SYNTHESES, CHARACTERIZATION, AND STRUCTURAL AND KINETIC-STUDIES OF HALF-OPEN CHROMOCENES AND THEIR LIGAND ADDUCTS

The syntheses of a variety of half-open chromocenes from a chromium(II) chloride complex are reported. These complexes may contain either the C5H5 or the C5Me5 ligand, as well as a variety of pentadienyl ligands (C5H7, 3-C6H9, 2,3-C7H11, 2,4-C7H11, 1,5-(C6H5)2C5H5, and 1,5-(Me3Si)2C5H5 for C6H9 = methylpentadienyl and C7H11 = dimethylpentadienyl). A structural study of Cp*(C5H7)Cr was undertaken, revealing much shorter Cr-C(pentadienyl) than Cr-C(Cp*) bonds. The space group is P2(1/n) (No. 14) with a = 7.593 (3) angstrom b = 23.073 (7) angstrom, c = 8.530 (4) angstrom, beta = 113.76 (2)degrees, and V = 1367.7 angstrom3, for Z = 4. These 16-electron complexes react with various 2-electron donor ligands (e.g., CO, PF3, RNC, and PMe2C6H5) to yield 18-electron adducts. These species are unusual in that they all contain eta-5-S-pentadienyl (S = sickle) ligands. This has been confirmed by a single-crystal X-ray diffraction study Of Cp(C5H7)Cr(2,6-xylyl isocyanide). The space group is P2(1/n) (No. 14), with a = 10.379 (2) angstrom, b = 7.629 (2) angstrom, c = 19.961 (4) angstrom, beta = 93.88 (2)degrees, V = 1576.9 angstrom3, and Z = 4. Additionally, a number of 18-electron bisadducts have been prepared in which a eta-3-pentadienyl coordination is present. This mode of coordination has been confirmed by an X-ray structural determination of Cp(eta-3-C5H7)Cr(CO)2, which crystallizes in space group Pna2(1) (No. 33), with a = 19.653 (6) angstrom, b = 6.545 (2) angstrom, c = 17.126 (5) angstrom, V = 2202.9 angstrom3, and Z = 8. The eta-3-C5H7 ligands were found to adopt the W configuration. Oxidation of the 18-electron monoadducts by NO+ to 17-electron monocations regenerates the eta-U-pentadienyl coordination, revealing that the sickle coordination has only a very small window for stability. Kinetic studies of the CO exchange reactions for the monocarbonyls have been carried out. The exchanges take place by a first-order (dissociative) pathway, with the rates being reasonably comparable to those of their vanadium analogues. Unusually low (even negative) values for DELTA-S have been observed and may be attributed to an eta-5-S --> eta-5-U conversion of the pentadienyl ligands.