Transition metal bonding functions and their applications in catalytic adsorptions and reactions: Part III. Effect of metal valence band and periodic trend of CO σ-π bonding functions on transition metals

Abstract

Our model of metal valence band and our new concept of σ-π coordination are further discussed and confirmed in this paper. The infrared stretching frequencies of C-O decrease in the order 2056, 1886 and 1786 cm −1 in Ni(CO) 4, Co(CO) 4 −1 and Fe(CO) 4 −2, which parallels the increase in d electron back-donation functions (B metal bonding functions) from 1.539, 2.121 to 2.895 on Ni, Co and Fe metals, respectively. On the other hand, the M-C bond orders increase from 1.33, 1.89 to 2.16 for Ni(CO) 4, Co(CO) 4 −1 and Fe(CO) 4 −2, which parallel the increase in A(CO5σ-Mσ)-B(CO2π-Mπ) metal bonding functions from 24.61, 30.01 to 33.19, respectively. They are in agreement with our new concept of σ-π coordination proposed in the previous paper. This new concept has also been used to analyze the mechanism of the formation of Ni(CO) 4, Co(CO) 4 −1 and Fe(CO) 4 −2, and to explain why they can automotively hybridize each other despite the energy differences between 3d and 4s, 4p, which are very large. The effects of metal valence bands have been accounted for on all transition metals (d 1 to d 8), and it is demonstrated that d orbitals increase from the Vd band upward to the Vs band, and s orbitals from the Vs band downward to the Vd band, which is equivalent to a change in orbital potential, and would modify their orbital overlap integrals with the adsorbate M.O.s and the A, B metal bonding functions significantly. The effective potentials and the percentage s, d functions of Vs, Vd and d occ bands are the most important factors for determining the effect of the metal valence band. The effects of promoter and support are also altered by changes in the above factors. For Group VIII metals, the valence band provides various s and d orbitals at various potentials, in which a certain number of s and d orbitals can match better with CO adsorbate M.O.s, which explains why CO adsorbed species on Group VIII metals are all stable and adsorption rates are all relatively rapid. The periodic trends of metal A, B, AB and D c bonding functions depend on the structures of the metal valence band, i.e. the potential levels and s, d percentage functions of Vs, Vd and d occ bands. For 4d and 5d metals, the potential levels of the Vs band are high, which cannot form a strong CO 5σ-M σ bond, but the potential levels of Vd band are higher and the width of the d band is wider than those of 3d metal, so their B bonding functions are larger, and they can be used to activate saturated and unsaturated hydrocarbons. In contrast, for 3d metals, the potentials of the Vs band are lower, which favour formation of strong CO 5σ-M σ and M-C bonds, i.e. their A and D c bonding functions are larger, which can promote coke formation. While ABD cD o can be used to characterize CO dissociation, B/A can be used to characterize C-C formation. The characteristics of various metal bonding functions on each transition metal are useful for designing catalyst composition. A typical example has been illustrated, using the possibility to select non-noble metals instead of noble metals in hydrocarbon reactions.