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Abstract

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(NO₂) redox couple. The neutral Pt(NO) species adopts a bent ²A' ground state, while the cationic [Pt(NO)]⁺ species adopts a linear ¹Σ⁺ ground state. The B3LYP/SDD- predicted Pt-N bond lengths are 2.016 and 1.777 Å for Pt(NO) (²A') and [Pt(NO)]⁺ (¹Σ⁺), respectively, while the ∠Pt-N-O bent angle for [Pt(NO)] (²A') is 119.6°. On the other hand, the anionic [Pt(NO)]^- species adopts the bent ¹A' ground state with a Pt-N bond length of 1.867 Å and a ∠Pt-N-O bent angle of 122.5°. The computed binding energies of the NO, NO⁺ and NO^- 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(NO₂)]⁺ component of the Pt(NO)/Pt(NO₂) redox couple and its transformation to [Pt(NO)]⁺ upon reaction with CO was analysed in the framework of the DFT theory. The coordination of the CO ligand to [Pt(NO₂)]⁺ affords the cationic mixed-ligand [Pt(CO)(NO₂)]⁺ complex, which is stabilized by 66.6 (60.5) kcal/mol, with respect to the separated [Pt(NO₂)]⁺ and CO in their ground states. The O-transfer reaction from the coordinated NO₂ to the coordinated CO ligands in the presence of the [Pt(NO₂)]⁺ species corresponds to an exothermic process; the heat of the reaction (∆_RH) 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 NO₂ ligand located on the potential energy surfaces of the [Pt(NO),O₂], [Pt(NO)⁺,O₂], and [Pt(NO)^-,O₂] systems were analysed in the framework of the DFT theory. The computed interaction energies of O₂ with Pt(NO), [Pt(NO)]⁺ and [Pt(NO)]^- species were found to be 106.9 (105.3), 49.2 (48.4) and 26.9 (26.5) kcal/mol, respectively. The O₂ ligand is coordinated to the Pt central atom in an end-on mode for [Pt(NO),O₂] and [Pt(NO)^-,O₂] systems and in a side-on mode for the [Pt(NO)⁺,O₂] system. The transformation of NO to NO₂ in [Pt(NO)]^- species upon reaction with dioxygen corresponds to an exothermic process; the heat of the reaction (∆_RH) 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.

Keywords: DFT; Nitrosylplatinum; Nitroplatinum; Redox couple; Electronic structure; Ab initio calculations.

References: 61 live references.