Thermally induced endo to exo isomerization in the o-xylylene complexes of zerovalent ruthenium and osmium, M( η 6-C 6Me 6){ η 4- o-(CH 2) 2C 6Me 4} (M=Ru, Os) 1 Dedicated to Professor Akira Nakamura on the occasion of his retirement from Osaka University. 1