Quadrupole Coupling and Anisotropic Chemical Shift in Re2(CO)10, Mn2(CO)10, and ReMn(CO)10

Single crystals of the bimetallic carbonyls Mn2(CO)10, Re2(CO)10, and ReMn(CO)10 were studied at room temperature by broadline NMR. Earlier work on the orientation of the principal axis system of the electric field-gradient tensor in Re2(CO)10 and Mn2(CO)10 was corrected. The z axis of the tensor in Re2(CO)10 was found to lie within 6° of the Re–Re bond. In Mn2(CO)10 the y axis of the field gradient tensor was within 10° of the Mn–Mn bond. The coupling constant and asymmetry parameter for 55Mn in MnRe(CO)10 were determined as e2qQ/h=8.6 MHz and η=0.61. The z axis of the field gradient tensor at the 55Mn site was within 2° of the Mn–Re bond. The orientation of the principal axis system of the field gradient tensor in Mn2(CO)10 and Re2(CO)10 was determined at liquid nitrogen temperature. The angle between the z axis and the Re–Re bond in Re2(CO)10 was found to double by going from room temperature to liquid nitrogen temperature. The orientation of the principal axis system in Mn2(CO)10 was unchanged within experimental error by changing the temperature from 25°C to 77°K. A small anisotropy in the chemical shift was found for Re2(CO)10 while the shift in Mn2(CO)10 was essentially isotropic. An NMR probe for studies of air unstable compounds and constant temperature is described.