Crystal chemistry of light metal borohydrides

TL;DR

This review explores the crystal structures, bonding, and phase behavior of light metal borohydrides, highlighting their structural diversity, stability factors, and similarities to metal-organic frameworks.

Contribution

It provides a comprehensive analysis of the structural chemistry of light metal borohydrides, including recent insights into their phase transitions and potential for chemical modification.

Findings

BH4 groups generally adopt tetrahedral geometry

Porous and framework structures observed in less electropositive metal borohydrides

Directional BH4...M interactions drive structural complexity

Abstract

Crystal chemistry of M(BH4)n, where M is a 2nd-4th period element, is reviewed. It is shown that except certain cases, the BH4 group has a nearly ideal tetrahedral geometry. Corrections of the experimentally determined H-positions, accounting for the displacement of the electron cloud relative to an average nuclear position and for a libration of the BH4 group, are considered. Recent studies of structural evolution with temperature and pressure are reviewed. Some borohydrides involving less electropositive metals (e.g. Mg and Zn) reveal porous structures and dense interpenetrated frameworks, thus resembling metal-organic frameworks (MOFs). Analysis of phase transitions, and the related changes of the coordination geometries for M atoms and BH4 groups, suggests that the directional BH4...M interaction is at the origin of the structural complexity of borohydrides. The ways to influence their stability by chemical modification are discussed.