Inert gas configuration

When the outer orbitals of all of the atoms in the molecules are filled—giving inert gas configurations—then the electrons of another molecule cannot approach the nuclei closely. When molecules of this sort approach each other, the energy is lowered only a few tenths of a kilocalorie per mole. This weak interaction is typical of van der Waals forces. 

Zachariasen, W. H. (1931). A set of empirical crystal radii for ions with inert gas configuration. Z. Kristallogr. 80, 137-153. 

It is of interest that a Co (II) dimer of the composition Co2(CN)ro contains the same number, 34, of electrons in the combined outer shells of the two cobalt atoms as does Co2(CO)g. In the absence of strong cobalt-cobalt bonding in these complexes, it would appear that each is just two electrons short of a closed-shell configuration. Similarly, Co(CN)6, with 17 outer electrons is one electron short of an inert-gas configuration. 

In general, it is observed that the charges on simple positive ions are limited to a maximum of three even if an inert gas configuration is not reached. Thus the elements of the transition series all may lose two electrons (the s electrons are lost first) to give dipositive ions in ionic compounds, and a number of them give -1-3 ions. Similarly, as a rule, the lanthanide... 

A convenient tool for understanding organometallic catalysis mechanisms is the 16 and 18 electron rule, whereby valence electrons are counted in order to ascertain whether or not complexes are coordinatively unsaturated. An 18 electron complex possesses an inert gas configuration and must first undergo dissociation to achieve the coordinative unsaturation necessary for reactivity. The number of valence electrons for various transition metals is readily seen from their position in the periodic table (e.g., Mn has 7, Fe has 8). The number counted for a particular metal is independent of its oxidation state. 

We have already established that the carbene carbon is an electrophilic center and, hence, it should be very easily attacked by nucleophiles. In most reactions we believe that the first reaction step probably involves attachment of a nucleophile to the carbene carbon. In some cases, for instance with several phosphines (49) and tertiary amines (50), such addition products are isolable analytically pure under certain conditions (1 in Fig. 3). For the second step there exists the possibility that the nucleophilic agent may substitute a carbon monoxide in the complex with preservation of the carbene ligand (2 in Fig. 3). One can also very formally think of the carbene complex as an ester type of system [X=C(R )OR with X = M(CO)j instead of X = 0], because the oxygen atom as well as the metal atom in the M (CO) 6 residue are each missing 2 electrons for attainment of an inert gas configuration. So, it is not surprising that the... 

In the hexavalent state these elements are similar to uranium in the quadrivalent state however they have, like uranium and protactinium in this valency state, externally the inert gas configuration of the thorium ion in the trivalent state that of the trivalent actinium ion. The name 5f series may perhaps be the best in place of actinides, thorides or uranides. 

In the metals of the Short Periods and of the A-subgroups (main series), such as the alkali metals, positive ions with the inert gas configurations are also produced while the surplus electrons form the degenerate gas of the conduction electrons. Also the configuration with a filled d-subshell is characterized by a certain preference as is demonstrated by the common 18-electron configuration of many positive ions of the B-subgroups (subseries) e.g. 

The complex ions, in which the central ion has not the inert gas configuration, are very numerous these are especially formed with readily polarizable ions, such as Cl, (Br, I), CN, CNS and S. In these cases the polarization as well as the Van der Waals-London energy can contribute to the heat of formation of the complex. The following are examples K4CdCl6, K2[Hg(CN)4], Ks[Ag(SCN)J, KFeS2 etc. 

The above statements formed the basis of the descriptive theory of G. N. Lewis and I. Langmuir on atomic bonding by the formation of electron pairs, in which each atom at the same time forms as far as possible an octet of eight electrons (only hydrogen merely two, He configuration) in connection with the stability of the inert gas configuration (Octet theory). 

Everywhere there appears the strong tendency to the formation of complete octets as required by the Lewis-Langmuir theory (see also p. 200), owing to the relative stability of the inert gas configuration. 

Plane structures are, on the other hand, according to both conceptions of a bond, to be expected for an arrangement of three atoms or a tetrahedral arrangement for four, when there are only 3.8 or 4.8 electrons available outside the closed shell so that no non-bonding pair occurs on the central atom. The possibility of a formulation as ions with inert gas configurations is in fact always present in these cases. Thus NO3-, C032" and S03 are indeed plane (see pp. 156 and 160). 

It can be expected that an inert gas atom in an excited state is able to make an atomic bond when indeed one of the electrons is in a higher level then however there is no longer any question of an inert gas configuration (He2, HeNe). 

Maximum and Minimum Bond Lengths, Core Repulsions and Inert Gas Configurations... 

The Grignard reagent is a classical organometallic compound. The magnesium ion in Group IIA of the periodic table needs to lose two and only two electrons to achieve the inert gas configuration. This metal h ts a strong tendency to form ionic bonds by electron transfer ... 

Iron has 6 electrons in the 3d orbital, 2 in the 45, and none in the 4p orbital. The inert gas configuration requires 18 electrons—ten 3d, two 45, and six 4p electrons. Iron pentacarbonyl enters this configuration by accepting two electrons from each of the five carbonyl groups, a total of 18 electrons. Back-bonding of the d-rr type distributes the excess electrons among the five carbon monoxide molecules. 

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