bridging group.In a seventh, especially preferred embodiment of the twenty-fourth, twenty-fifth, twenty-sixth, twenty-seventh or twenty-eighth aspect, the Group 8-10 transition metal is nickel and the bidentate or variable denticity ligand is selected from Set 13: wherein:R7a-d are groups other than H or alkyl; provided that at least one of R7a-d is an aryl or heteroaryl group;R2x and R2y are eac