Multinuclear NMR spectroscopy of transition metal complexes.

Abstract

95 Mo n.m.r. spectroscopic data for a variety of inorganic and organometallic compounds have been obtained, extending the known chemical shift range to -2953 ppm ([Mo(n-C5H5)2H3]+) (chapter 6) to 4185 ppm ([MO2(02CCF3)4] in pyridine) (chapter 8). Results from photoelectron spectroscopy, electronic spectroscopy, and 13C n.m.r. have been used to interpret 95Mo chemical shift differences (chapters 4-9). Initial investigations have suggested a correlation between temperature independent paramagnetism and molybdenum deshielding (chapter 12). The following trends in molybdenum shielding have been noted: Greater pi-delocalisation of a polyhapto-benzene, an increase in the size of a polyhapto ring or an increase in the chain length of an acyclic polyene deshields the molybdenum (chapters 4 ,5 ,7); Protonation of a molybdenum species leads to increased metal shielding if the protonation involves stabilisation of an electron pair previously localised on the metal, to form a metalhydrogen a-bond (chapters 6 ,7); Trends in molybdenum shielding with varying group V ligands observed for substituted molybdenum carbonyl compounds have been seen in other types of organometallic molybdenum compounds (chapter 7); For molybdenum-molybdenum bonded dimers with similar ligands preliminary studies indicate that molybdenum shielding decreases Mo—1— Mo > Mo—2— Mo > Mo—3— Mo > Mo—4— Mo (chapter 8 ). Molybdenum-molybdenum sulphur bridged dimers and clusters have a large chemical shift dispersion and their study by 95Mo n.m.r. suggests that the technique may be useful in catalytic and enzymatic investigations (chapter 9). Piano-stool compounds often give narrow molybdenum n.m.r. signals presumably related to a low electric field gradient about the metal. When the resonances are sharp molybdenumphosphorus coupling may be resolved, the variations in magnitude of which may be traced to changes in the Fermi contact term (chapters 4 ,5). Direct observation of 183W resonances of organometallic compounds presents several problems but the use of the DEPT pulse sequence greatly enhances the sensitivity enabling signals to be obtained for tungsten hydrides (chapter 11).