The successive protonation. steps occurring in 2,2';6',2 ''-terpyridine (L1) are characterized by a strong affinity for the first entering proton (Delta G(connect)(H,L1) = -17 kJ/mol) followed by allosteric anticooperativity (Delta E-interaction(H,H,L1) = 6 kj/mol), a behavior mirrored by 2,6-bis(azaindolyl)pyridine (L2) despite the extension of the chelate ring size from five members (L1) to six members (L2; Delta G(connect)(H,L2) = -28 kJ/mol and Delta E-interaction(H,H,L2) = 7 kj/mol). On the contrary, 2,6-bis(8-quinolinyl)pyridine (L3) is less eager for the initial protonation (Delta G(connect)(H,L3) = -10 kj/mol), but the fixation of a second proton in [H(2)L3](2+) is driven to completion by positive cooperativity (Delta E-interaction(H,H,L3) = -5 kj/mol). Because of its unusual ability to adopt a cis-cis conformation with a large affinity for the entering protons, L2 has been selected for exploring the reactivity of a terdentate fused six-membered chelate with labile metallic cations possessing increasing electrostatic factors along the series Mz+ = Li+ < Mg2+ approximate to Zn2+ < Y3+. Spectroscopic, thermodynamic, and structural studies demonstrate that covalency is crucial for stabilizing the complexes [Zn(L-2)(n)](2+). With the highly charged Y3+ cation, hydrolysis drastically competes with ligand complexation, but anhydrous conditions restore sufficient selectivity for the successful coordination of neutral fused six-membered polyaromatic terdentate chelates with large 4f-block cations.