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- _nMe_n)] 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 C_3v 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.