Mechanism of generation of volatile species by aqueous boranes: Towards the clarification of most controversial aspects

The state of knowledge of the mechanisms involved in the chemical generation of volatile species (CHG) arising from aqueous phase reaction of classical hydride forming elements, transition and noble metals with borane complexes (mainly NaBH4), has been critically reviewed in the light of evidences and literature data published in the last fifty years. The mechanisms, which are necessary to describe the reactivity of CHG system, are essentially: (i) the mechanism of hydrolysis of borane complexes, (ii) the mechanism of formation of volatile species, (iii) the mechanism of liquid phase interference and (iv) the mechanism of action of additives. Only the mechanisms (i) and (ii) have reached a good degree of rationalization, whereas more experimental evidences are necessary for the mechanisms (iii) and (iv). A more general reaction model for analytical CHG can be drawn according to the present state of knowledge, which is valid for both classical hydride forming elements and transition and noble metals. It is based on the formation of analyte–borane complex (ABC) intermediates through which takes place the direct, stepwise transfer of hydrogen atoms from boron to analyte substrate MLn, (M is a metal or semi-metal, L is a ligand). By this way the original analyte substrate is stepwise converted to hydrido metal complexes MHxLy, then to the final products (hydride, metal atoms, etc). The clarification of several controversial aspects and the ruling out of wrong concepts, among them the “nascent” hydrogen theory, can been achieved in the light of the present state of knowledge.