. . . "Other preferred molecules are illustrated by Formula XXI: where K1, K2, K3, and K4 are independently selected from the group consisting of N, O, S, Se, Te, and CH; M is a metal or (H,H); L1, L2, and L3, and L4 are independently present or absent and, when present, are linkers; and X1, X2, X3, and X4 are independently selected from the group consisting of a substrate, a reactive site that can coval" . .