Predominance of covalency in water-vapor-responsive MMX-type chain complexes revealed by ¹²⁹I Mössbauer spectroscopy

¹²⁹I Mössbauer spectroscopy was applied to water-vapor-responsive MMX-type quasi-one-dimensional iodide-bridged Pt complexes (MMX chains) in order to investigate their electronic state quantitatively. Two sets of octuplets observed in K₂(H ₃NC₃H₆NH₃)[Pt₂(pop) ₄I]·4H₂O (2·4H₂O) and one octuplet observed in K₂(cis-H₃NCH₂CHCHCH ₂NH₃)[Pt₂(pop)₄I]·4H ₂O (1·4H₂O) and dehydrated complexes (1 and 2) indicate a unique alternating charge-polarization + charge-density-wave (ACP + CDW) electronic state and a charge-density-wave (CDW) electronic state, respectively. These spectra correspond to their crystal structure and the change of electronic states upon dehydration. Since these complexes consist of an alternating array of positively charged and negatively charged layers, the charge on the iodide ion (ρ_IS) was discussed on the basis of the isomer shift (IS). The ρ_IS of the water-vapor-responsive MMX chains was mainly -0.13 to -0.21, which are the smallest of all MMX chains reported so far. Hence, it indicates that the negative charge on the iodide ion is strongly donated to the Pt ion in these complexes. This covalent interaction predominates in the ACP + CDW state as well as in the CDW state. Therefore, the ACP + CDW state is in fact the CDW state with the ACP-type lattice distortion. Because the ρ_IS became smaller with the decreasing Pt-I-Pt distance, it can be concluded that the covalent interaction plays an important role in determining the electronic states of the MMX chains with pop (= P ₂H₂O₅^2-) ligands.