In a first preferred embodiment of this thirtieth aspect, the bidentate or variable denticity ligand is selected from Set 15: wherein:R6a and R6b are each independently an aromatic or heteroaromatic ring wherein the ortho positions of said ring(s) are substituted by groups other than H or alkyl; provided that at least one of the ortho positions of at least one of said aromatic or heteroaromatic ri