COMPLEXATION OF POLYETHERS AND PYRIDYL LIGANDS WITH MONOPOSITIVE TRANSITION-METAL IONS IN THE GAS-PHASE

The formation, ligand exchange reactions, and dissociation of various types of mixed ligand/metal complexes are studied in a quadrupole ion trap. The monopositive complexes are generated by ion-molecule associa tion reactions between the polydentate organic ligands and laser-desor bed monopositive metal ions (M+) and incorporate at least one polyethe r or at least one pyridyl ligand bound to a transition metal ion. Liga nd exchange and collisional activated dissociation (CAD) techniques ar e used to investigate the nature of the binding interactions and the r elative binding free energies of the various ligands. There are two do minant influences on the strength of the binding interactions in the c omplexes. First, the number of coordination sites of each ligand and i ts flexibility or ability to participate in cooperative binding intera ctions are reflected in the relative coordination capabilities of the ligands. Second, the ability of a ligand to approach the metal ion and successfully adopt a favorable binding conformation differs greatly w hen the metal is already bound to one versus two ligands. For example, when the metal is bound to two pyridyl ligands, the ability of a larg e flexible polyether with multiple oxygen binding sites to displace ev en one of the pyridyl ligands is greatly reduced relative to the case when the metal is only bound to a single pyridyl ligand. In addition t o these two trends, the electronic configuration of the metal ion also plays a subtle role in terms of influencing the stabilities of certai n monomer (pyridyl ligand + M+) and heterodimer (pyridyl ligand + M+ polyether) complexes. However, the nature of the metal ion does not a ppear to change the trends in relative binding free energies of the va rious ligands; it simply influences the formation of certain types of complexes based on the optimum coordination number and favored coordin ation geometry of the metal ion. (Int J Mass Spectrom 176 (1998) 39-61 ) (C) 1998 Elsevier Science B.V.