Electronic states spin orientation

Iron can assume the oxidation states+2, +3, and +6, the last being rare, and represented by only a few compounds, such as potassium ferrate, KaFeOj. The oxidation states +2 and +3 correspond to the ferrous ion, Fe ", and ferric ion, Fe, respectively. The ferrous ion has six electrons in the incomplete 2>d subshell, and the ferric ion has five electrons in this subshell. The magnetic properties of the compounds of iron and other transition elements are due to the presence of a smaller number of electrons in the 3td subshell than required to fill this subshell. For example, ferric ion can have all five of its 2>d electrons with spins oriented in the same direction, because there are five 2>d orbitals in the 3d subshell, and the Pauli principle permits parallel orientation of the spins of electrons so long as there is only one electron per orbital. The ferrous ion. is easily oxidized to ferric ion by air or other oxidizing agents. Both bipositive and terpositive iron form complexes, such as the ferrocyanide ion, Fe(CN)e and the ferricyanide ion, Fe(CN)e, but they do not form complexes with ammonia. 

Figure 2.14. The molecular orbitals of gas phase carbon monoxide, (a) Energy diagram indicating how the molecular orbitals arise from the combination of atomic orbitals of carbon (C) and oxygen (O). Conventional arrows are used to indicate the spin orientations of electrons in the occupied orbitals. Asterisks denote antibonding molecular orbitals, (b) Spatial distributions of key orbitals involved in the chemisorption of carbon monoxide. Barring indicates empty orbitals.5 (c) Electronic configurations of CO and NO in vacuum as compared to the density of states of a Pt(lll) cluster.11 Reprinted from ref. 11 with permission from Elsevier Science.

The revision leads to a difference of 0.06 A. between the interatomic distance in the normal oxygen molecule and the sum of the double-bond radii. This may be attributed to the presence of an unusual structure, consisting of a single bond plus two three-electron bonds. We assign this structure both to the normal 2 state, with ro = 1.204 A., and to the excited 2 state, with ro = 1.223 A., the two differing in the relative spin orientations of the odd electrons in the two three-electron bonds. We expect for the double-bonded state the separation n 1.14 A. 

Ti A neutral titanium atom has 22 electrons. The ground-state configuration is (Ar] A 3 cf. The spins of the 4 electrons cancel, but the two electrons in 3 orbitals have the same spin orientation, so... 

Electron configurations of transition metal complexes are governed by the principles described in Chapters. The Pauli exclusion principle states that no two electrons can have identical descriptions, and Hund s rule requires that all unpaired electrons have the same spin orientation. These concepts are used in Chapter 8 for atomic configurations and in Chapters 9 and 10 to describe the electron configurations of molecules. They also determine the electron configurations of transition metal complexes. 

Superexchange describes interaction between localized moments of ions in insulators that are too far apart to interact by direct exchange. It operates through the intermediary of a nonmagnetic ion. Superexchange arises from the fact that localized-electron states as described by the formal valences are stabilized by an admixture of excited states involving electron transfer between the cation and the anion. A typical example is the 180° cation-anion-cation interaction in oxides of rocksalt structure, where antiparallel orientation of spins on neighbouring cations is favoured by covalent... 

According to the Hund rule, only the states with maximal spin multiplicity have the lowest energy among all possible states of a given electron configuration. As for multiplicity, it is defined with parallelism in spin orientations. Therefore, the interaction depicted in scheme EE3 is the most probable, because it, in contrast to that of schemes EE2 and EE4, leads to the state with the maximal spin multiplicity. That is the picture of microscopic ferromagnetism of an ion radical salt. 

Phosphorescence is emission of light from triplet excited states, in which the electron in the excited orbital has the same spin orientation as the ground state electron. Transitions to the GS are forbidden and the emission rates are slow, so that phophoresence life times are typically milliseconds to seconds. 

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