7t back-bonding

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

Protonation, and likewise alkylation, leads to weakening of the respective S —> Fe a bonds and an inductive withdrawal of electron density from the Fe centers. The newly formed thiol or thioether donors have 71-acceptor capability and partial Fe—S n back-bonding leads to a further decrease of electron density at Fe. However, the weakening of the Fe— S a bonds and formation of the Fe —> S 7t back-bonds compensate each other so that the Fe—S distances remain invariant. 

Tertiary phosphines—and the analogous arsines-are able to stabilize transition metals in a variety of oxidation states and coordination geometries. Investigations of complexation with P-ligands were promoted by the high stabilization of metal by P ligands, which is mainly due to 7t-back bonding. 

Calculations for trigonal bipyramidal ML4(NO) systems with axial NO-like [Ir(NO)(PPh3)3H ] give a d orbital sequence of xz,yz < xy so that in such an IrNO system, the orbital is unoccupied not only does bending not produce any stabilization but in fact d z, d — 7T back-bonding is lost, favouring a linear Ir—N—O bond. 

---