CHARACTERIZATION OF (ETA-C5ME5)CR(NO)(O-I-PR)2 - COORDINATION SATURATION FROM ALKOXIDE PI-DONATION

Treatment of Cp*Cr(NO)2CH2Cl or Cp*Cr(NO)2Cl with excess Na(O-i-Pr) in 2-propanol gives a volatile, deep blue-green complex that analyzes correctly for the empirical formula [Cp*Cr(NO)(O-i-Pr)2] in 86-94% yield (Cp* = eta(5)-C5Me5). Spectroscopic evidence indicates the new material to be a mononuclear, diamagnetic Cr(II) complex that contains a terminal NO ligand. A single-crystal X-ray structural analysis shows the molecule to adopt a nearly C(s) piano-stool structure with the nitrosyl ligand being bound in a terminal (NO+) mode: monoclinic space group P2(1)/n, a = 9.535 (3) angstrom, b = 9.789 (4) angstrom, c = 20.108 (9) angstrom, beta = 91.89 (5)-degrees, Z = 4, Mo K-alpha radiation, T = 294 K, R/R(w) = 5.71%/6.49% for 2267 independent reflections (F > 4-sigma-F) and 190 parameters. The isopropoxide ligands are oriented so as to make use of a p-pi-orbital perpendicular to the Cr-O-C plane for donation into an empty Cr d orbital that is nonbonding to the nitrosyl ligand. The Cr-O bond lengths observed average 1.815 (4) angstrom and are consistent with significant Cr-O pi-bonding. Although formally a 16-electron complex, Cp*Cr(NO)(O-i-Pr)2 is unreactive in solution with excess alkoxide, CO, or PPh3, indicating isopropoxide pi-donation to essentially render the complex coordinatively saturated.