Bonding, homopolar

Contents Formal Oxidation Numbers. Configurations in Atomic Spectroscopy. Characteristics of Transition Group Ions. Internal Transitions in Partly Filled Shells. Inter-Shell Transitions. Electron Transfer Spectra and Collectively Oxidized Ligands. Oxidation States in Metals and Black Semi-Conductors. Closed-Shell Systems, Hydrides and Back-Bonding. Homopolar Bonds and Catenation. Quanticule Oxidation States. Taxological Quantum Chemistry. 

I presume that Ziegler s notation C—Na was only symbolical, since when it was proposed the homopolar and heteropolar nature of chemical bonds was not as clearly understood as it is today. 

Heitler, W., London, F. (1927). Wechselwirkung neutraler Atome und hombopolare Bindung nach der Quantenmechanik [Interactions Between Neutral Atoms and Homopolar Bonding According to the Quantum Theory]. ZeitschriftfurPhysik, 44, 455 72. 

We have attributed the origin of the HF symmetry breaking of homopolar simple and multiple bonds or in symmetric homoatomic clusters to umealistic constraints on the coefficients of the different VB components. Going to a valence CASSCF (ie an optimal valence Cl function) should restore the symmetry. 

The smaller electrostatic charge of the halophosphates, i.e. 2 and 1 resp., compared with the triple charge in [PO ] , gives rise to a certain anisotropy of the bonding system, which especially in the dihalophosphates causes the formation of bonds with evidently homopolar character. This is certainly one of the reasons for their stmctural variety. 

An interesting feature of 5-tocopherylacetic acid (51) and its derivatives was their appreciable thermal stability up to 200 °C. In contrast to 5a-substituted tocopherols carrying an electronegative substituent at C-5a, the homopolar C—C bond in the C2-unit at the 5-position of the tocopherol skeleton was shown to be very stable. Thermal decomposition of 51 at temperatures above 250 ° C caused a complete breakdown of the chroman structure, the C3-unit consisting of C-2, C-2a, and C-3 being eliminated as propyne, the side chain as 4,8,12-trimethyltridec-l-ene (Fig. 6.38). Fragmentation... 

While this definition of electron-pair covalency neglects the effective charges on the bonded atoms, the designation "homopolar bond appears to be reserved for the special case of an electron-pair bond... 

However, is intrinsically restricted to homopolar bonding. In order to... 

Even cursory inspection will show that conjugation alters the properties of the participating pi bonds. For example, the vinyl 7tcc NBO of 6 is not homopolar as in ethylene, but instead becomes rather strongly polarized toward C2,... 

The NBO picture of 3c/2e T-bonding may be developed along lines parallel to those for 3c/4e tu-bonding in Eqs. (3.194) et seq. Let us first consider the special case of a homopolar two-center bond aAc = 2-1/2(hA + hB) interacting with a vacant one-center orbital nB = hB to form a three-center bond tABc of the form... 

Bohr s hydrogen atom model of 1913 had provided inspiration to a few physicists, like Kossel, who were interested in chemical problems but to very few chemists concerned with the explanation of valence. First of all, the Bohr atom had a dynamic character that was not consistent with the static and stable characteristics of ordinary molecules. Second, Bohr s approach, as amended by Kossel, could not even account for the fundamental tetrahedral structure of organic molecules because it was based on a planar atomic model. Nor could it account for "homopolar" or covalent bonds, because the radii of the Bohr orbits were calculated on the basis of a Coulombic force model. Although Bohr discussed H2, HC1, H20, and CH4, physicists and physical chemists mainly took up the problem of H2, which seemed most amenable to further treatment. 11... 

According to Lewis s (1916) definition, a covalent bond exists whenever two atoms are linked in a stable molecule by sharing two outer electrons. We distinguish between homopolar and heteropolar covalent bonds on the basis of whether or not the bonded atoms are of the same type. Pauling (1960) has shown that in covalent compounds the distance between two elements A and B, independent of the nature of the two atoms, is the same in all compounds containing... 

If the wave function for a localized state is such that the probability of encountering the electron on the foreign atom is the same as that of encountering it in the crystal then, when such a state is doubly occupied, we have a purely homopolar surface bond. The quantity R, defined in terms of the wave-function coefficients by the equation... 

On the homopolar line between the A(P and the C(P regions, for example, the usual anionic chemisorption of the last section and the unusual cationic chemisorption of this section coalesce, and a homopolar bond is formed between the foreign atom and the lattice. One electron is lost from an impurity level for each foreign atom adsorbed, and this homopolar chemisorption is depletive. 

By its nature, the acceptor bond, like the donor bond, may be purely ionic or purely homopolar or, in the general case, a mixed one. As we shall see below, this depends on how the electron or the hole captured by the particle and participating in the bond is distributed between the adsorbed particle and the adsorption center. In other words, this depends on the type of localization of the electron or the hole, which in turn, is determined by the nature of the adsorbate and the adsorbent. 

Figures la or Id correspond to so-called weak bonding. Which of these two cases (a or d) actually takes place depends on the nature of the particle C and the lattice. Figures lb and Ic depict the strong acceptor bond. These two figures represent two limiting cases, the first of which (Fig. lb) corresponds to a purely homopolar and the second (Fig. Ic) to a purely ionic bond. In reality, as a rule, we deal with cases intermediate between Figs, lb and Ic. Figures le and If depict a strong donor bond. These figures also correspond to two limiting cases. In reality, we deal, as a rule, with intermediate cases.

---