Dinudear metal complexes

Using and P NMR in the model [Pd]/Ph2S2/(OtPr)3P catalytic system trans-and ds-mononuclear and dinudear metal complexes were detected. The equilibrium constant for the dissociation of the dinudear metal complexes was measured, K= 0.066 0.007 M at 30 °C [119]. Although the equilibrium is shifted towards dinudear complexes, in excess of the (OiPr)3P ligand the mononudear trans-[Pd (SPh)2((OiPr)3P)2] complex has been isolated and characterized by X-ray analysis (Figure 3.1). 

As mentioned above, the intermediary of metallacydopentadienes has been widely recognized in many [2 -i- 2 -i- 2] cydotrimerizations of alkynes and related cydocotri-merizations [9]. Metallacydopentadienes are generally produced by the oxidative cydization of two alkyne molecules on a low-valent metal center. Various ruthena-cyclopentadienes were synthesized by this method. For example, heating a decalin solution of Ru3(CO)i2 and diphenylacetylene at 200 °C gave rise to the dinudear mthenacyclopentadiene complex 1 (Scheme 4.1) [10]. The similar dinudear complex 2 was obtained from dimethyl acetylenedicarboxylate (DMAD) at lower temperature [11]. Dinudear ruthenacyclopentadienes were also obtained, when conjugated... 

As with dinudear metal carbonyls ( 13.2.4.2.) the prevalent mode of photoreaction of trinuclear metal-carbonyl complexes is cleavage of a metal-metal bond. Subsequent decay of the bond-cleaved intermediate can yield fragmentation or substitution of the complex. These reactions are discussed in 13.3. 

Among the more interesting metal-metal multiple bonded complexes are the hexaalkoxo dinudear tungsten and molybdenum complexes, [M->(OR)A] (M — Mo. W) i >... 

Alkyl halides also add to dinudear compounds. Additions can occur across the metal-metal bond of homodinuclear complexes, as shown in Equation 7.22, or across the metal-metal bond of heterodinudear compounds, as shown in Equation 7.23. Oxidative additions of dihaloalkanes across two metal centers to generate bridging methylene complexes - have been reported for complexes of iron (Equation 7.24) and osmium. Addition of a higher alkyl dihalide to an odd-electron, dinudear cobalt system (Equation 7.25) " has also been reported. In this case, the stereochemical outcome suggests that the first substitution process occurs by a radical mechanism. 

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