Polarized single-crystal optical spectroscopy, together with a scattered wave Xα analysis, provides an understanding of the intriguing blue color and the metal-metal interactions in the Pt(2.25) chain complex cis-diammineplatinum α-pyridone blue (PPB), cis-[Pt(NH3)2(C5H4N0)]4(N03)5H20. Pt-Pt bonding is found to be mainly due to σ overlap between Pt dz2, s hybrid orbitals. The net σ-bonding interaction between the end pairs of Pt atoms in the chain is stronger than between the middle pair. The PPB HOMO and LUMO are Pt-Pt σ* in character and are delocalized over all four Pt atoms. Immediately below the HOMO are two orbitals with Pt-pyridone oxygen π* character. These orbital characteristics are used to rationalize the redox chemistry of PPB. Of the unpaired spin density 94% is contained within the Pt spheres, with 43% in the two end spheres and 51% in the two inner spheres. PPB is therefore a Robin-Day class III-A compound. The observed [and calculated] optical transition energies (eV), intensities, and polarizations are as follows: ~1.55 vw,? [1.63 w, x]; 1.82 s, z [1.50 s, z; 1.53 s, z; 2.14 s, z]; 2.25 vw, x [2.15 vw, x; 2.16 vw, x; 2.39 vw, x]; 2.58 m, z [2.80 s, z]; 2.69 wm, x [2.39 m, x]; 3.10 w, x [2.84 w, x; 3.10 w, x], The blue color of PPB is due to the intense z-polarized transitions at 1.82 (680 nm) and 2.58 eV (480 nm). These may be described, respectively, as inner Pt-Pt bonding →→→ inner Pt-Pt σ* and outer Pt-Pt π → outer Pt-Pt σ*. Both inner Pt → outer Pt and outer Pt → inner Pt charge transfer make important contributions to the intensity of the 680-nm band, while outer Pt → inner Pt charge transfer is the main contributor to the intensity of the 480-nm band.
ASJC Scopus subject areas
- Colloid and Surface Chemistry