Sixteen-electron rule

Another remarkable exception to the eighteen electron rule is found among the d transition-metal ions, such as Ni(ll), Pd(II), Pt(ll), Rh(l), lr(l), and Au(ni), which often appear as four-coordinate square planar complexes with only 16 valence electrons. These are said to comply with the sixteen electron rule. Finally, d ions such as Cu(l), Ag(l), and Au(l) can also form sixteen electron three-coordinate complexes, or two-coordinate linear complexes that obey the fourteen electron rule. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

Eighteen-electron complexes react more slowly than similar complexes with either more or less electrons. The eighteen-electron rule explains why some reactions are associative and others dissociative. Complexes in which the metal has sixteen or less valence electrons tend to react by associative mechanisms, since the metal has vacant low-energy orbitals which can be used to form a bond with the entering ligand. This orbital can accept an electron pair from an entering ligand and provide a path for associative substitution. Substitution reactions in square planar complexes illustrate this point, reaction (40). 

With regard to the valence electron count, this number determines whether the transition metal ion is using its full complement of valence shell orbitals— i.e., the five nd s, the (n + l)s, and the three (n + l)p s. If the valence electron count is eighteen, all of the orbitals are fully utilized in bond formation and electron pair storage, the effective atomic number (EAN) rule is fulfilled and the metal ion is said to be saturated. If it is seventeen, the metal ion is covalently unsaturated, and if it is sixteen or less, the metal ion possesses at least one vacant coordination site and is said to be coordinatively unsaturated. The importance of the valence electron count in homogeneously catalyzed reactions has been discussed by Tolman (7). 

As a final step, let us do some "electron accounting." There are eight electron pairs, and they correspond to sixteen valence electrons (8 pair X 2e /pair). Furthermore, there are eight electrons around each atom and the octet rule is satisfied. Therefore... 

The number of electrons in the valence shell of each complex or intermediate is shown. This illustrates the important rule that diamagnetic complexes of the transition metals from Groups IV—VIII have either sixteen or e hteen electrons in their outer shell. This rule is well known for stable complexes, being a modification of the inert gas rule. In addition Tolman 38) has recently pointed out that it works quite well for reactive intermediates or transition states. Exceptions are known, however. 

To understand this particular research, we need to pay special attention to the Kekule structure for pyrene (Fig. 14.17). The total number of tt electrons in pyrene is 16 (8 double bonds = 16 tt electrons). Sixteen is a non-Hiickel number, but Hiickel s rule is intended to be applied only to monocyclic compounds and pyrene is clearly tetracyclic. If we disregard the internal double bond of pyrene, however, and look only at the periphery, we see that the periphery is a planar ring with 14 T7 electrons. The periphery is, in fact, very much like that of [I4]annulene. Fourteen is a Hiickel number An A- 2, where n = 3), and one might then predict that the periphery of pyrene would be aromatic by itself, in the absence of the internal double bond. 

O2. The sixteen (8 X 2) electrons are distributed as shown (o-i,) (o-i, ) (o-2 ) (o M ) (7rv) (7rj,) (n2p) (irj, ) (7rj )b Note that in accordance with Hund s rule, the last 2 electrons are distributed, one each, into the equal-energy itJ and it MO s. These 2 antibonding electrons cancel 2 of the 6 bonding electrons in TTyY Tt and (bonding electrons in two pairs. Thus the MO theory accurately predicts both the diradical nature and the double-bond character of the O2 molecule. From this analysis it is evident that neither Lewis formula... 

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