Bands, bonds, and charge-density waves in the Nb<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Se</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>family of compounds

Through detailed examination of the crystal structure and bonding conditions in the trichalcogenides, it has been possible to gain a deeper understanding of the band structure of these complex materials, and with this of their unusual electronic behavior. The character of the observed semimetallicity in Ta${\mathrm{Se}}_{3}$ and Nb${\mathrm{Se}}_{3}$ is much clarified. Each material in the group-V family is seen to achieve a very individually tailored band structure, enfolding the various structural and energy-level adjustments. The nonuniformity and the molecularization of these structures contrasts strongly with the dichalcogenide behavior. The structural, if not the electronic, dimensionality of the group-V trichalcogenides remains closer to two than to one. It is found that the periodic structural distortions developed by Nb${\mathrm{Se}}_{3}$ are probably more suited to a metal-metal bonding description than to the traditional Fermi surface determined instability of a charge-density wave. The field-induced sliding of these distortions can then be described in terms of cooperative bond flipping through a discommensurate superlattice. We examine in crystal chemical terms why this motion is so unusually weakly pinned to defects and impurities in Nb${\mathrm{Se}}_{3}$. In fact, the investigation shows that it will be very hard to find a material better suited to the realization of this phenomenon.