Relaxation mechanisms, bonding, and molecular reorientation in CpRe(CO) 3

Abstract

Carbonyl 13C and 17O and cyclopentadienyl 13C NMR spin-lattice relaxation times of CpRe(CO) 3 were measured as a function of temperature in the solvent CDCl 3. Analysis of T 1( 13CO) revealed that, unlike in most metal carbonyls, where chemical shift anisotropy is the dominant mechanism, the major contribution to 13C relaxation is via scalar coupling to the directly bonded rhenium. The oxygen-17 quadrupole coupling constant [determined from T,( 17O)], and theirπ★ antibonding orbital population was very close to values reported in several (arene)Mo(CO) 3 complexes, indicating similar pi bond strengths. The C-K force constants ( k co), on the other hand, were quite dissimilar, indicating that k co is not a pure measure of π bonding in metal carbonyls. Comparison of rotational correlation times of the cyclopentadienyl group and M(CO) 3 unit revealed substantial internal rotation of the Cp ring. The rate, however, is slower than expected for afree rotor, indicating the prese