The chemical bonding topology of ternary and quaternary transition metal nitrides containing low-coordinate metal atoms

Abstract

Solid state ternary and quaternary metal nitrides containing a transition metal and one or two very electropositive metals such as alkali and (or) alkaline earth metals exhibit a number of structures containing low-coordinate transition metals in the transition metal – nitrogen subnetwork. Transition metal – nitrogen double and even triple bonds are found in these structures involving dπ → pπ bonding from a filled transition metal d orbital to an otherwise empty nitrogen p orbital. Such multiply bonded nitrogen atoms function as strong field ligands in contrast to amido ligands such as (Me3Si)2N−, which generally function as weak field ligands in transition metal chemistry. The transition metal environment in these ternary nitrides can be modelled by "banana bonds" from the polyhedron using the atomic orbitals required for both the σ- and π-bonding to the nitride ligands, e.g., the trigonal prism for the trigonal planar derivatives MIIIN36− (M = V, Cr, Mn, Fe) with three M=N double bonds as well as FeIIN24− in Li4FeIIN2 with two Fe≡N triple bonds and the planar square for the discrete linear CoIN25− in LiSr2CoN2. Reasonable electron counts are also obtained for the anionic metal–nitrogen networks in Ca2FeN2, Sr2LiFe2N3, Ba2LiFe2N3, Li3FeN2, CaNiN, Li3Sr3Ni4N4, BaNiN, and Ba8Ni6N7. Keywords: chemical bonding, topology, nitrides, transition metals.