Eighteen electron systems

In this work we discuss the transferability properties of certain quantities related to the LMOs. Beside the one-electron electric moments certain one-electron energy contributions are also investigated. The actual calculations were performed on some ten- and eighteen-electron systems. The transferable quantities are investigated on a series of saturated hydrocarbons, too. The possibility of using the electric moments obtained for LMOs will also be presented. 

Each Mn in Mn2(CO)io would therefore be called an eighteen-electron system. Eighteen-electron complexes have the EAN of a noble gas and the corresponding stability. 

Finally, the iron compound haem, part of the haemoglobin molecule we use to carry oxygen around in our bloodstream. It contains the aromatic porphyrin ring system with its eighteen electrons arranged in annulene style. Chlorophyll, mentioned earlier in this chapter, has a similar aromatic ring system. 

In 1959, V(C0)6 was prepared. It is a black paramagnetic solid that decomposes at 70°C. It is the only stable metal carbonyl containing one metal atom that does not obey the eighteen-electron rule. It is a seventeen-electron system which is readily redueed to the stable eighteen-electron anion [V(C0)6] , reaction (2). The seventeen-eleetron V(C0)6 is very reactive for example, it... 

Addition reactions. Certain square planar c/ (sixteen-electron) complexes such as [RhH(CO)L2] play a particularly important role in catalysis. These are sixteen-electron systems, having two less electrons than needed to complete a noble gas electronic configuration. It should not be surprising that such complexes add one ligand to become eighteen-electron complexes, reaction (18). 

In a formal sense, these are oxidation reactions, because rhodium(l) is oxidized to rhodium(ni). These are also addition reactions because two ligands are added to square planar sixteen-electron systems which are transformed into octahedral eighteen-electron complexes. These reactions can also be viewed as an insertion reaction (see (4) below) in which a metal atom is inserted into a bond between two nonmetals. 

Porphyrins form a planar macrocycle that contains a conjugated, cyclic delocalized system of 22tt-electrons [17]. Eighteen electrons can be assigned to the perimeter of a l,16-diaza[18]aimulene, an arrangement which is also found in the chlorin and bacteriochlorin systems. Porphyrins and chlorins are intensely colored (1 red to violet, kmax 500-700 nm 2-4 green, X ax 600-700 nm). 

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 four-membered chelate rings of type C, the Mobius-type (pd) systems formed have the favourable twelve-membered ring into which eighteen n electrons have to be fitted, with three antibonding MO s occupied, whereas the six-membered chelate ring of type C, the Mason-type system, contains eighteen MO s into which twenty-four jr electrons have to be placed, i.e. two antibonding MO s are filled. 

The idea that dehydroannulenes should be aromatic compounds was suggested as far back as 1948 [57]. In a molecule such as (X) there is a continuous conjugated system of eighteen tr-electrons the extra electrons of the triple bonds are orthogonal to this conjugated system. Benzyne is the simplest dehydroannulene, being dehydro-[6]annulene. 

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