The eighteen-electron rule

For case (1) complexes, examples of which include many first transition series compounds (Table 8.8.3), the lt2g orbitals are essentially nonbonding and A0 is small. In other words, the 2eg orbitals are only slightly antibonding and they may be occupied without much energy cost. Hence, there is little or no restriction on the number of d electrons and the eighteen-electron rule has no influence on these complexes. 

For case (3) complexes, examples of which include many metal carbonyls and their derivatives (Table 8.8.3), the l/2g orbitals are strongly bonding due to back donation and the 2eg orbitals are strongly antibonding. Thus, while it is still imperative not to have electrons occupying the 2eg orbitals, it is equally 

Type (1) complex Number of valence electrons Type (2) complex Number of valence electrons Type (3) complex Number of valence electrons 

Coordinate system for a square-planar complex MLq, where the ligand a orbitals are labelled as o, . . 74, and the ligand it orbitals as v, . .V4 and h, . . 4. 

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In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

In a similar vein, we observe nickel(O), possessing ten electrons in its valence shell, to require four carbonyl ligands to satisfy the eighteen electron rule and form [Ni(CO)4l, whilst chromium(O), with six electrons in its valence shell forms [Cr(CO)6]. These latter compounds are tetrahedral and octahedral respectively. 

When using the eighteen electron rule, we need to remember that square-planar complexes of centers are associated with a 16 electron configuration in the valence shell. If each ligand in a square-planar complex of a metal ion is a two-electron donor, the 16 electron configuration is a natural consequence. The interconversion of 16-electron and 18-electron complexes is the basis for the mode of action of many organometallic catalysts. One of the key steps is the reaction of a 16 electron complex (which is coordinatively unsaturated) with a two electron donor substrate to give an 18-electron complex. 

Table 8.3 3. Three types of complexes in relation to the eighteen-electron rule...

In 1937 English chemist Nevil V. Sidgwick suggested a rule (the octet rule for first-row p-block elements) for complex formation tmder which a metal can acquire ligands until the total number of electrons around it is equal to the number surrounding the next noble gas. This rule was later expanded as the eighteen-electron rule under which a d-block transition metal atom has eighteen electrons in its nine valence orbitals [five n d one (n -I-... 

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). 

These and other apparent exceptions to the eighteen-electron rule prompted an addition to the rule which states substitution reactions of eighteen-electron transition metal compounds may proceed by an associative mechanism provided the metal can delocalize a pair of electrons onto one of its ligands. 

A variety of metal carbonyls are known. The eighteen-electron rule and Sidgwick s effective atomic number rule (Section 2.3) are very successful in explaining their stoichiometries. Simple monomeric carbonyls are expected for transition metals with even atomic numbers Cr(CO)6, Fe(CO)s, Ni(CO)4. The heavier members of the Cr and Fe families also form monomeric carbonyls of the predicted stoichiometry. 

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... 

Mitchell PR, Parish RV (1969) The eighteen-electron rule. J Chem Educ 46 811-814... 

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. 

Effective atomic number rule A rule that applies to covalent complexes, which states that the sum of the number of electrons of a metal ion and of the electrons donated by the coordinated ligands should correspond to the atomic number of the noble gas of the same period. Also formulated as the eighteen electron rule. 

Electron precise complexes Complexes that obey the eighteen electron rule. See ective atomic number rule. 

Transition metals have to fill an s orbital, three p orbitals and five d orbitals. This requires eighteen electrons. This is the eighteen-electron rule. These electrons must either belong to the metal atom already or must be supplied by the ligand. We must also adjust for the charge. 

Pyykkb P. Understanding the eighteen-electron rule. J Qrganomet Chem. 2006 691 4336-4340. 

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