Ligand effects on the heats of protonation of metal-metal bonds in organometallic complexes

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1997
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Nataro, Chip
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Robert J. Angelici
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Chemistry

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The Department of Chemistry was founded in 1880.

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Calorimetric determination of the metal center basicity in organometallic complexes has proven to be a useful tool for understanding reactivity trends. Similar studies conducted on metal-metal bonded species to determine discrete metal-metal bond basicity could offer a simple model of reactivity for transition metal surfaces. However, few examples of dinuclear compounds exist in which protonation occurs at the metal-metal bond to form the bridging hydride. In this research, the basicities of the metal-metal bond in Ru2Cp'2(CO)3L and Mo2Cp2(CO)4L2 have been determined. Basicities have been measured by the heat evolved ([delta] HMHM) when a metal complex is protonated by CF3SO3H in 1,2-dichloroethane (DCE) to form either Ru2Cp'2(CO)3L([mu]-H)+ (eq 1) or Mo2Cp2(CO)4L2([mu]-H)+ (eq 2) at 25°C.*;Previous protonation studies of mononuclear cyclopentadienyl complexes have found that the basicity of a metal center is increased by replacing Cp with a more electron donating Cp' ligand. Complexes which contain a Cp* ligand have been found to be significantly more basic than the Cp analog. Similarly, the basicity of the metal center in mononuclear complexes when L = PR3 has been found to be substantially more basic than when L = CO. In these studies it was found that the metal-metal bond basicity is affected by the electron donor ability of the Cp' or L ligands. However, the observed data could not be explained based solely on electronic arguments. Another factor, which could not be observed in mononuclear species, was found to influence the overall metal-metal bond basicity;In the ruthenium system, the complexes can have either four terminal carbonyls, two bridging and two terminal carbonyls or there can be a mixture of these two isomers. The [delta] HMHM can be considered to be the sum of H a and H b (eq3).*;The relative amounts of the bridging and non-bridging isomers affect the overall basicity of the complex. This can be seen in complexes that are expected to have similar basicity based on electronic arguments, but in fact have very different heats of protonation. It was determined that the Ru-Ru bond in Cp2Ru2(CO)4 is substantially more basic than the Ru in the related mononuclear CpRu(CO)2H.* ftn Please refer to the dissertation for diagrams.

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Wed Jan 01 00:00:00 UTC 1997