Orbitals energy differences between subshells

The principal characteristic of the transition elements is an incomplete electronic subshell that confers specific properties on the metal concerned. Ligand systems may participate in coordination not only by electron donation to the 3d levels in the first transition series but also by donation to incomplete outer 4s and 4p shells. Figure 5.1 shows that the differences in orbital energy levels between the 4s, 4p and 3d orbitals are much smaller than, for example, the difference between the inner 2s and 2p levels. Consequently, transitions between the 4s, 4p and 3d levels can easily take place and coordination is readily achieved. The manner in which ligand groups are oriented in surrounding the central metal atom is determined by the number and energy levels of the electrons in the incomplete subshells. 

Equations 8-11 can be immediately applied also to the y of the orbitals, corresponding to a given 1. For instance, the d shell of the central ion divides into two subshells, consisting of two orbitals with the same energy, and y consisting of three orbitals with the same energy. We denote the energy difference between these two subshells by A, but many authors have used other symbols, such as lODq or E — E ). 

The problem with expanded valence-shell structures is, of course, to explain where the "extra" electrons go. This expansion has been rationalized by assuming that after the 3s and 3p subshells of the central atom fill to capacity (eight electrons), extra electrons go into the empty 3d subshell. If we assume that the energy difference between the 3p and 3d levels is not very large, the valence-shell expansion scheme seems reasonable. But is this a valid assumption The use of the 3d orbitals for valence-shell expansion is a matter of scientific dispute. Although unresolved questions about the expanded... 

Because tbe shapes of the 2s and 2p electron clouds are different, we distinguish between them by saying that an electron with the 2s waveform is in the 2s sublevel, and an electron with any of the three 2p waveforms is in the 2p sublevel. Orbitals that have the same potential energy, the same size, and the same shape are in the same sublevel. The sublevels are sometimes called subshells. Thus there is one orbital in the Ir sublevel, one orbital in the 2s sublevel, and three orbitals in the 2p sublevel. The following orbital diagram shows these orbitals and sublevels. 

The subshells also include a certain number of stable orbitals and a maximum number of electrons. The difference in energy between different subshells determines the shape of the orbitals, and the orbital quantum number describes the orbital shape. 

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