The 16-18-electron rule

Fe(0) = 8 valence electrons 2Cp = 2 X 5 valence electrons Total = 18 electrons 

The number of valence electrons for a zero oxidation state metal centre is equal to the group number (e.g. Cr, 6 Fe, 8 Rh, 9). Some commonly encountered ligands donate the following numbers of valence electrons  

Worked example 24.2 18-Electron rule metal-metal bonding 

Metal-metal bonding in multinuclear species is not always clear-cnt. Solely on the basis of the 18-electron rule, snggest whether (ifi -Cp)Ni(p,-PPh2)2Ni(ifi -Cp) might be expected to contain a metal-metal bond. 

The formula is instructive in terms of drawing a structure, except in respect of an M—M bond. Thus, we can draw an initial structure  

Cr(0) (group 6) contributes 6 electrons ri -CeHg contributes 6 electrons 3 CO contribute 3x2 = 6 electrons 

Confirm that the Fe centres in H2Fe(CO)4 and [(t -C5H5)Fe(CO)2] obey the 18-electron rule. 

Show that Fe(CO)4(t -C2H4), HMn(CO)3(PPh3)2 and [(t -C6H5Br)Mn(CO)3] contain 18-electron metal centres. 

The 18-electron rule is essentially a statement about the kinetic stability of complexes. In this it resembles the octet rule in the chemistry of elements of the First Short Period, including carbon. In 18-electron compounds full use is made of 

Compounds Carbonyls, nitrosyls, organometallics phosphine complexes Halides, oxides, aquo complexes alkoxides, ammines. 

In main group chemistry, electron counts in molecules are often described by 

Several schemes exist for counting electrons in organometallic compounds. We will describe two of these using several examples. The first two examples are of classic 18-electron species. 

This method considers ligands to donate electron pairs to the metal. To determine the total electron count, one must take into account the charge on each 

A chromium atom has six electrons outside its noble gas core. For transition metals the only electrons that are counted are the 5 and d electrons beyond the noble gas core. Each CO is considered to act as a donor of 2 electrons (from an electron dot standpoint, C=0 , the donated electrons correspond to the lone pair on carbon). Thus, the total electron count is 

Cr(CO)6 is therefore considered an 18-electron complex. It is thermally stable for example, it can be sublimed without decomposition. Cr(CO)5, a 16-electron species, is much less stable and known primarily as a reaction intermediate 2 20-electron Cr(CO)7 is not known. Likewise, the 17-electron [Cr(CO)6]+ and 19-electron [Cr(CO)6] are much less stable than the neutral, 18-electron Cr(CO)6. The bonding in Cr(CO)6, which provides a rationale for the special stability of many 18-electron systems, is discussed in Section 3-2. 

When the metals have 18 electrons, they are referred to as coordinatively saturated. They cannot easily take on another ligand. Metals with lower electron counts are coordinatively unsaturated. If their geometries allow, these metals can react with additional ligands to bring the electron count up to 18. These metals are considered to have open coordination sites, defined as positions on the metal where another ligand can add. Attachment of another ligand increases the coordination number (the number of ligands attached) of the metal by one. 

We now look at the 18-electron rule and at the alternative ionic and covalent bonding models on which this metal valence electron counting procedure is based. We then examine the ways in which binding to the metal can perturb the chemical character of a ligand, an effect that lies at the heart of organometallic chemistry. 

Just as organic compounds follow the octet or eight valence electron rule, typical organometallic compounds tend to follow the 18e rule. This is also known as the noble-gas or effective atomic number (EAN) rule because the metals in an 18e complex achieve the noble-gas configuration— for example, in the Werner complexes, the cobalt has the same EAN as Kr, meaning it has the same number of electrons as the rare gas. We first discuss the covalent model that is the most appropriate one for counting compounds with predominant covalency, such as most organometallics. 

To show how to count valence electrons by forming a compound from the neutral atomic components, we first apply the method to CH4, where the simpler octet rule applies (Eq. 2.1). 

The Organometallic Chemistry of the Transition Metals, Sixth Edition. Robert H. Crabtree. 

An octet is appropriate for carbon, where one 2s and three 2p orbitals make up the valence shell 8e fill all four orbitals. 

In cases where we start with an odd number of electrons on the metal, we can never reach an even number, 18, by adding 2e ligands such as CO. In each case the system resolves this problem in a different way. In V(CO)6, the complex is 17e but is easily reduced to the 18e anion V(CO)6. Unlike V(CO)6, the Mn(CO)3 fragment, also 17e, does dimerize, probably because, as 

M—M bond contributes le to each metal all the CO groups are teiminal Trigonal bipyramidal 

IM-CO contributes le to each metal, and there is also an M—M bond Tetrahedral 

These different ways of assigning electrons are simply models. Since all bonds between dissimilar elements have at least some irniic and some covalent character, each model reflects a facet of the truth. The covalent model is probably more appropriate for the majority of low-valent transition metal complexes, especially with Ae unsaturated ligands we will be studying. On the other hand, the ionic model is more appropriate for high-valent complexes with N, O, or Cl ligands, such as are found in coordination chemistry or in the organometallic chemistry described in Chapter IS. In classical coordination chemistry, the oxidation state 

TABLE 2.2 Common Ligands and Their Electron Counts 

The basis of the above classification is the 18-electron rule and it is appropriate to discuss this rule here. 

