18-electron rule for

A persistent feature of qualitative models of transition-metal bonding is the supposed importance of p orbitals in the skeletal hybridization.76 Pauling originally envisioned dsp2 hybrids for square-planar or d2sp3 hybrids for octahedral bonding, both of 50% p character. Moreover, the 18-electron rule for transition-metal complexes seems to require participation of nine metal orbitals, presumably the five d, one s, and three p orbitals of the outermost [( — l)d]5[ s]1[ p]3 quantum shell. 

Clusters with up to four metal atoms generally obey the 18-electron rule for each metal atom. This is no longer the case for six-atom clusters. For instance, in an octahedral cluster a total of 84 electrons (provided by the metal atoms and the ligands) would be predicted for a stable configuration, counting each edge of the octahedron as a metal-metal bond. The 84-electron count for an octahedral cluster is realized in only one known case, namely [HCufPPhj)] 42, 92), and almost aU other cases, as listed in ref. 20), have 86 electrons. 

The power of the 18-electron rule for predicting structures of complexes involving unsaturaiec ligands can be illustrated with W(CO)2(C Ho2- If both CSH, ligands were pentahapto. the compound would have 20 electrons two more than the optimum for stability. However, if one of the ligands is presumed to be pentahapto and the other trihapto, we have an 18-electron complex ... 

The primary difference between covalent and ionic bonding is that with covalent bonding, we must invoke quantum mechanics. In molecular orbital (MO) theory, molecules are most stable when the bonding MOs or, at most, bonding plus nonbonding MOs, are each filled with two electrons (of opposite spin) and all the antibonding MOs are empty. This forms the quantum mechanical basis of the octet rule for compounds of the p-block elements and the 18-electron rule for d-block elements. Similarly, in the Heider-London (valence bond) treatment... 

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 ... 

Criteria and guidelines useful in network elucidation and supplementing the rules derived in this chapter include considerations of steric effects, molecularities of postulated reaction steps, and thermodynamic constraints as well as Tolman s 16- or 18-electron rule for reactions involving transition-metal complexes and the Woodward-Hoffmann exclusion rules based on the principle of conservation of molecular orbital symmetry. Auxiliary techniques that can be brought to bear include, among others, determinations of isomer distribution, isotope techniques, and spectrophotometry. 

Verify the 18-electron rule for five of the binary carbonyls [other than V(CO)g, Cog(CO)]5 and RhgfCOlig] shown in Figure 13-16. 

Determine the metal-metal bond order consistent with the 18-electron rule for the following ... 

As an additional example of the practical utility of the 18-electron rule for the analysis of relationships between composition, structure and properties of uranyl compounds, let us discuss the problem of influence of tetrahedral cations of the X04 anions (X = Si, P, S) upon the structure of [1102X04] complexes. In particular, we look to the answers to the following questions. 

Other metallocenes have similar structures but do not necessarily obey the 18-electron rule. For example, cobaltocene, (t)5-C5H5)2Co, and nickelocene, (Tl5-C5H5)2Ni, are structurally similar 19- and 20-electron species. The extra ... 

In a manner similar to the relationship between the 18-electron rule for octahedra and other structures, and the 16-electron rule for square planar complexes, isolobal relationships between 16-electron fragments of octahedra and 14-electron fragments of square planar structures can also be demonstrated. 

By analogy with the octet rule, it has been proposed that a transition metal tends to be surrounded by the number of valence electrons equal to that of the following rare gas (electron configuration nd °(n + l)s (n + l)p ). One thereby obtains the 18-electron rule, for which we shall provide a first theoretical justification in this chapter ( 1.6.3). However, in light of the examples given above, one must note that there are many exceptions to this rule we shall analyse them in greater detail in the following chapters. 

Earlier we talked about the octet rule, which says that elements like carbon and oxygen are most stable when they have eight valence electrons. For transition metals, as we ve mentioned, the d-orbitals start to become important, so eight electrons just aren t enough anymore. Since there are five d-orbitals, we need an additional ten electrons to fill the valence shell. And 8 -t 10 = 18, hence the 18-electron rule for transition metals. 

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