POP-type MMX-chain compounds with binary countercations and vapochromism

Pyrophosphito-bridged diplatinum complex is one of the most studied dinuclear complexes in a paddle-wheel structure. The chemistry of pyrophosphito-bridged diplatinum complex began at the discovery of the Pt(II)-Pt(II) complex, K₄[Pt₂(pop)₄]·2H₂O (pop = P₂H₂O ₅ ^2-) by Roundhill et al. in 1977 [1]. Since [Pt₂(pop)₄]^4- shows intense long-lived phosphorescence, photochemistry and excited-state chemistry of [Pt₂(pop)₄]^4- has been attracted much attention (see following early reviews and references therein [2, 3]). Although no chemical bond exist between two Pt(II) atoms in the ground state, 5dσ → 6pσ transition should induce the bonding character between them. This excited-state structure has been confirmed by several optical methods [4-9]. The excited state of [Pt₂(pop)₄]^4- is powerful one-electron reductant, therefore, it can be used as a photochemical catalyst for converting ethanol to acetaldehyde and hydrogen [10] and for the transfer hydrogenation of alkenes and alkynes [11, 12]. The Pt(III)-Pt(III) complex, [Pt₂(pop)₄H₂]^4-, is the active species of the catalytic reaction [13].