Metal-Hydride and Hydride-Hydride Distances from Measurements of Selective and Nonselective 1H T1 Relaxation Times of Hydride Ligands: Relaxation in NbCp2H

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• 作者：Vladimir I. Bakhmutov ; Evgenii V. Vorontsov ; Georgii I. Nikonov ; and Dmitry A. Lemenovskii
• 刊名：Inorganic Chemistry
• 出版年：1998
• 出版时间：January 26, 1998
• 年：1998
• 卷：37
• 期：2
• 页码：279 - 282
• 全文大小：53K
• 年卷期：v.37,no.2(January 26, 1998)
• ISSN：1520-510X

文摘

The detailed ¹H NMR relaxation study,performed forCp₂NbH^X₂H^A(1) and its partially-deuterated derivatives intoluene-d₈, has demonstrated howT₁ and T_1selmeasurements can be used for accurate determination of thehydride-hydride and metal-hydride distances in Nb complexes. TheT_1sel method was independentlysupported by therelaxation studies of the partially deuterated derivatives of1. The application of theT_1sel approach is limitedbythe presence in the H^X/H^A pair of the slowhydride/hydride site exchange characterized by saturationtransferexperiments: k²⁹⁶ = 1.34s^-1, ages/gifchars/Delta.gif" BORDER=0 >G^{ages/entities/thermod.gif">}(298K) = 17.3 kcal/mol, ages/gifchars/Delta.gif" BORDER=0 >H^{ages/entities/thermod.gif">} = 13.1 kcal/mol,and ages/gifchars/Delta.gif" BORDER=0 >S^{ages/entities/thermod.gif">} = -14.2 eu.Theexchange does not show a pronounced isotopic effect, andk(H)/k(D) = 1.1 at 295 K. It has beensuggested thatthe T_1sel approach can be used forindication of slow hydride/hydride exchanges. The experimentalestimationshave shown Nb-H dipole-dipole interactions to be very important forthe hydride relaxation in 1. The Nb-Hcontribution shortens significantly the hydrideT₁ times, providing, for example, up to 64% ofthe H^X relaxationrate. The relaxation H^X-H^A and Nb-Hdistances have been discussed.