Mononuclear and binuclear rhenium carbonyl nitrosyls: comparison with their manganese analogues

The structures and energetics of Re(NO)(CO)n (n = 5, 4, 3, 2) and Re2(NO)2(CO)n (n = 7, 6) have been investigated using density functional theory. For Re(NO)(CO)4 the preferred structure is an equatorially substituted trigonal bipyramid analogous to the known structure of the manganese analogue. The lowest energy structures for the unsaturated Re(NO)(CO)n (n = 3, 2) species can be derived from this structure by removal of carbonyl groups. A structure is found for Re(NO)(CO)5 in which the NO ligand has attached to one of the CO ligands by forming a C-N bond to give an unprecedented eta(2)-OCNO ligand. However, this structure is predicted to undergo exothermic CO loss to give Re(NO)(CO)4. The preferred structures for the binuclear derivatives Re2(NO)2(CO)n (n = 7, 6) are structures unprecedented for the manganese analogues and consist of a Re(CO)5 unit linked to a Re(NO)2(CO)(n-5) unit. However, only slightly higher in energy are structures of the type Re2(mu-NO)2(CO)n with two bridging nitrosyl groups, similar to the global minima for the manganese analogues. These results predict extensive areas of new rhenium carbonyl nitrosyl chemistry. Thus the synthesis of Re(NO)(CO)4 by methods related to the synthesis of the manganese analogue appears to be feasible. In addition, the existence of an extensive series of Re(NO)2(CO)2X derivatives, as well as a Re2(NO)4(CO)4 dimer, is predicted.