Applications of 95Mo N.M.R-Spectroscopy. XVI. Structure and Bonding in [Mo(Co)3(η6-Arene)] Complexes From 95Mo,13C and 17O Relaxation-Times

Abstract

Spin-lattice relaxation times (T1) have been measured for the 95Mo, 17O and methine and carbonyl 13C nuclei for a series of [Mo(CO)3(η6-C6H6- nMen)] complexes. In addition to the expected decrease in T1 with increasing molecular volume, the 95Mo and methine 13C data are interpreted in terms of variations in arene -molybdenum bonding. The methine 13C T1 values decrease in the order o- xylene > toluene > p- xylene > m- xylene > mesitylene . The methyl substitution pattern determines the distribution of electron-rich sites on the arene ligand , affecting the molybdenum- arene bond strength and the barrier to arene rotation. These observations support an earlier proposal that downfield 95Mo chemical shifts are associated with increased molybdenum- arene bond strength. 95Mo T1 values are unaffected by the rotational barriers, since they depend only on molecular tumbling perpendicular to the pseudo C3v symmetry axis. While these T1 values decrease as the number of methyl groups increases, an effect due to increasing molecular volume, the order of 95Mo T1 for the xylene complexes is opposite to that of the 13C T1, due to a reduction in the electric field gradient at the 95Mo nucleus. 95Mo quadrupole coupling constants for the series are 1.2�0.2 MHz. The 17O T1 values have also been measured for this series. They are considerably longer, and quadrupole coupling constants smaller (0.87�0.07 MHz), than those of other metallocarbonyls , indicating a significant increase in metal- ligand dπ-pπ - back bonding in the present system. A correlation between carbonyl force constants and 17O quadrupole coupling constants is presented.