We here present an analysis of valence distributions and Peierls-type distortions in one-dimensional halogen-bridged chains MMX. In those systems which result from the linking of metal dimers, the phenomenological chain and cluster models show that Peierls distortions can be associated with internal degrees of freedom in dimers and not with the displacements of bridging halogen atoms. On the other hand, our semiempirical quantum-chemical band calculations suggest that the role of internal Peierls mode in Pt-2(dta)(4)I chains can be played by the twisting distortion of dta ligand. The resulting distribution of valence states is -M+2-M+3-X-M+3-M+2, in agreement with some experimental data.