Dihalogeno(diphosphane)metal( ii ) complexes (metal = Co, Ni, Pd) as pre-catalysts for the vinyl/addition polymerization of norbornene – elucidation of the activation process with B(C 6 F 5 ) 3 /AlEt 3 or Agcloso-1-CB 11 H 12 and evidence for the in situ formation of “naked” Pd 2+ as a highly active species