The large majority of the transition metal complexes with metal-carbon organic, -carbonyl or -hydride ligands isolable at room temperature can be regarded as having 18 electrons in their valence shell. This is the 

In order to test the application of the 18-electron rule to a compound it is necessary to count the number of electrons which are formally in the valence shell of the metal atom. This is most easily achieved in the following manner  

Finally, consider the complex C7H7Co(CO)3 whose structure is unknown but may be predicted using the 18-electron rule. At first sight the complex appears to be a 7 -f 9 -f 6 = 22 electron complex. It is therefore proposed that the C7H7 ring only contributes three electrons to the cobalt, rather than seven, and that instead of being the jr-cycloheptatrienyl complex, I.l, it is the Ti-enyl complex 1.2  

A problem in the coimting of electrons may arise with complexes containing metal-metal bonds. In complexes for which X-ray diffraction clearly shows there to be metal-metal bonding, e.g. (CO)sMn—Mn(CO)s, the bond is represented by an unbroken line. In these complexes each metal atom acts as a one-electron ligand to the other. 

In main group chemistry, we have encountered the octet rule, in which electronic structures can be rationalized on the basis of a valence shell requirement of 8 electrons. Similarly, in organometallic chemistry, the electronic structures of many compounds are based on a total valence electron count of 18 on the central metal atom. As with the octet rule, there are many exceptions to the 18-electron rule, but the rule nevertheless provides useful guidelines to the chemistry of many organometallic complexes, especially those containing strong 7T-acceptor ligands. 

A glance at Table 2.1 will show how the first-row carbonyls mostly follow the 18e rule. Each metal contributes the same number of electrons as its group number, and each CO contributes 2e for its lone pair -ir back bonding makes no difference to the electron count for the metal. In the free atom, it had one atomic orbital (a.o.) for each pair of electrons it uses for back bonding in the complex it still has one filled molecular orbital (m.o.), now delocalized over metal and ligands. 

In Section 20.4, we apphed molecular orbital theory to octahedral complexes containing w-acceptor ligands and gave a rationale for the fact that low oxidation state organometallic complexes tend to obey the 18-electron rule. This rule often breaks down for early and late J-block metals as examples later in the chapter show 16-electron complexes are common for e.g. Rh(I), Ir(I), Pd(0) and Pt(0). The majority 

As mentioned in Chapter 1, many transition metal complexes containing organic, carbonyl or hydride ligands, which are isolable at room temperature, can be regarded as having 18 electrons in the valence shell of the metal. This is the empirical basis of the rule—a valence shell containing 

18 electrons gives rise to stable compounds. Exceptions to the rule are discussed later. 

To exemplify this procedure consider first some simple carbonyl compoimds  

Metal Carbonyl Number of electrons contributed by metal Number of electrons contributed by ligands Total 

When the manganese atom (Z = 25) is considered, we see that the addition of five CO molecules would bring the total number of electrons to 35, whereas six CO ligands would bring the total to 37. In neither case is the 18-electron mle obeyed. In accord with these observations, neither Mn(CO)5 nor Mn(CO)6 is a stable complex. What is stable is the complex [Mn(CO)5]2 (sometimes written as Mn2(CO)10) in which there is a metal-metal bond between the manganese atoms, which allows the 18-electron rule to be obeyed. 

The 18-electron rule is especially useful when considering complexes containing ligands such as cyclo-heptatriene, C7H8, abbreviated as cht. This ligand, which has the following structure, can bond to metals in more than one way because each double bond can function by donating two electrons  

On the other hand, in the complex [Ni(CO)2cht] the cht ligand is a four-electron donor, so two double bonds are functioning as electron pair donors and the structure is 

In the complex [Cr(CO)3cht], the cht ligand functions as a six-electron donor because three CO ligands donate six electrons and 12 are needed by the chromium atom to reach a total of 36. Therefore, this complex has the structure 

The number of complexes for which the 18-electron rule is an overriding bonding condition is very large. It is an essential tool for interpreting the bonding in organometallic compounds and it will be considered many times in the chapters to follow. 

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Many transition metal complexes including Ni(CO)4 obey the 18 electron rule, which IS to transition metal complexes as the octet rule is to mam group elements like carbon and oxygen It states that... 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Not all ligands use just two electrons to bond to transition metals Chromium has the electron configuration [Ar]4s 3rf (6 valence electrons) and needs 12 more to satisfy the 18 electron rule In the compound (benzene)tricarbonylchromium 6 of these 12 are the tt elec Irons of the benzene ring the remammg 6 are from the three carbonyl ligands... 

