Two electron bond

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Boron s electron deficiency does not permit conventional two-electron bonds. Boron can form multicenter bonds. Thus the boron hydrides have stmctures quite unlike hydrocarbons. The B nucleus, which has a spin of 3/2, which has been employed in boron nuclear magnetic resonance spectroscopy. 

The H3+ ion is the simplest possible example of a three-centre two-electron bond (see discussion of bonding in boranes on p. 157) and is also a model for the dihapto bonding mode of the ligand r] -H2 (pp. 44-7) ... 

What happens if there are not enough electrons to form conventional two-electron bonds Diborane (B2H6 provides a good example. Were the molecule to look like ethane, how many valence electrons would be required tc hold it together How many valence electrons does diborane possess Examine the actual structure for diborane. 

We have performed full-potential calculations on TisSia in its proposed stable crystal structure. The enthalpy of formation obtained from these calculations agrees well with the value deduced from experiment. Due to the low crystal symmetry, the possibility of a more complex bonding character arises. The charge density in this phase differs considerably from that in the hypothetical unstable structure, so two-electron bonds can be excluded in this phase. We have also showed that the opening of a quasigap in the Si DOS has its origin in the Ti-Si interaction. 

Jorgensen, Ch. K. The Problems for the Two-electron Bond in Inorganic Compounds, 124, 1-31... 

Just as there are two ways in which a bond can break, there are two ways in which a covalent two-electron bond can form. A bond can form in an electronically symmetrical way if one electron is donated to the new bond by each reactant or in an unsymmetrical way if both bonding electrons are donated by one reactant. 

This formula has two unpaired electrons, representing unused bonding capacity. This objectionable situation can easily be rectified by allowing the two unpaired electrons to pair, and thus form an additional two-electron bond. Now the carbon atoms are joined by a double bond, just as the oxygen atoms in O2 are double bonded to each other... 

The problems of the two-electron bond in inorganic compounds analysis of the coordination number, N. C. K1 Jorgensen, Top. Curr. Chem., 1984,124,1 (125). 

One can only wonder what sense some students make of the instruction that the reality involves eight electrorrs distributed over the three C-0 bonds that is, 2 /3 electrorrs per bond, or each bond equivalent to IV3 classical two-electron bonds ... 

It is the only example of a free, persistent phosphirenylium ion, and also, only one stable transition-metal complex of this species was published [78,79]. Quantum chemical calculations [80,81] indicated that in the halogeno-phosphirenes the P-X bonds already possesses a high ionic character and can be described as interactions between phosphirenylium and halide ions. The aromatic character of the phosphirenylium ion was shown to be based on a three-centre two-electron bond of 7i-type and the resonance energy was assessed by calculation to 38 kcal/mol. Before the generation of 32, substituted phosphirenylium ions were... 

The sharing of electrons between carbon and hydrogen is an example of atom to-atom bonding known as covalency and the two-electron bond is called a covalent bond. 

To be more exact, every bond is a multi-center bond with contributions of the wave functions of all atoms. However, due to the charge concentration in the region between two atoms and because of the inferior contributions %H2, Xm> and Xh4> the bond can be taken to a good approximation to be a two-center-two-electron bond (2c2e bond) between the atoms C and HI. From the mathematical point of view the hybridization is not necessary for the calculation, and in the usual molecular orbital calculations it is not performed. It is, however, a helpful mathematical trick for adapting the wave functions to a chemist s mental picture. 

In a-B12 the icosahedra are arranged as in a cubic closest-packing of spheres (Fig. 11.16). In one layer of icosahedra every icosahedron is surrounded by six other icosahedra that are linked by three-center two-electron bonds. Every boron atom involved contributes an average of electrons to these bonds, which amounts to -6 = 4 electrons per icosahedron. Every icosahedron is surrounded additionally by six icosahedra of the two adjacent layers, to which it is bonded by normal B-B bonds this requires 6 electrons per icosahedron. In total, this adds up exactly to the above-mentioned 10 electrons for the inter-icosahedron bonds. 

Links between atoms serve to compensate for the lack of the electrons which are necessary to attain the electron configuration of the next noble gas in the periodic table. With a common electron pair between two atoms each of them gains one electron in its valence shell. As the two electrons link two centers , this is called a two-center two-electron bond or, for short, 2c2e bond. If, for an element, the number of available partner atoms of a different element is not sufficient to fill the valence shell, atoms of the same element combine with each other, as is the case for polyanionic compounds and for the numerous organic compounds. For the majority of polyanionic compounds a sufficient number of electrons is available to satisfy the demand for electrons with the aid of 2c2e bonds. Therefore, the generalized 8 —N rule is usually fulfilled for polyanionic compounds. 

For low values of the valence electron concentration (VEC< 4 for main group elements), covalent 2c2e bonds are not sufficient to overcome the electron deficiency. We have the case of electron-deficient compounds . For these, relief comes from multicenter bonds. In a three-center two-electron bond (3c2e) three atoms share an electron pair. An even larger number of atoms can share one electron pair. With increasing numbers of... 

Bivalent tin compounds can easily be transformed to fourvalent tin compounds by oxidizing agents. Mechanistically, this reaction can be understood as an insertion of a six-electron system into a two-electron bond, resulting in a tetrahedrally tetra-coordinated tin atom. This process is often also regarded as an oxidative addition, a distinction being made between additions to a-bonds and to it-bonds. 

Stable stannylenes, base-stabilized stannylenes and unstable stannyles are known to react easily with two-electron bonds, e.g. with molecule Y—Z ... 

The many higher boranes such as B5H9 and BgH 2 are similarly electron deficient and cannot be described by a single Lewis structure. They can often be described in terms of a combination of two- and three-center bonds. Alternatively, their structures can be rationalized by electron-counting schemes such as those proposed by Wade. Analysis of the electron density of these molecules by the AIM method shows that there are bond paths between all adjacent pairs of atoms. So from the point of view of the AIM theory there are bonds between each adjacent pair of atoms, but these cannot all be regarded as Lewis two-center, two-electron bonds as is the case in B2H6. 

It is known that a carborane framework involves three-center two-electron bonding and is thus an elelctron-deficient cluster. Shatenshtein76 reported that the pKa value of the C—H proton of carborane is 23. Therefore, a proton attached to the carbon of o-carborane could be deprotonated easily by a weak base, and the resulting... 

---