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I searched on the internet but failed to find a clear solution for this question:

Regarding transition metal sulfides like NiS or FeS, is there metallic bonding within their crystal structure?

If yes, how does it influence the macroscopic properties of the crystal and the possible reactions of the ions? Does the S atom affect the construction of the energy band and how do we calculate the certain effect?

If no, then how do we explain the low reflect rate, metallic luster and (semi-)conductivity?

My thoughts: there is metallic bonding in the crystals.

  1. The polarization effect is most apparent in compounds of transitive metal which represents the covalent bonding in this type of compound. In other words, it signifies the extent to which the electrons are localized between the S atom and the metal atom. However, the formation of metallic bond needs the elections to be localized between metallic atoms. The paradox means that either the metallic bond can stabilize the S atoms, or the S atoms are involved in the formation of energy band, or the S atoms can form a special structure that stabilizes themselves.

  2. Due to the structure of NiS crystal, this compound may have a layered structure. Hence, its fragility may be explained by a layered metallic bonding structure in the crystal.

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    $\begingroup$ modern chemistry does not consider metallic bonding a thing, it is a variation of electron-defiicient covalent bonding. Metallic behaviour is exactly this: a macroscopic behaviour, manifesting in a relatively high electrical conductivity. LIsting of some data on elecrical conductivitly of sulfides can be found here, together with bits of relevant theory ela-iet.com/IronGlen/PearceMagSulfides127.pdf $\endgroup$ – permeakra May 5 '17 at 14:04

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