Electrons in bonds

Only electrons in bonds that are f3 to the positively charged carbon can stabilize a car bocation by hyperconjugation Moreover it doesn t matter whether H or another carbon IS at the far end of the (3 bond stabilization by hyperconjugation will still operate The key point is that electrons m bonds that are (3 to the positively charged carbon are more stabilizing than electrons m an a C—H bond Thus successive replacement of first one... 

Simple Hiickel calculations on benzene, in contrast, place all the n electrons in bonding MOs. The 7t-electron energy of benzene is calculated by summing the energies of the six 71 electrons, which is 6a -F 8/S, lower by 2/S than the value of 6a -F 6/S for three isolated double bonds. Thus, the HMO method predicts a special stabilization for benzene. 

Formal charge is the charge an atom would have if valence electrons in bonds were distributed evenly. 

There are eight electrons in bonding orbitals, six in antibonding orbitals. 

Bond order = ix (number of electrons in bonding orbitals —... 

A concept that is important when considering bonds between atoms is the bond order, B. The bond order is a measure of the net number of electron pairs used in bonding. It is related to the number of electrons in bonding orbitals (Nb) and the number in antibonding orbitals (Na) by the equation... 

The methyl carbanion, CH3, has bond angles close to that in a tetrahedral arrangement of atoms, 109°, indicating 4 electron groups around the central C atom and sp3 hybridization. The lone pair of electrons exerts significant repulsive force on the electrons in bonding orbitals and must be counted as an electron group. 

The bond order is ( electrons in bonding MOs — electrons in anti-bonding MOs)/2. 

We can deduce the charge associated with ammonia and water by simply considering that the component atoms are neutral, that all we have done is share the electrons, so the molecules must also be neutral. The formal charge on an individual atom can be assessed more rigorously by subtracting the number of valence electrons assigned to an atom in its bonded state from the number of valence electrons it has as a neutral free atom. Electrons in bonds are considered as shared equally between the atoms, whereas unshared lone pairs are assigned to the atom that possesses them. 

The three elements to be treated in this chapter (V, Cr, Mn) are the third, fourth, and hfth members of the first transition series. The first two members (Sc, Ti) have been treated in previous chapters (Chapters 12 and 13). The ten elements of this first transition series (Sc through Zn) are characterized by electron activity in the 3d-4s levels. All elements in the 3d transition series are metals, and many of their compounds tend to be colored as a result of unpaired electrons. Most of the elements have a strong tendency to form complex ions due to participation of the d electrons in bonding. Since both the 4s and the 3d electrons are active, most of the elements show a considerable variety of oxidation states (Sc and Zn being exceptions). For the first five (Sc through Mn), the maximum oxidation number is the total number of electrons in the 4s and 3d levels. Complexing is often so strong that the most stable oxidation state for simple compounds may differ from that for complex compounds. 

In Fig. 1, we have plotted the oxidation numbers of the actinides and of the lanthanides. We see that for the lanthanides the valence 3 is the most stable valence throughout the series. There are exceptions Ce displays for instance tetravalency in many compounds Eu and Yb display divalency. These exceptions are understood e.g., Eu and Yb are at the half-filling and at the filling of the 4f shell, which are stable electronic configurations. There is a tendency for both to share just the two outer 5 s electrons in bonding, displaying therefore, divalency, and preserve these stable configurations. 

Bismuth is the fifth member of the nitrogen family of elements and, like its congeners, possesses five electrons in its outermost shell, 6s 6p. In many compounds, the bismuth atom utilizes only the three 6p electrons in bond formation and retains the two 6x electrons as an inert pair. Compounds are also known where bismuth is bonded to four, five, or six other atoms. Many bismuth compounds do not have simple molecular structures and exist in the solid state as polymeric chains or sheets. 

---