Atomic orbitals, stability sequence

This sequence of events may be illustrated by the homogeneous hydrogenation of ethylene in (say) benzene solution by Wilkinson s catalyst, RhCl(PPh3)3 (Ph = phenyl, CeH5 omitted for clarity in cycle 18.10). In that square-planar complex, the central rhodium atom is stabilized in the oxidation state I by acceptance of excess electron density into the 3d orbitals of the triphenylphosphane ligands but is readily oxidized to rhodium (III), which is preferentially six coordinate. Thus, we have a typical candidate for a catalytic cycle of oxidative addition and subsequent reductive elimination ... 

The Structural Basis of the Magic Numbers.—Elsasser10 in 1933 pointed out that certain numbers of neutrons or protons in an atomic nucleus confer increased stability on it. These numbers, called magic numbers, played an important part in the development of the shell model 4 s it was found possible to associate them with configurations involving a spin-orbit subsubshell, but not with any reasonable combination of shells and subshells alone. The shell-model level sequence in its usual form,11 however, leads to many numbers at which subsubshells are completed, and provides no explanation of the selection of a few of them (6 of 25 in the range 0-170) as magic numbers. 

The difference 0.48 is negative which means an additional stabilization of the butadiene molecule associated with the non-localization of the tt molecular orbitals. This is a consequence of an interaction between the 2pz a.o.s of atoms C(2) and C(3) which prevents the mere existence of two independent tt bonds localized in C(1)C(2) and C(3)C(4). In this and similar situations one refers to conjugation of double bonds, a phenomenon clearly related to the molecular topology, that is to the sequence and geometric arrangement of atoms in the molecule. 

The type of the central metal atom is a dominant factor influencing the catalytic activity of the M-N4 macrocyclic complexes for ORR. At early time, it had been known that the ORR activity in acid electrolyte would decrease in the order of Fe > Co > Ni > Cu > Mn for the MPc, while in the order of Co > Mn > Fe > Ni > Cu for the MTAA macrocycle complexes [27, 47]. Figure 2 shows an example electrochemical measurement result systematically examining the ORR activity on various metal tetra-sulfonated-phthalocyanines (MTsPc) in alkaline electrolytes. It can be observed in Fig. 2 that the ORR activity decreases in the order of Fe > Mn > Co > Cr > Ni Zn > Cu for the MTsPc macrocycles. Importantly, the results in Fig. 2b revealed a correlation between the observed ORR activity and the number of the d electrons in the central transition metal of the M-N4 macrocycles. It appears that the MTsPc macrocycles could possess superior ORR catalytic activity when their central transition metal atoms, such as Fe, Mn, and Co, have nearly half-filled d orbitals. Such a correlation had also been confirmed for the ORR on the MPc macrocycles in alkaline medium, namely the ORR activity of the MPc were found to decrease following the sequence of Fe > Co > Mn, Pd, Pt > Zn [79]. It notes that Mn-N4 macrocycle complexes exhibit low stability in both acid... 

The periodic table tells us that five hd orbitals are next available for electron occupancy. The next three elements fill in order, as predicted, to vanadium (V, Z = 23) [Ar]4 3 i. Chromium (Cr, Z = 24) is the first element to break the orderly sequence in which the lowest-energy orbitals are filled. Its configuration is [Ar]4 3fi, rather than the expected [Ax]As l>d. This is generally attributed to an extra stability found when all orbitals in a sublevel are half-filled or completely filled. Manganese (Mn, Z = 25) puts us back on the track, only to be derailed again at copper (Cu, Z = 29) [Ar]4 3d °. Zinc (Zn, Z = 30) has the expected configuration [Ar]4 3(i °. Examine the sequence for atomic numbers 21 to 30 in Figure 11.15 and note the two exceptions. 

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