Fulvalene tetracarbonyldiruthenium. Storage of light energy

In the last twelve years, special attention has been devoted to photochemistry of the dinuclear organometallic complexes, since different structures undergo light excitation leading to ligand replacements, disproportionation, or to different redox reactions. Detailed data on the nature of the primary photo reaction and on the reaction intermediates are known and have been published for various metal n complexes, including complexes of the [Cp2M2(CO)4] type (M = Fe or Ru). 

Extensive studies of iron dimers suggest at least three independent primary processes  

Thermally reversible photoisomerization of complex 1. - Solution of the yellow crystals of 1 in THF, when exposed to light, as described, converts to the complex 2 in 80% yield. The structure of 2 has been confirmed by IR and H NMR spectra, which show that the metal-metal bond is broken, which has been definitively proven by structural X-ray analysis. 

The reverse reaction 2 — 1, which takes place in THF at only 65 °C, or in the crystal at 208 °C, causes crystals to bounce a few centimeters high, which is known as Hupfeffekt (hUpfen in German means to bounce) English expression is thermosalient . Vollhardt etal. carried out 10 cycles of these conversions with no noticeable decomposition of the substrate, announcing a possible practical application. 

The 13= 2 interconversion involves a very interesting problem of the metal-metal bond breaking and also Cp-Cp bond breaking (between cyclopentadienyls), and their subsequent bond making with the metal, as r -C5H4-M. It should be stressed that 1 absorbs light more effectively than 2, and that we actually deal with a very unusual oxidative addition 3=t reductive elimination reaction. The reaction is claimed to follow a concerted mechanism and not to involve intermediates, though the formation of diradicals cannot be excluded.  

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