Density functional calculations at the B3LYP level of theory, using the SDD basis set, provide satisfactory description of geometric, energetic, electronic and spectroscopic properties of the Pt(NO)/Pt(NO<sub>2</sub>) redox couple. The neutral Pt(NO) species adopts a bent <sup>2</sup>A' ground state, while the cationic [Pt(NO)]<sup>+</sup> species adopts a linear <sup>1</sup>Ξ£<sup>+</sup> ground state. The B3LYP/SDD- predicted Pt-N bond lengths are 2.016 and 1.777 Γ… for Pt(NO) (<sup>2</sup>A') and [Pt(NO)]<sup>+</sup> (<sup>1</sup>Ξ£<sup>+</sup>), respectively, while the β� Pt-N-O bent angle for [Pt(NO)] (<sup>2</sup>A') is 119.6Β°. On the other hand, the anionic [Pt(NO)]<sup>-</sup> species adopts the bent <sup>1</sup>A' ground state with a Pt-N bond length of 1.867 Γ… and a β� Pt-N-O bent angle of 122.5Β°. The computed binding energies of the NO, NO<sup>+</sup> and NO<sup>-</sup> ligands with Pt(0) were found to be 29.9 (32.8), 69.9 (78.4) and 127.4 (128.7) kcal/mol at the B3LYP/SDD and CCSD(T)/SDD (numbers in parentheses) levels of theory, respectively. Moreover, the structure of the [Pt(NO<sub>2</sub>)]<sup>+</sup> component of the Pt(NO)/Pt(NO<sub>2</sub>) redox couple and its transformation to [Pt(NO)]<sup>+</sup> upon reaction with CO was analysed in the framework of the DFT theory. The coordination of the CO ligand to [Pt(NO<sub>2</sub>)]<sup>+</sup> affords the cationic mixed-ligand [Pt(CO)(NO<sub>2</sub>)]<sup>+</sup> complex, which is stabilized by 66.6 (60.5) kcal/mol, with respect to the separated [Pt(NO<sub>2</sub>)]<sup>+</sup> and CO in their ground states. The O-transfer reaction from the coordinated NO<sub>2</sub> to the coordinated CO ligands in the presence of the [Pt(NO<sub>2</sub>)]<sup>+</sup> species corresponds to an exothermic process; the heat of the reaction (β�†<sub>R</sub><i>H</i>) is -85.2 (-80.5) kcal/mol and the activation barrier amounts to 27.7 (33.0) kcal/mol. Finally, the equilibrium structures of selected stationary points related to the transformation of NO to NO<sub>2</sub> ligand located on the potential energy surfaces of the [Pt(NO),O<sub>2</sub>], [Pt(NO)<sup>+</sup>,O<sub>2</sub>], and [Pt(NO)<sup>-</sup>,O<sub>2</sub>] systems were analysed in the framework of the DFT theory. The computed interaction energies of O<sub>2</sub> with Pt(NO), [Pt(NO)]<sup>+</sup> and [Pt(NO)]<sup>-</sup> species were found to be 106.9 (105.3), 49.2 (48.4) and 26.9 (26.5) kcal/mol, respectively. The O<sub>2</sub> ligand is coordinated to the Pt central atom in an end-on mode for [Pt(NO),O<sub>2</sub>] and [Pt(NO)<sup>-</sup>,O<sub>2</sub>] systems and in a side-on mode for the [Pt(NO)<sup>+</sup>,O<sub>2</sub>] system. The transformation of NO to NO<sub>2</sub> in [Pt(NO)]<sup>-</sup> species upon reaction with dioxygen corresponds to an exothermic process; the heat of the reaction (β�†<sub>R</sub><i>H</i>) is -60.6 (-55.8) kcal/mol, while the activation barrier amounts to 35.5 (30.2) kcal/mol. Calculated structures, relative stability and bonding properties of all stationary points are discussed with respect to computed electronic and spectroscopic properties, such as charge density distribution and harmonic vibrational frequencies.
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