Chemical Bonding Analysis on Amphoteric Hydrogen - Alkaline Earth Ammine Borohydrides

TL;DR

This study investigates the unique amphoteric behavior of hydrogen in alkaline earth ammine borohydrides using computational methods, revealing hydrogen's variable oxidation states and covalent bonding implications for hydrogen storage.

Contribution

It provides the first detailed computational analysis of hydrogen's amphoteric nature in AABH compounds, highlighting its non-integer oxidation states and covalent character.

Findings

Hydrogen exhibits amphoteric behavior with positive and negative oxidation states.

Covalent bonding influences hydrogen's oxidation state, making it non-integer.

Implications for hydrogen storage applications are discussed.

Abstract

Usually the ions in solid are in the positive oxidation states or in the negative oxidation state depending upon the chemical environment. It is highly unusual for an ion having both positive as well as negative oxidation state in a particular compound. Structural analysis suggest that the alkaline earth ammine borohydrides (AABH) with the chemical formula M (BH4)2(NH3)2 (M = Mg, Ca, or Sr) where hydrogen is present in +1 and -1 oxidation states. In order to understand the oxidation states of hydrogen and also the character of chemical bond present in AABH we have made charge density, electron localization function, Born effective charge, Bader effective charge, and density of states analyses using result from the density functional calculations. Our detailed analyses show that hydrogen is in amphoteric behavior with hydrogen closer to boron is in negative oxidation state and that closer to nitrogen is in the positive oxidation state. Due to the presence of finite covalent bonding between the consitutents in AABH the oxidation state of hydrogen is non-interger value. The confirmation of the presence of amphtoric behavior of hydrogen in AABH has implication in hydrogen storage applications.