Outer shell bonding

The extended valence bond description of inner shell and outer shell bonding for octahedral cobalt(III) complexes, required as a result of different d-electron arrangements on the metal. The six empty hybridized orbitals can in each case accommodate six bonding lone pairs from six ligand donor atoms. 

The common feature of these schemes is the assumed separability of certain groups of electrons, inner-outer shells, bonds, etc., and their invariance from one atom or molecule to another. These assumptions are valid (see below) for the orbital pictures as such, but must be justified for the correlation part. Semi-empirical theories ought to be based on a theory which includes electron correlation at the outset and then does the parametri-zation in a well defined fashion. 

In the case of carbon the stable number of electrons for the outer shell is eight and for hydrogen, two. Thus all the atoms possess or share the number of electrons required for stability. Where a pair of electrons is shared between two atoms, it is stated that the atoms are bound by a single bond. If there are two pairs a double bond is formed and if there are three pairs a triple bond. 

Phosphorus and sulfur are the third-row analogs of nitrogen and oxygen, and the bonding in both can be described using hybrid orbitals. Because of their positions in the third row, however, both phosphorus and sulfur can expand their outer-shell octets and form more than the typical number of covalent bonds. Phosphorus, for instance, often forms five covalent bonds, and sulfur occasionally forms four. 

Draw an electron-dot structure for acetonitrile, C2H3N, which contains a carbon-nitrogen triple bond. How many electrons does the nitrogen atom have in its outer shell How many are bonding, and how many are non-bonding ... 

The octet rule tells us that eight electrons fill the outer shell of an atom to give a noble-gas ns1ns(l valence-shell configuration. However, when the central atom in a molecule has empty d-orbitals, it may be able to accommodate 10, 12, or even more electrons. The electrons in such an expanded valence shell may be present as lone pairs or may be used by the central atom to form additional bonds. 

The three eigenfunctions which would take part in the formation of a triple bond can be made symmetrical about the bond direction, for an atom of the type considered above, with only four eigenfunctions in the outer shell but since the group attached by the fourth valence lies on the axis of the triple bond, there is no way of verifying the resulting free rotation about the triple bond. 

The electron-pair bond as postulated by Lewis consists of two electrons held jointly by two atoms. By assuming that atoms tend to surround themselves with an outer shell of either shared or unshared electron pairs, usually four in number, but sometimes more or less, Lewis... 

In bivalent nickel, palladium, or platinum there are eight electrons in the outer d subshell. Putting them two to an orbital, they occupy a minimum of four of the five d orbitals, leaving only one d orbital available for bond formation through combination with s, px, py, and pz of the next outer shell. It is found that only four strong bond orbitals can be formed. These four lie in a plane and are directed towards the four comers of a... 

In NF3, with each atom having four orbitals in its outer shell, there is no way for one fluorine atom to be attached to the nitrogen atom by a double bond and the other two by single bonds and to provide orbitals for the... 

Here the phosphorus atom has four shared electron pairs and one unshared pair, using five orbitals. (In PC15, eg, the transargononic phosphorus atom has five shared pairs in its outer shell.) However, because of the electroneutrality principle such a structure is allowed only for structure 1. Transargononic structures do not occur for first-row atoms, so this phenomenon is not found in NF3. These ideas concerning the bonding in NF3 and PF3 are implicit in the discussion by Marynick, Rosen and Liebman61 of the inversion barriers of these molecules. 

The symbol ( S—]+ represents tercovalent argononic sulfur. Like Si, it is argononic in that, counting shared as well as unshared electron pairs, it has four pairs in its outer shell, giving it the electron number of argon. A tercovalent argononic sulfur atom resembles a normal (neutral) tercovalent phosphorus atom. The bond orbitals of S + are similar to those of S. ... 

Once the number of valence electrons has been ascertained, it is necessary to determine which of them are found in covalent bonds and which are unshared. Unshared electrons (either a single electron or a pair) form part of the outer shell of just one atom, but electrons in a covalent bond are part of the outer shell of both atoms of the bond. First-row atoms (B, C, N, O, F) can have a maximum of eight valence electrons, and usually have this number, although some cases are known where a first-row atom has only six or seven. Where there is a choice between a structure that has six or seven electrons around a first-row atom and one in which all such atoms have an octet, it is the latter that generally has the lower energy and that consequently exists. For example, ethylene is... 

Electron population parameters of inner monopoles were constrained to be equal for all 40 non-H atoms. Single exponentials r exp(-ar) were adopted as radial functions for the higher multipoles, with n = 2, 2, 3 respectively for dipole, quadrupole, and octopole of the species C, N and 0, and n = 4, 4, 4 for the same multipoles of the S atom. A radial scaling parameter k, to shape the outer shell monopoles, and the exponential parameter a of all non-H atomic species were also refined. H atoms were initially given scattering factors taken from the H2 molecule [15] and polarised in the direction of the atom to which they are bonded. 

The outer shell is called the valence shell because it is these electrons that are involved in bond formation and give the atom its valence. 

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