r heteroatom-containing hydrocarbyl group to form a metallocycle containing five, six or seven members, including M, provided that R21 can be joined to the divalent hydrocarbyl or heteroatom-containing hydrocarbyl group connecting E??? to X4 as part of an aromatic ring, each n??? is 1 or 2, and each n??? is 0 or 1, depending on the bond order of the adjacent bonds, and provided that optionally two