Matrix Isolation Infrared Spectroscopic And Theoretical Study Of Noble Gas-Transition Metal Oxide Complexes

Noble gas-transition metal oxide complexes have been investigated by matrixisolation infrared spectroscopy and theoretical calculations. The complexes wereprepared by co-deposition of transition metal oxides with noble gas atoms at 12 K,and were characterized on the basis of frequency shifts via noble gas doping and ¹⁸O₂isotopic substitution, as well as quantum chemical calculations. The metal oxideswere generated either via the reactions of laser-evaporated metal atoms with oxygenor by laser-evaporation of bulk metal oxide targets. The bonding mechanism andperiodic trends in these transition metal noble gas complexes were discussed.The experimentally characterized noble gas-transition-metal complexes include:(1) The ScO⁺ cation coordinates five noble gas atoms in forming the[ScO(Ng)₅]⁺(Ng=Ar, Kr, Xe) complexes, and YO⁺ cation coordinates six Ar or Kr andfive Xe atoms in forming the [YO(Ng)₆]⁺ (Ng=Ar, Kr) and [YO(Xe)₅] complexes innoble gas matrixes. (2) The 3d transition metal monoxides coordinate one noble gasatom in forming the linear NgMO(M=Cr, Mn, Fe, Co, Ni; Ng=Ar, Kr, Xe) complexes.(3) The group VB metal oxides MO₂ and MO₄ coordinate two and one noble gasatoms in forming the MO₂(Ng)₂ and MO₄(Ng)(M=V, Nb, Ta; Ng=Ar, Kr, Xe)complexes. (4) A chromium(V1) oxo-superoxide complex (η²-O₂)₂CrO₂ reacts withXe in forming the chromium(â…¥) oxo-peroxide-Xe complex. The bonding ofabove-characterized noble gas-transition metal oxide complexes involves the Lewisacid-base interactions, in which electron density of the Ng lone pair is donated intothe vacant or partially occupied MO's of the transition metal oxides.According to molecular orbital theory, the main bonding orbitals of 3d transitionmetal monoxides MO are 9σ, 1δand 4Ï€molecular orbitals. The 9σis primarily anonbonding hybrid of the metal 4s and 3d_z² orbitals that is directed away from the Oatom; The 1δmolecular orbital is largely 3d orbital of metal that is mainlynonbonding; the doubly degenerated 4Ï€molecular orbitals are the combination of themetal 3dÏ€and O 2pÏ€atomic orbitals, which are M-O antibonding in character. The 9σand 1δorbitals of ScO⁺ are the primary acceptor orbitals for donation from noble gasatoms, which lead to the formation of the [ScO(Ng)₅]⁺ (Ng=Ar, Kr, Xe) complexes.The yttrium-based nonbonding LUMOσorbital of YO⁺ is virtually at the same energy level as the 9σLUMO of ScO⁺, but the LUMO+1 nonbondingδorbital ofYO⁺ is significantly higher in energy than that of the 1δorbital of ScO⁺. Hence,donation from the valence p orbitals of noble gas atoms onto theσLUMO of YO⁺dominates the bonding interactions, which results in the formation of [YO(Ar)₆]⁺,[YO(Kr)₆]⁺ and [YO(Xe)₅]⁺ complexes. The interaction between 3d transition metalmonoxide neutrals and noble gas atoms is less efficient. It was found that earlytransition metal monoxides ScO, TiO and VO do not form noble gas complexes, whilethe late transition metal monoxides coordinate one noble gas atom in forming thelinear NgMO(M=Cr, Mn, Fe, Co, Ni) complexes. Similarly, the group VB metaldioxides are able to coordinate two noble gas atoms to form the MO₂(Ng)₂ (M=V,Nb, Ta) complexes, while MO₄ form MO₄(Ng) (M=V, Nb, Ta) complexes with onenoble gas atom. The calculated results show that the binding energies of the NgMOcomplexes increase in the order of CrO＜MnO＜FeO＜CoO＜NiO, those of theMO₂(Ng)₂ (M=V, Nb, Ta) complexes increase in the order of Ta＜Nb＜V, andMO₂(Ng)₂＞MO₄(Ng). The results show that the heavier noble gas atoms can replacethe lighter noble gas atoms in coordination sphere of the complexes in noble gasmatrixes, as experimentally observed.The coordination of noble gas atoms leads to a shift of the M-O stretch frequency.In general, donation of electron density into a bonding orbital of metal oxides inducesa blue-shift of the M-O stretch frequency, while donation of electron density into aantibonding orbital of metal oxides results in a red-shift of the M-O stretch frequency.But if a nonbonding orbital of metal oxides is involved, no significant shift will beobserved. Our results show that the observed frequency shifts are largely depended onthe nature of valence orbitals of the metal oxides involved in bonding.