A Square-Planar Dinickel(II) Complex with a Noninnocent Dinucleating Oxamate Ligand: Evidence for a Ligand Radical Species

The metal-assisted cycloisomerization of (omega)-alkynols to oxacycloalkylidene complexes is reviewed. A variety of low-valent coordinatively unsaturated transition metal templates from group 6 to 10 mediate the cyclization of (omega)-butynols, -pentynols, and -hexynols to give the corresponding oxacycloalkylidene complexes. In some cases kinetic hydroxyalkylvinylidene complex intermediates can be isolated and characterized.