Dichromium complexes

In the reduction of C-C triple bonds with chromous sulfate in water, the key intermediate consists of a dichromium complex with the alkyne (Scheme 20.29) [119]. This configuration assures the selective formation of trans double bonds. Various substrates have been reduced in excellent yields without the occurrence of isomerizations or byproduct formation (Table 20.4). 

Figure 60 shows the molecular structure of the dichromium complex [Cr2(DPPC)4] and its electrochemical behaviour.111... 

In the 1960s quadruple bonds between metallic atoms were discovered in species such as Re2Clg . There would be no obvious way to explain quadruple bonds with cubic atoms or tetrahedra. In 2005, evidence was presented for a quintuple bond between the two chromium atoms in a crowded dichromium complex (RCrCrR). ... 

One objective of exploring the reactivity of dichromium complexes with proposed fivefold bonding is to obtain additional data to support these high bond orders. 

Bridged Dichromium Complexes.— The thermodynamics and kinetics of the equilibria between di-/U-hydroxo and -hydroxo dinuclear complex ions of the type [(en)aCr-//-(OH)a-Cr(en)a + and [(H20)(en)2Cr-i -(0H>Cr(en)2(0H2) + have been investigated. In acid solution (31) is rapidly converted into a mixture... 

While metallocene-derived complexes 103 deserve interest mainly because of their redox properties and applications resulting therefrom, half-sandwich complexes 104 and 105 show more diverse chemical reactions. Based on their synthesis of the fulvalene dianion 102, Smart et al. [99] prepared some molybdenum complexes such as 108 [62% yield based on Na(DME)Cp] by treatment of 102 with molybdenum hexacarbonyl followed by oxidation with bromine. Vollhardt reported the synthesis of a variety of homo- and heterodinuclear fulvalene complexes such as 109-112 by treatment of dihydrofulvalene and respective metal carbonyls. The dichromium complex 110 has some catalytic activity in the hydrogenation of conjugated dienes [110]. The authors report the reaction with iron carbonyls being unsuccessful, whereas the reaction with dicobaltoctacarbonyl resulted in the formation of fulvalene complex 112 lacking a metal-metal bond in 80% yield (Scheme 10.38) [100, 101]. Later, the diiron complex corresponding to 109 was reported [111]. Hermann et al. and Cuenca etal. prepared zirconium(IV) and zirconium(III) fulvalene complexes [112,113]. 

Bridged Dichromium Complexes.— Aquation of the hydroxo-bridged complex... 

Sulphate exchange with the i -sulphato-dichromium complexes (39) and (40) has been monitored by S labelling. Rates depend on pH ... 

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