Sulfur-nitrogen rings electronic structure/aromaticity

Thiophene (see Fig. 2), a five-membered aromatic ring containing sulfur, is comparable in structure to pyrrole and furan, which contain nitrogen and oxygen, respectively, but it possesses some unique qualities. Sulfur is an electron-donating heteroatom and it contributes two electrons to the 6jt electron-system of the ring and additionally has a lone pair of electrons in an sp -hybridized orbital, in the plane of the ring. Thiophene is thus an electron-rich aromatic heterocycle. 

Metallaheterocycles with the metal atom capable of supporting a carbon-metal double bond, and with oxygen, nitrogen, sulfur, or selenium as the second heteroatom are the only structures capable of full conjugation. These compounds show furan-like aromaticity with the heteroatom /i-electrons participating. No examples of this type of ring system were uncovered, undoubtedly due to the stability of the metalla-carbon double bonds required for their formation. 

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