Hydrido Complexes of Calcium: A New Family of Molecular Alkaline‐Earth‐Metal Compounds

Abstract

The application of solid calcium hydride CaH has been mostly confined to its use as a desiccant, although its catalytic activity has long been known. Since the first isolation of a well-defined molecular calcium hydride in 2006, the past decade has witnessed a gradual emergence of this new family of compounds. Although the detrimental Schlenk equilibrium has kept the number of examples low, the novelty of their reactivity, especially in small-molecule activation, holds great promise. This Minireview gives an overview of the molecular calcium hydrides reported to date, highlighting their synthesis, structure, and reactivity.

Super‐Bulky Penta‐arylcyclopentadienyl Ligands: Isolation of the Full Range of Half‐Sandwich Heavy Alkaline‐Earth Metal Hydrides

“…The bridging hydrido ligand (H1) was located by difference Fourier syntheses and refined isotropically. The Ca−H distances are identical (2.29(4) and 2.28(4) Å), which fall in the range (2.09(4)–2.60(3) Å) found in the previous molecular calcium hydride complexes …”

“…Among the limited number of well‐defined molecular Group 2 metal–hydride complexes, magnesium hydrides dominate with over 40 examples . Only a handful of calcium hydrides are known . The heavier strontium and barium hydrides are scarce, with a record of three and two respectively.…”

“…[5] Only ah andful of calcium hydrides are known. [6] Theh eavier strontium and barium hydrides are scarce,w ith ar ecord of three [7] and two [8] respectively.T he tapering distribution is consistent with the increasing difficulty in isolating Group 2 metal hydride species (Mg < Ca < Sr < Ba), mainly attributed to the increasing ionic character and bond lengths,a nd decreasing bond energies as the group is descended. To prevent the formation of insoluble metal hydride through Schlenk equilibrium in solution, suitable ligands are required.…”

Heterolysis of Dihydrogen by Nucleophilic Calcium Alkyls

“…[29,30] Although largely ac onsequence of the highly polarized nature of the Ca À Cbonding,more generally these observations underscore the view that group 2o rganometallic reactivity is not simply "lanthanide mimetic", but displays features that are unique to each of the available elements.W eare continuing to examine these possibilities. [29,30] Although largely ac onsequence of the highly polarized nature of the Ca À Cbonding,more generally these observations underscore the view that group 2o rganometallic reactivity is not simply "lanthanide mimetic", but displays features that are unique to each of the available elements.W eare continuing to examine these possibilities.…”

Low‐Coordinate Barium Boryloxides: Synthesis and Dehydrocoupling Catalysis for the Production of Borasiloxanes

“…[1] These developments can be paralleled with the many breakthroughs in the organometallic chemistry of calcium, [2][3][4] while in comparison progress with strontium [5] and barium [6] has been slow owing to greater synthetic challenges. Ca 2+ (rionic = 1.00 Å), Sr 2+ (1.18 Å) and Ba 2+ (1.35 Å) are large, electropositive ions that generate highly ionic d 0 complexes.…”