The complete miscibility of N2 and H2 with supercritical fluids is used to stabilize unstable, and previously unknown, dinitrogen and dihydrogen complexes which would normally decay rapidly at ambient temperatures. UV photolysis of a series of (CnRn)M(CO)3 compounds [M = Fe, Mn, Re or Cr; R = H for n = 4; R = H or Me for n = 5 or 6] dissolved in supercritical Xenon (scXe) under a high pressure of N2 or H2 leads to the formation of (CnRn)M(CO)2(N2) and (CnRn)M(CO)2(H2) complexes at room temperature. Re and Fe undergo facile substitution of more than one CO group by N2, while Cr and Mn do not. For all elements except Re, the spectroscopic evidence suggests that the (CnRn)M(CO)2(H2) complexes contain the “nonclassical” η2-H2 ligand. No IR bands directly associated with η2-H2 in these compounds have been observed, but the experiments were sufficiently sensitive to detect the v(H-H) band of W(CO)5(H2) under similar conditions. Most of these dihydrogen complexes decompose by reaction with CO to regenerate the (CnRn)M(CO)3 starting material, but significant amounts of Fe(CO)5 are formed in the decay of (C4H4)Fe(CO)2(H2). Kinetic measurements between 11 and 80 °C show that (C6H5Me)Cr(CO)2(H2) reacts with CO with an activation energy of 70 ± 5 kJ mol-1, which may correspond to the Cr-(η2-H2) bond dissociation energy. © 1990, American Chemical Society. All rights reserved.
