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 met al ions laser-ablated from a metal substrate in vacuum with ammonia-wa ter binary clusters in a molecular beam injected nearby [the laser-abl ation-molecular beam (LAMB) method]. Relative abundances of M+(NH3)(m) (H2O)(n) are characterized by intensity gaps which indicate limited (t ypically 2 or 3) coordination (solvation) numbers in the first coordin ation (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 o f stagnation ratios of two component gases. The coordination numbers a s judged from the intensity gaps remain the same throughout the stagna tion ratios studied, A model simulation of the dynamic processes invol ved was made under simple-minded assumptions: (1) the ensemble of meta l complex ions starting from the reaction region is characterized with a temperature T-start (its value being taken as an adjustable paramet er), (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 bind ing energy of the leaving ligand, and (4) temperature drop rate of com plex ions per one microsecond is constant (its value being taken as an adjustable parameter). Such a simulation procedure is found successfu l in reproducing the positions of intensity gaps, together with the qu alitative features; of the metal-specific coordination (solvation) pat terns observed. (C) 1998 American Institute of Physics. [S0021-9606(98 )00110-X].