A Comparison of the Influences of Alkoxide and Thiolate Ligands on the Electronic Structure and Reactivity of Molybdenum(3+) and Tungsten(3+) Complexes. Preparation and Structures of M2(O
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- 作者：Malcolm H. Chisholm ; Ernest R. Davidson ; John C. Huffman ; and Kristine B. Quinlan
- 刊名：Journal of the American Chemical Society
- 出版年：2001
- 出版时间：October 3, 2001
- 年：2001
- 卷：123
- 期：39
- 页码：9652 - 9664
- 全文大小：880K
- 年卷期：v.123,no.39(October 3, 2001)
- ISSN：1520-5126
文摘
M2(OtBu)6 compounds (M = Mo, W) react in hydrocarbon solvents with an excess of tBuSH togive M2(OtBu)2(StBu)4, red, air- and temperature-sensitive compounds. 1H NMR studies reveal the equilibriumM2(OtBu)6 + 4tBuSH 
M2(OtBu)2(StBu)4 + 4tBuOH proceeds to the right slowly at 22 
deg.gif">C. The intermediatesM2(OtBu)4(StBu)2, M2(OtBu)3(StBu)3, and M2(OtBu)5(StBu) have been detected. The equilibrium constantsshow the M-OtBu bonds to be enthalpically favored over the M-StBu bonds. In contrast to the M2(OtBu)6compounds, M2(OtBu)2(StBu)4 compounds are inert with respect to the addition of CO, CO2, ethyne, tBuC
d1.gif">CH, MeCd1.gif">N, and PhCd1.gif">N. Addition of an excess of tBuSH to a hydrocarbon solution of W2(OtBu)6(
-CO)leads to the rapid expulsion of CO and subsequent formation of W2(OtBu)2(StBu)4. Addition of an excess oftBuSH to hydrocarbon solutions of [Mo(OtBu)3(NO)]2 and W(OtBu)3(NO)(py) gives the structurally relatedcompounds [Mo(StBu)3(NO)]2 and W(StBu)3(NO)(py), with linear M-N-O moieties and five-coordinate metalatoms. The values of 
(NO) are higher in the related thiolate compounds than in their alkoxide counterparts.The bonding in the model compounds M2(EH)6, M2(OH)2(EH)4, (HE)3Md1.gif">CMe, and W(EH)3(NO)(NH3) andthe fragments M(EH)3, where M = Mo or W and E = O or S, has been examined by DFT B3LYP calculationsemploying various basis sets including polarization functions for O and S and two different core potentials,LANL2 and relativistic CEP. BLYP calculations were done with ZORA relativistic terms using ADF 2000.The calculations, irrespective of the method used, indicate that the M-O bonds are more ionic than the M-Sbonds and that E p
to M d bonding is more important for E = O. The latter raises the M-M orbitalenergies by ca. 1 eV for M2(OH)6 relative to M2(SH)6. For M(EH)3 fragments, the metal dxz,dyz orbitals aredestabilized by OH p bonding, and in W(EH)3(NO)(NH3) the O p to M d donation enhances W d to NO* back-bonding. Estimates of the bond strengths for the Md1.gif">M in M2(EH)6 compounds and Md1.gif">C in (EH)3Md1.gif">CMe have been obtained. The stronger donation of the alkoxide ligands is proposed to enhance back-bonding to the * orbitals of alkynes and nitriles and facilitate their reductive cleavage, a reaction that is notobserved for their thiolate counterpart.
M2(OtBu)2(StBu)4 + 4tBuOH proceeds to the right slowly at 22 deg.gif">C. The intermediatesM2(OtBu)4(StBu)2, M2(OtBu)3(StBu)3, and M2(OtBu)5(StBu) have been detected. The equilibrium constantsshow the M-OtBu bonds to be enthalpically favored over the M-StBu bonds. In contrast to the M2(OtBu)6compounds, M2(OtBu)2(StBu)4 compounds are inert with respect to the addition of CO, CO2, ethyne, tBuCd1.gif">CH, MeCd1.gif">N, and PhCd1.gif">N. Addition of an excess of tBuSH to a hydrocarbon solution of W2(OtBu)6(-CO)leads to the rapid expulsion of CO and subsequent formation of W2(OtBu)2(StBu)4. Addition of an excess oftBuSH to hydrocarbon solutions of [Mo(OtBu)3(NO)]2 and W(OtBu)3(NO)(py) gives the structurally relatedcompounds [Mo(StBu)3(NO)]2 and W(StBu)3(NO)(py), with linear M-N-O moieties and five-coordinate metalatoms. The values of (NO) are higher in the related thiolate compounds than in their alkoxide counterparts.The bonding in the model compounds M2(EH)6, M2(OH)2(EH)4, (HE)3Md1.gif">CMe, and W(EH)3(NO)(NH3) andthe fragments M(EH)3, where M = Mo or W and E = O or S, has been examined by DFT B3LYP calculationsemploying various basis sets including polarization functions for O and S and two different core potentials,LANL2 and relativistic CEP. BLYP calculations were done with ZORA relativistic terms using ADF 2000.The calculations, irrespective of the method used, indicate that the M-O bonds are more ionic than the M-Sbonds and that E p to M d bonding is more important for E = O. The latter raises the M-M orbitalenergies by ca. 1 eV for M2(OH)6 relative to M2(SH)6. For M(EH)3 fragments, the metal dxz,dyz orbitals aredestabilized by OH p bonding, and in W(EH)3(NO)(NH3) the O p to M d donation enhances W d to NO* back-bonding. Estimates of the bond strengths for the Md1.gif">M in M2(EH)6 compounds and Md1.gif">C in (EH)3Md1.gif">CMe have been obtained. The stronger donation of the alkoxide ligands is proposed to enhance back-bonding to the * orbitals of alkynes and nitriles and facilitate their reductive cleavage, a reaction that is notobserved for their thiolate counterpart.