Abstract
The structural stabilities of the disulphides, diarsenides and sulpharsenides of iron, cobalt and nickel are explained on the basis of ligand field theory. The structural stabilities can be correlated with the number of non-bonding d electrons of the metal atom in the structure, and can be explained by the tendency of the compounds to form structures in which maximum electron spin-pairing takes place. The pyrite structure, which is favoured by metals with six or more non-bonding d electrons, and which includes pyrite, cattierite, vaesite, cobaltite and gersdorffite, is characterized by metal-sulphur octahedra joined at corners, with no apparent interaction between the d electrons of neighbouring metal atoms. The other structures are all characterized by shared octahedral edges along one direction, so that the metal atoms are brought into relatively close proximity. In the marcasite structure, which includes marcasite and rammelsbergite, both with six non-bonding d electrons, the metal atoms repel each other because of completely filled t 2, levels. ln the arsenopyrite structure, which includes arsenopyrite and safflorite, both of which have live d electrons that do not participate in metal-sulphur bonding, pairs of metals are drawn together to permit spinpairing of the odd electrons. ln lollingite, in which the iron atom is assumed to have four non-bonding d electrons, the d orbitals in the c crystallographic direction are emptied, permitting close iron-iron approaches in this direction, as well as complete spin-pairing of the electrons in the two remaining t 2 , orbitals.
Published Version
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