Liquid Phase Partial Oxidations Over Active Transition Metals Grafted on Different Heterogeneous Supports pp. 361-400
Authors: (Mahasweta Nandi, Asim Bhaumik, Dept. of Materials Science, Indian Association for the Cultivation of Science, Kolkata, India, and others)
Abstract: To start with, let us recall the definition of transition metals. According to the modern definition, given by the International Union of Pure and Applied Chemistry (IUPAC) a transition metal is ‗an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.‘ Electrons are fed into the d-orbitals starting from Group 3. For the first and second transition series, the Group 3 elements, scandium (Sc3+) and yttrium (Y3+), have a single d electron in their outermost shell but usually they are not considered as transition metals. In all of their compounds they exist as Sc3+ and Y3+ ions where there are no d electrons. Other elements with d1 configuration are lanthanum and actinium, but they are classified under lanthanoid and actinoid series of elements, respectively. On the other hand, the Group 12 elements, namely zinc, cadmium, and mercury have an outer shell electronic configuration d10s2 with no incomplete d shell and hence they are not transition metals according to the above definition. In their +2 oxidation state they have a d10 electronic configuration while in the +1 oxidation state; there are no unpaired electrons because of the formation of dimer with a covalent bond between the two atoms. An interesting property of these transition metals is that they can exhibit two or more oxidation states which usually differ by one in their compounds. They have electrons of similar energy in both the 3d and 4s levels and thus a particular element can form ions of nearly the same stability by losing different numbers of electrons.