. A mechanistic study reveals that the latter reaction proceedsvia initial dissociation of PMe3, followed by oxidative addition to five-coordinate [W(PMe3)3I2], rather thandirect oxidative addition to trans-W(PMe3)4I2. Conversely, reductive elimination of H2 does not occur directlyfrom W(PMe3)4H2I2 but rather by a sequence that involves dissociation of PMe3 and elimination from theseven-coordinate species [W(PMe3)3H2I2].