Chromium orbital energies

Figure 8.21 An energy level diagram for dibenzenechromium. The positions of ring and chromium orbitals on this diagram are only approximate.

The rigorous explanation of the electron configuration of chromium, which requires knowledge that is beyond the scope of an introductory course, involves the details of the electron interactions. It turns out that orbital energies are not constant for a given atom but depend on the way that the other orbitals in the atom are occupied. Thus there is no simple explanation for why chromium has the 4s 3d5 configuration rather than the 4s 3d4 configuration. Suffice it to say that for all the first-row transition metals, because the 4s and... 

In the sequence of orbital energies shown above the 4s orbitals have a lower energy than the 3d orbitals and so they will be filled first in keeping with the minimum energy principle. For example, the electron configuration of the outer 10 electrons of calcium (atomic number Z = 20) is 3s 3p 3d 4s. In the filling of the electron orbitals for elements 21 to 29, there are two irregularities, one at 24 (chromium) and one at 29 (copper). Each of these elements contains one 4s electron instead of two. The reason... 

In the case of element 20, calcium, the new electron also enters the 4s orbital. But in the case of element 21, scandium, the orbital energies have reversed so that the 3d orbital has a lower energy. Textbooks typically claim that since the 4s orbital is already fuU, the next electron necessarily begins to occupy the 3d orbital. This pattern is supposed to continue across the first transition series of elements, apart from the elements chromium and copper, where further anomalies occur (table 9.1). 

As you can see from Figure 6.9, the electron configurations of several elements (marked ) differ slightly from those predicted. In every case, the difference involves a shift of one or, at the most, two electrons from one sublevel to another of very similar energy. For example, in the first transition series, two elements, chromium and copper, have an extra electron in the 3d as compared with the 4s orbital. 

The stability of sexivalent chromium, in the chromate ion and related ions, can also be understood. The chromic complexes, involving tervalent chromium, make use of d2sp3 bond orbitals, the three remaining outer electrons of the chromium atom being in three of the 3d orbitals, with parallel spins. The resonance energy of these three atomic electrons in a quartet state helps to stabilise the chromic compounds. However, if all of the nine outer orbitals of the chromium atom were available for bond formation, stable compounds might also be expected... 

The d block includes all the transition elements. In general, atoms of d block elements have filled ns orbitals, as well as filled or partially filled d orbitals. Generally, the ns orbitals fill before the (n - l)d orbitals. However, there are exceptions (such as chromium and copper) because these two sublevels are very close in energy, especially at higher values of n. Because the five d orbitals can hold a maximum of ten electrons, the d block spans ten groups. 

The HO-energy of a ir-system can be changed, e.g. by occupied orbitals of hetero-atoms which are relative donors or acceptors for the HO s. This can also be true of interactions of the LUMO s of the tr-system with vacant AO s of heteroatoms. The consequences e.g. for the preference of certain conformers relative to the type and position of perturbations in tricarbonyl-chromium benzene complexes (organo-metallic example) are described in Figure 2 of Scheme 2.1-4 together with the consequences for the reactivities in benzene derivatives (example of organic chemistry) due to rektive donor- or acceptor-perturbations (see also Scheme 2.1-2 Fig. 2). 

In the case of dibenzene chromium and its cation, the most complete investigation of the electronic spectra is due to Yamada and co-workers (76) who report both solution and crystal spectra in detail for dibenzene chromium iodide and ditoluene chromium iodide. Four intense bands were observed in the region 3 to 6 eV, and a detailed discussion of these in terms of molecular-orbital assignments has been given by Berry (78), although no direct comparison with the energy-level scheme of Shustorovich and Dyatkina (75) is reported. The intense band at 5.51 eV (log max = 4.14) for... 

In the case of the tricarbonylarene metals, enhancement of nucleophilic substitution relative to the free arene is reported 106), In contrast to earlier reports 106) Friedel-Crafts acylation of tricarbonylbenzene chromium occurs under mild conditions 18), Molecular-orbital calculations of the 7r-electron activation energies for these reactions 63) confirm enhanced nucleophilic reactivity and suggest electrophilic activity similar to that of the free arene. The nucleophilic displacement of halide by methoxide ion... 

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