Reactivity pattern, ligand property

The current high level of interest in binuclear metal complexes arises from the expectation that the metal centers in these complexes will exhibit reactivity patterns that differ from the well-established modes of reactivity of mononuclear metal complexes. The diphosphine, bis(diphenylphosphino)methane (dpm), has proved to be a versatile ligand for linking two metals while allowing for considerable flexibility in the distance between the two metal ions involved (1). This chapter presents an overview of the reaction chemistry and structural parameters of some palladium complexes of dpm that display the unique properties found in some binuclear complexes. Palladium complexes of dpm are known for three different oxidation states. Palladium(O) is present in Pd2(dpm)3 (2). Although the structure of this molecule is unknown, it exhibits a single P-31 NMR reso-... 

In summary, bis(dithiolene) complexes are clearly distinct from traditional inorganic or organometallic complexes in which the chemical reactivity is dominated by the metal center. The unique properties of dithiolene ligands such as redox activity, aromaticity, and unsaturation of the metal-ligand chelate rings, in combination with the metal-centered reactivity paths, have generated many unusual reactivity patterns for this class of complexes. 

Free alkoxide and aryloxide anions are Bronsted bases with pK values of the corresponding alcohols ranging from 5 to 20 in water. The basicity is highly dependent on the electronic properties of the alkyl or aryl moieties. For example, the pK value of hexafluoro-tert-butanol, (CF3)jMeCOH, is 9.6, which is considerably lower than the pK value of tert-butanol (19.2), but roughly the same as that of phenol (9.9). Such differences in electronic, as well as steric, environments often leads to the different structures and reactivity patterns for compounds containing similar ancillary ligands, but different alkoxides or aryloxides. 

Complexation profoundly alters ligand properties and can even invert normal reactivity patterns seen in the free organic ligands (Section 2.6). 

Substitution at one or more metal sites will generally break the symmetry of the cluster core, and can greatly influence its electronic properties and reactivity. Consider, for example, the possible substitutions of a metal M into an octahedral core of composition MfiX v (x = 8, 12). The first substitution will afford an MsM X core, for which the symmetry has been lowered to C4v. A second substitution generates an M4M 2Xx core with two possible isomers One in which the M atoms are positioned at trans vertices (D4/,) and another where they are positioned at cis vertices (C2v). With a third substitution to give an M3M 3Xx core, fac and mer isomers become possible, while further substitutions simply repeat the pattern with M and M interchanged. Here again, the substitutions can be anticipated to alter the basic electronic properties of the cluster. Moreover, the outer-ligand substitution chemistry could potentially be quite different... 

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