Axially asymmetric metal alkyls. Part 2. Synthesis of the di-Grignard reagent [{Mg(thf)nCl}2R][thf = tetrahydrofuran, R =(2-CH2C6H4)22–] and 6,7-dihydro-5H-dibenzo[c,e]-silepines and -stannepines; X-ray crystal structures of [SnPh2R] and δ-SS- and λ-RR-[SnPh2{[2-CH(SiMe3)C6H4]2}]