1H NMR T1(min) data and structure in a series of rhenium polyhydride complexes and the contribution of M-H dipole–dipole relaxation

✍ Scribed by Xiao-Liang Luo; Judith A. K. Howard; Robert H. Crabtree

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 482 KB
• Volume: 29
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260291317

✦ Synopsis

Abstract

The unusual species [ReH~5~(H~2~){P(p‐tolyl)~3~}~2~], for which a neutron diffraction study shows an intermediate HH bond distance of 1.357 Å, is studied by variable‐temperature ^1^H NMR spin‐lattice (T~1~) relaxation time measurements. The T~1~ relaxation rate goes through a maximum at low temperature and the resulting T~1~ (min) value is 66 ms. This is within the range of T~1~(min) values found for classical ReH~7~L~2~ complexes, so H~2~ complexes with such long HH bond distances are undetectable by the T~1~ method. The observed T~1~(min) value is compared with that calculated from the neutron diffraction data. We consider not only the usual proton–proton dipole–dipole (HHDD) contributions to the relaxation but also the metal‐hydride dipole–dipole (MHDD) contributions. The latter are shown to be negligible for most transition metals, but not for Nb, V, Re, Mn, Co and Ta. Inclusion of the MHDD contributions leads to better agreement between the calculated and observed T~1~(min) values for a series of rhenium polyhydride complexes. The MHDD relaxation also accounts for most of the disparity previously noted between the calculated and observed T~1~(min) values for Re~2~H~8~(PEt~2~Ph)~4~.