Limited coordination number and competitive coordination in ammonia-water mixed-ligand complexes of monopositive metal ions as studied by the laser-ablation molecular beam method: Experiment and simulation

Ammonia-water mixed-ligand complexes of monopositive metal ions M+ (M=Mg, Al, Mn, and Co) were prepared in the gas phase by reactions of metal ions laser-ablated from a metal substrate in vacuum with ammonia-water binary clusters in a molecular beam injected nearby [the laser-ablation-molecular beam (LAMB) method]. Relative abundances of M+(NH3)(m)(H2O)(n) are characterized by intensity gaps which indicate limited (typically 2 or 3) coordination (solvation) numbers in the first coordination (solvation) sphere. Three patterns of competitive coordination (solvation), i.e., selective, nonselective, and magic-number-like, are observed. The patterns are metal-specific and relatively independent of stagnation ratios of two component gases. The coordination numbers as judged from the intensity gaps remain the same throughout the stagnation ratios studied, A model simulation of the dynamic processes involved was made under simple-minded assumptions: (1) the ensemble of metal complex ions starting from the reaction region is characterized with a temperature T-start (its value being taken as an adjustable parameter), (2) only evaporation of component ligands one by one occurs after metal complex ions start from the reaction region into the quadrupole, (3) activation energy of each evaporation step is determined by binding energy of the leaving ligand, and (4) temperature drop rate of complex ions per one microsecond is constant (its value being taken as an adjustable parameter). Such a simulation procedure is found successful in reproducing the positions of intensity gaps, together with the qualitative features; of the metal-specific coordination (solvation) patterns observed. (C) 1998 American Institute of Physics. [S0021-9606(98)00110-X].