Transition element complexes, photochemistry

Photochemistry of Metal-Metal-Bonded Transition Element Complexes... 

The photochemical studies of transition metal hydride complexes that have appeared in the chemical literature are reviewed, with primary emphasis on studies of iridium and ruthenium that were conducted by our research group. The photochemistry of the molybdenum hydride complexes Mo(tj5-C5H5)2M2] and [MoH4(dppe)2] (dppe = Ph2PCH2CH2PPh2), which eliminate H2 upon photolysis, is discussed in detail. The photoinduced elimination of molecular hydrogen from di-and polyhydride complexes of the transition elements is proposed to be a general reaction pathway. 

Molecules that contain heavy elements (in particular 5d transition metals) play an important role in the photochemistry and photophysics of coordination compounds for their luminescent properties as well as for their implication in catalysis and energy/electron transfer processes. Whereas molecular properties and electronic spectroscopy of light molecules can be studied in a non-relativistic quantum chemical model, one has to consider the theory of relativity when dealing with elements that belong to the lower region of the periodic table. As far as transition metal complexes are concerned one has to distinguish between different manifestations of relativity. Important but not directly observable manifestations of relativity are the mass velocity correction and the Darwin correction. These terms lead to the so-called relativistic contraction of the s- and p- shells and to the relativistic expansion of the d- and f- shells. A chemical consequence of this is for instance a destabilisation of the 5d shells with respect to the 3d shells in transition metals. 

Geoffroy, George, L., Photochemistry of Transition Metal Hydride Complexes George, J. W., Halides and Oxyhalides of the Elements of Groups Vb and VIb George, Philip and McClure, Donald S., The Effect of Inner Orbital Splitting on the Thermodynamic Properties of Transition Metal Compounds, and... 

Rn(II) and Os(II) Complexes. We restrict ourselves again to the same class of compounds, as with Pd and Pt, that is, cyclometallated species with N- and C-donor atoms, that are parts of aromatic systems. Whereas cyclometallated complexes of this type have been reported for Ru(II), almost nothing is known about analogous Os(II) compounds. This is somewhat surprising, since third row transition metals in the other groups seem to be even more apt to form stable cyclometallated species compared to second row elements. There is probably still a lot of chemistry to be done in this field, with potentially very interesting applications to photochemistry. 

---