Molecular transition-metal complexes in solids

To give more breadth to our discussion of the connections between molecules and solids we now consider examples of systems in which one can see transition-metal complexes. They are of pedagogical interest because their simple electronic structures can be used to good effect in elementary treatments of the electronic structure of coordination compounds. 

Ternary metal hydrides of formula A2MH6 (A = Mg, Ca, Sr, Eu and M = Fe, Ru and Os) are known. Full characterization requires knowledge of the distribution of the H atoms within the structure of the A2M host. Using what we have learned, let s try to deduce their arrangement employing Mg2FeH6 as an example. 

But with your experience of the earlier examples, you are probably wondering if the perturbations by the electropositive Mg2+ cations invalidate this simple model. Band-structure calculations indicate that, although the valence orbitals of Mg reinforce stability through covalent interactions with the Fe and the H atoms, the molecular properties essentially survive. That is, the occupied t2g and vacant eg levels of the [FeH6]4- anions give rise to separated bands of low dispersion (flat) in the solid in agreement with the Zintl-Klemm analysis and the behavior of the material as an insulator. 

Exercise 7.6. The solid-state compound Mg2CoH5 has been prepared and exhibits the same metal lattice as MgFeH6. Using the Zintl-Klemm approach and your knowledge of molecular coordination chemistry, suggest a logical geometry for the H atoms around the Co center. 

Answer. As we deal again with Mg2+ ions, the anion is formulated as [CoH5]4 with Co1 and five [H] ligands. Thus, it is an 18-electron compound. The limiting geometries adopted by five-coordinate complexes are trigonal bipyramidal (D3h symmetry and most common) for which Fe(CO)s provides an example and square pyramidal (C4v symmetry) for which [Ni(CN)s]3- provides an example, albeit slightly distorted. These, and distorted structures in between, have similar stabilities. 

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