Stoichiometric barium pernitride, BaN(2), was prepared from the elements under N(2) pressure of 5600 bar in an autoclave at 920 K. The compound is isotypic to ThC(2) (space group C2/c, Z = 4) according to powder X-ray (neutron) diffraction data with a = 7.1712(1), b = 4.3946(1), c = 7.2362(1) A, and beta = 104.864(1) degrees (a = 7.1745(1), b = 4.3963(1), c = 7.2393(1) A, beta = 104.876(1) degrees ). The N-N distance of 1.221(4) A (based on the neutron diffraction data) is indicative of a double bond in the N(2)(2-) dumbbells. BaN(2) is metallic according to magnetic susceptibility measurements and TB-LMTO band structure calculations.
Compounds of the general formula MCu(2n)X(n)(+1), where M is a monovalent metal and X is a chalcogen, exhibit relatively high conductivity and an interesting structural pattern of copper-chalcogen layers. The electronic structure of a series of copper-sulfur layers with the Cu(2n)S(n)(+1) stoichiometry was studied using the extended Hückel method. Attention was focused on the unoccupied states at the top of the valence band. These states are Cu-S and Cu-Cu antibonding, which accounts for the observed contraction in the plane of the layers. The same states turn out to be strongly delocalized in the plane of the layers, with both copper and sulfur contribution; high mobility of holes in these states is responsible for the substantial conductivity observed in the corresponding materials. The idea of isodesmic reactions, borrowed from computational organic chemistry, was developed to address the relative stabilities of the copper-sulfur layers. We found the Cu(2)S(2)(-) layer to be less stable than the Cu(4)S(3)(-) layer, in accord with experiment.
The aluminum substructure of the AeM2Al9
(Ae = Ba and M = Fe, Co, Ni; Ae = Sr and M = Co,
Ni; Ae = Ca and M = Co) and CaNiAl9 compounds is a
beautiful three-dimensional network of vertex-sharing aluminum octahedra. Bonding in this network is analyzed at
the extended Hückel level by studying
the effect of vertex sharing between isolated aluminum octahedral
clusters in several model systems: a linear
dimer of two clusters, a linear one-dimensional chain of clusters, a
two-dimensional square sheet, and a three-dimensional cubic network of clusters. We find that the number of
skeletal electrons per aluminum cluster
optimal for Al−Al bonding is reduced from 14 for an isolated cluster
to 12 for the cluster dimer and cluster
chain, 10 for the two-dimensional cluster sheet, and 8 for the cubic
network in which all cluster vertices are
shared. Two effects are responsible for the reduction in the
optimum electron count: First, vertex-sharing
reduces the number of skeletal bonding orbitals per cluster (through
restrictions due to translational symmetry
in extended structures). Second, the levels just above the
skeletal bonding states become Al−Al antibonding
due to next-nearest-neighbor intercluster interactions. According
to our calculations, Al−Al bonding in the
aluminum network of the BaFe2Al9-type
compounds is maximized for approximately 9 skeletal electrons
per
aluminum octahedral cluster, which is qualitatively consistent with the
results obtained for the model systems
and our assignment of formal charges. Connections are made to a
number of related structures containing
networks of main group octahedra.
Ce2MnN3 was prepared by reaction of cerium nitride and manganese with nitrogen gas at 900 °C. It crystallizes isotypic to AC2MN3 (Ac = U, Th; M = Cr, Mn) and Ce2CrN3, space group Immm (No. 71), a = 3.74994(6) Å, b = 3.44450(6) Å and c = 12.4601(2) Å. The manganese atoms are coordinated in a nearly square planar fashion by four nitrogen atoms. These corner-connected MnN4 units form infinite 1∞[MnN2N2/2] chains, which run parallel to each other along the crystallographic a-axis, forming the motif of hexagonal rod packing. Cerium atoms connect the chains into a three-dimensional network. The results of measurements of the magnetic susceptibility, as well as of the electrical resistivity suggest metallic behavior. Electronic effects leading to shorter bonds between manganese and bridging nitrogen atoms than between manganese and terminal nitrogen atoms in the 1∞[MnN2N2/2] chains were investigated through extended Hückel and LMTO band structure calculations. Issues pertaining to stability of this and some other nitridometallate structures are discussed.
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