Group 8 meta-metal bonds

Metal or Bismuth(III)-Main Group Meta Bonds.783... 

The compounds discussed in 6.2.3 are significant as examples of metal-meta bonding in group IIIB, but their value for chemical utilization is restricted by their insolubility and their tendency to disproportionate to E(0) or E(I) and E(III) states. 

Aromatic substituents at C2 and C4 are to some extent hindered in rotation. This leads to a third type of isomerism in bispidine chemistry (i.e., atropisomer-ism). The activation energy for the rotation around the C2/C4—aryl bond for bispidones with various meta-substituted phenyl groups was determined by various NMR methods and found to be 70-75 kJ mol (23). For a rotation of 180°, which is usually necessary for the coordination of bispidine ligands to metal ions (e.g., 14, see Scheme 14), two energy barriers have to be overcome. The higher is the result of an interaction of the ortho-disposed proton of the aromatic ring with the proton or the alkyl substituent at the N3 amine nitrogen atom. The... 

In the selective formation of palladacycle 114, electronic factors may also play a role. It is known that the presence of a 3-methoxy substituent in an aryne gives the triple bond a partial positive charge meta to the methoxy group [2,3]. It is therefore reasonable to expect an electron-rich metal to attack at this position, resulting in the formation of intermediate 114 (Scheme 28). 

---