Synthesis, structure, and reactivity of polynuclear group VI metal complexes
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1984
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Abstract
Only in the past decade have trimers of the early transition metals been observed. In these clusters the metal atoms are in their higher oxidation states and are bound to simple electron donating ligands. Compounds of this type have been called "electron poor", meaning that the total number of metal valence electrons is approximately sufficient to form single bonds between adjacent pairs of metal atoms. Seven molybdenum and tungsten complexes with the general formula {M(,3)((mu)(,3)-O)(,2)(O(,2)CR)(,6)L(,3)}('n(+OR-)) have been prepared by a variety of synthetic methods. In addition to spectroscopic studies, five of the clusters have been crystallographically characterized. The nitration of the benzoate bridged trimers has demonstrated the robust nature of the complexes. From comparison of bond lengths in the compounds it is seen that the nature of the bridging carboxylate does not have a significant effect on the metal-metal bond lengths. However, the electon donation strength of the terminal ligands, L, does play a role in determining the M-M distance. Two new synthetic pathways to the Mo('IV)(aq) ion, Mo(,3)O(,4)(H(,2)O)(,9)('4+), have been devised. The advantages of each method over the other and previously reported preparative schemes are reported. In a deliberate attempt to promote the formation of uranium-uranium bonds, four uranium alkoxide complexes, three of which were previously unknown, with the uranium atoms in the plus four or five oxidation state were prepared and crystallographically characterized. It has been found that in these complexes there is insufficient 5f-5f overlap to stabilize uranium-uranium bonds. The terminal alkoxide groups in all four compounds display nearly linear U-O-C arrangements, with U-O-C bond angles between 158(DEGREES) and 179(DEGREES). This suggest a high degree of O(pi)(--->)U contribution to the metal-oxygen bonds, which may be competitive with the formation of U-U bonds.
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Chemistry