Why 18 Electrons

An oversimplified rationale for the special significance of 18 electrons can be made by analogy with the octet rule in main group chemistry. If the octet represents a complete valence electron shell configuration then the number 18 represents a filled valence shell for a transition metal Although perhaps a useful way to relate electron 

FIGURE 13.11 Molecular Orbital Energy Levels for a Square-Planar Complex. 

TT-acceptor characteristics, a 16-electron configuration is more stable than an 18-electron configuration. Sixteen-electron square-planar complexes may be able to accept one or two ligands at the vacant coordination sites (along the z axis) to achieve an 18-electron configuration. This is a common reaction of 16-electron square-planar complexes (Chapter 14). 

Metal and ligand orbitals can interact in several ways. The type of interaction depends on the orientation of the orbitals with respect to each other. Most of these interactions can be classified into three types, based on the role of the ligands a donor, n donor, and n acceptor. These classifications are discussed in the following sections. 

These ligands have an electron pair capable of being donated directly toward an empty (or partly empty) metal orbital. The following is an example of the interaction of a donor orbital of NH3, occupied by a lone pair of electrons, and an empty d orbital of suitable orientation on a metal. 

In a a donor interaction, the electron pair on the ligand is stabilized by the formation of a bonding molecular orbital, and the empty metal orbital (a d orbital in the example above) is destabilized in the formation of an antibonding orbital, as shown below. 

In some cases, ligands may donate electrons in a n fashion, for example, using a filled p orbital as indicated below. Halide ions may participate in this type of interaction, with an electron pair donated in a n fashion to an empty metal d orbital (this d orbital must have a different orientation than the d orbital used in a interactions). 

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Paper four first appeared in the Journal of Chemical Education and aimed to highlight one of the important ways in which the periodic table is not fully explained by quantum mechanics. The orbital model and the four quantum number description of electrons, as described earlier, is generally taken as the explanation of the periodic table but there is an important and often neglected limitation in this explanation. This is the fact that the possible combinations of four quantum numbers, which are strictly deduced from the theory, explain the closing of electron shells but not the closing of the periods. That is to say the deductive explanation only shows why successive electron shells can contain 2, 8, 18 and 32 electrons respectively. 

But does the fact that the third shell can contain 18 electrons, for example, which emerges from the relationships among the quantum numbers, also explain why some of the periods in the periodic system contain eighteen places Actually not exactly. If electron shells were filled in a strictly sequential manner there would be no problem and the explanation would in fact be complete. But as everyone is aware, the electron shells do not fill in the expected sequential manner. The configuration of element number 18, or argon is,... 

For example, if the first quantum number is 3 the second quantum number can take values of 2, 1, or 0. Each of these values of will generate a number of possible values of mt and each of these values will be multiplied by a factor of two since the fourth quantum number can adopt values of 1/2 or -1/2. As a result there will be a total of 2n2 or 18 electrons in the third shell. This scheme thus explains why there will be a maximum total of 2, 8, 18, 32, etc., electrons in successive shells as one moves further away from the nucleus. 

By then X-ray experiments allowed scientists to determine the charges of atomic nuclei and, because atoms were electrically neutral, find out the number of electrons in an atom of any element. For example, if the charge on a nucleus was +27, there had to be 27 electrons in the atom to balance that out. In 1916 the German physicist Walther Kossel had speculated that electrons in atoms arranged themselves into concentric shells. For example, argon, which had 18 electrons, had 2 in the innermost shell, 8 in a second shall that surrounded it, and 8 more in a third. But Kossel could not explain why this should be, and he considered no atoms with more than 27 electrons. 

Postulate geometries for bis-, ins-, and tetrakisUnpnenylphosphineipatladtum. Which of these obey the 18-electron rule. What is the geometry of IPdCIJ" 0 Why s it different from that of the lelrakisOnphenylphosphine) complex ... 

The structure of the cobalt reagents is worth a mention. Cobalt has nine electrons so the second reagent is easy nine from Co, five from the cyclopentadienyl, and two each from the two COs giving 18 in all. But why is the first reagent a dimer The monomer Co(CO)4 would have 9 + 8 = 17 electrons. 18-electron comPle < Co<°>... 

Knowing the information just presented, we can now predict the relative size of the atoms and ions presented. Because they all have 18 electrons we can look at the number of protons present as well. The ion with the greatest number of protons, Ca, will have the smallest radius because it has the greatest nuclear pull on the 18 electrons. Sulfur, with just 16 protons, will have the least nuclear attraction for the 18 electrons that it has. This helps explain why non-metal atoms are smaller than their respective ions. Just the same, it also explains why metal atoms are larger than their respective ions. 

In the chapters that follow you will find numerous exercises in counting electrons for clusters - elaborations of the eight- and 18-electron rules for these complex structures. The same factors that cause the eight- and 18-electron rules to fail will similarly limit cluster-counting rules based upon them. Like these fundamental rules, even when satisfied, the cluster-counting rules yield no detailed information on electronic structure. Hence, the bolder student occasionally asks, Why count by which he or she means Of what real value are these counting exercises if little is learned about where the electrons really are ... 

We begin with an obvious question. Given that a metal atom can accommodate larger coordination numbers than main-group atoms, why cannot clusters containing vertex connectivities greater than three be accommodated with two-center-two-electron bonding plus application of the 18-electron rule Consider an octahedral... 

Why study short-lived molecules Those who carry out such studies are perhaps driven by two main ambitions. First, to investigate the structures of compounds with unusual coordination numbers or oxidation states. We can thus make small molecules, for example, AlCl or OPCl (to take just two of very many examples), that are not stable under ambient conditions. In transition metal chemistry, it is becoming apparent that we caimot predict the structures of open shell molecules simply by looking at the structures of closed shell species, for example, we caimot predict what structure the unsaturated 16-electron carbonyl Cr(CO)s will adopt, simply by knowing that saturated 18-electron Cr(CO)6 is octahedral. The second point is to study reaction mechanism. We must know about stmctures of intermediates fully to understand reaction pathways kinetics alone is not enough. 

By this reaction, the nickel atom can reach an 18-electron shell, and this might be the reason why the cyclododecatriene molecule can be displaced so easily by cyclo-octadiene. 

A zinc atom contains a total of 18 electrons in its 3 s, 3p, and 3d orbitals. Why does its electron-dot structure show only two dots ... 

For Co, having one electron less than Ni, one finds that Co(CO)4 can accept one additional electron. This is the reason why dimerization occurs giving the stable [Co(CO)4]2 complex, but bonding of Co(CO)4 with an hydrogen atom can also occur. Because of the stability of the Co(CO)4 anion (18 electron rule), the hydrogen atom in the hydrogen cobalt carbonyl complex has an acidic character ... 

Octahedral transition metal complexes containing strong n acceptor ligands obey the 18-electron rule much more often than complexes containing n donor ligands. Why ... 

What is the valence electron count for the blue ion NiCI42 Why does this tetrahedral ion not obey the 18-electron rule ... 

What is the valence electron count in (CH3)3Re(=0)2 Suggest a reason why this compound does not obey the 18-electron rule. (See A. Haaland, W. Scherer et al., Organometallics, 2000,19, 22.)... 

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