The Ni2+O2 reaction: the IR spectrum and structure of Ni2O2. A combined IR matrix isolation and theoretical study.

The formation of Ni2O2 can be observed from the condensation of effusive beams of Ni and O2 in neon or argon matrices. Observation of 58Ni(2)16O2, 58Ni60Ni16O2, 60Ni2(16)O2, Ni(2)18O2 and Ni(2)16O18O isotopic data for five fundamental transitions enable a discussion of structural parameters for matrix-isolated Ni2O2 in its cyclic ground state. Analysis of the nickel isotopic effects on the 58,60Ni2(16)O18O fundamentals suggest an elongated rhombic structure with a Ni-O bond force constant (240+/-10 N m-1) and NiONi bond angles around 79 degrees. The latter points to a Ni-Ni internuclear distance shorter than the O-O one. Low-lying singlet, triplet and quintet states have been studied using density functional theory with an unrestricted wave function and broken symmetry formalism. The high spin states and closed shell singlet states have been also investigated at the CCSD(T) level. The Ni2O2 ground state is calculated to be an antiferromagnetic singlet state with all the hybrid functionals. The first order properties (energies, geometry) calculated with a hybrid functional are very similar when different exchange-correlation functionals with different exact exchange fractions are used and the calculated ground state geometry (NiONi bond angle near 80 degrees, NiO bond distance around 179.5 pm) is in good agreement with the experimental estimate. Nevertheless, a correct reproduction of the experimental vibrational properties is found only when a hybrid functional containing an exact exchange fraction in the 0.4-0.5 range is used. The orbital and topological bonding analyses of Ni2O2 reveal that the relatively short Ni-Ni internuclear distance within the molecule should not be interpreted as a remaining metal-metal bonding interaction, but clearly indicate that the bonding driving force is due to the formation of four strong and highly polarized Ni-O bonds. Even in such an early stage of metal oxidation, the Ni-Ni interaction has virtually disappeared.