We report the synthesis and properties of the first representative of a new class of PtL2 complexes with ambidentate mixed-donor cyanoxime ligands [L = 2-cyano-2-oximino-N,N'-diethylaminoacetamide, DECO (1)]. Three differently colored polymorphs of Pt(DECO)2 (35) were isolated, with the first two being crystallographically characterized. The dark-green complex [Pt(DECO)2]n (5) spontaneously forms in aqueous solution via aggregation of yellow monomeric complex 3 into the red dimer [Pt(DECO)(2)](2) (4), followed by further oligomerization into coordination polymer 5. A spectroscopic and light-scattering study revealed a poker-chips-type 1D polymeric structure of 5 in which units are held by noncovalent metallophilic interactions, forming a Pt-Pt wire. The polymer 5 shows a broad absorption at 400900 nm and emission at unusually long wavelengths in the range of 10001100 nm in the solid state. The near-infrared (NIR) emission of polymer 5 is due to the formation of a small amount of nonstoichiometric mixed-valence PtII/PtIV species during synthesis. A featureless electron paramagnetic resonance spectrum of solid sample 5 recorded at +23 and -193 degrees C evidences the absence of PtIII states, and the compound represents a solid solution containing mixed-valence PtII/PtIV centers. Exposure of KBr pellets with 5% 5 to Br-2 vapors leads to an immediate similar to 30% increase in the intensity of photoluminescence at 1024 nm, which confirms the role and importance of mixed-valence species for the NIR emission. Thus, the emission is further enhanced upon additional oxidation of PtII centers, which improves delocalization of electrons along the Pt-Pt vector. Other polymorph of the Pt(DECO)(2) complexmonomerdid not demonstrate luminescent properties in solutions and the solid state. An excitation scan of 5 embedded in KBr tablets revealed an emission only weakly dependent on the wavelength of excitation. The NIR emission of quasi-1D complex 5 was studied in the range of -193 to +67 degrees C. Data showed a blue shift of ?max and a simultaneous increase in the emission line intensity with a temperature rise, which is explained by analogy with similar behavior of known quasi-1D K2[Pt(CN)(4)]-based solids, quantum dots, and quantum wells with delocalized carriers. The presented finding opens a route to a new class of platinum cyanoxime based NIR emissive complexes that could be used in the design of novel NIR emitters and imaging agents.