The 18 electron rule is a general but not universal guide for assessing whether a certain transition metal complex is stable or not Both of the following are stable compounds but only one obeys the 18 electron rule Which one" ... 

Because the electron-counting paradigm incorporates the 18-electron rule when appHed to transition-metal complexes, exceptions can be expected as found for classical coordination complexes. Relatively minor exceptions are found in (Tj -C H )2Fe2C2BgHg [54854-86-3] (52) and [Ni(B2QH22)2] A [11141-32-5] (53). The former Q,n electrons) is noticeably distorted from an idealized stmcture, and the latter is reminiscent of the and complexes discussed above. An extremely deficient electron count is obtained for complexes such as P7036-06-9] which have essentially undistorted... 

For example, in Ni(CO) nickel metal having 28 electrons coordinates four CO molecules to achieve a total of 36 electrons, the configuration of the inert gas krypton. Nearly every metal forming a carbonyl obeys the 18-electron rule. An exception is vanadium, forming a hexacarbonyl in which the number of electrons is 35. This carbonyl, which has a paramagnetism equivalent to one unpaired electron, however, readily adds one electron to form a closed valence shell complex containing the V(CO)(, anion. 

Structure. The CO molecule coordinates in the ways shown diagrammaticaHy in Figure 1. Terminal carbonyls are the most common. Bridging carbonyls are common in most polynuclear metal carbonyls. As depicted, metal—metal bonds also play an important role in polynuclear metal carbonyls. The metal atoms in carbonyl complexes show a strong tendency to use ak their valence orbitals in forming bonds. These include the n + 1)5 and the n + l)p orbitals. As a result, use of the 18-electron rule is successflil in predicting the stmcture of most metal carbonyls. 

Like nickel, iron reacts with carbon monoxide to form a compound having the formula M(CO)n that obeys the 18-electron rule. What is the value of n in the formula Fe(CO)n ... 

Fenocene has an even more interesting stmcture. A central iron is ir-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes five electrons for a total of ten and iron, with an electron configuration of [Ar]45 34i contributes eight. Alternatively, fenocene can be viewed as being derived from Fe " (six valence electrons) and two aromatic cyclopentadienide rings (six electrons each). 

The borides (p. 145), carbides (pp. 297, 1074), and nitrides (p. 417) have been discussed previously. Binary hydrides are not formed but prolonged heating of powdered Mg and Fe under a high pressure of H2 yields MgFeH6 containing the octahedral hydrido anion, [FeH6] which satisfies the 18-electron rule. 

Flaving the d s configuration, the elements of this triad are able to conform with the 18-electron rule by forming mononuclear carbonyls of the type M(C0)5. These are volatile liquids which can be prepared by the direct action of CO on the powdered metal (Fe and Ru) or by the action of... 

Although the cyclopentadienyls dominate the aromatic chemistry of this group, bis(arene) compounds are also well established. They are able to satisfy the 18-electron rule as the dications, [M(arene)2] " or by the two rings adopting different bonding modes one tj the other tj". ... 

Simple ligand-field arguments, which will be elaborated when M ions of the Ni, Pd, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinate, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation state and the requirements of the 18-electron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Iri are predominantly square planar, although 5-coordination docs also occur. 

Because they possess an odd number of valence electrons the elements of this group can only satisfy the 18-electron rule in their carbonyls if M-M bonds are present. In accord with this, mononuclear carbonyls are not formed. Instead [M2(CO)s], [M4(CO)i2] and [M6(CO)i6] are the principal binary carbonyls of these elements. But reduction of [Co2(CO)g] with, for instance, sodium amalgam in benzene yields the monomeric and tetrahedral, 18-electron ion, [Co(CO)4] , acidification of which gives the pale yellow hydride, [HCo(CO)4]. Reductions employing Na metal in liquid NH3 yield the super-reduced [M(CO)3] (M = Co, Rh, Ir) containing these elements in their lowest formal oxidation state. 

On the basis of the 18-electron rule, the d s configuration is expected to lead to carbonyls of formula [M(CO)4] and this is found for nickel. [Ni(CO)4], the first metal carbonyl to be discovered, is an extremely toxic, colourless liquid (mp —19.3°, bp 42.2°) which is tetrahedral in the vapour and in the solid (Ni-C 184pm, C-O 115 pm). Its importance in the Mond process for manufacturing nickel metal has already been mentioned as has the absence of stable analogues of Pd and Pt. It may be germane to add that the introduction of halides (which are a-bonded) reverses the situation [NiX(CO)3] (X = Cl, Br, I) are very unstable, the yellow [Pd"(CO)Cl2]n is somewhat less so, whereas the colourless [Pt (CO)2Cl2] and [PtX3(CO)] are quite stable. 

A central theme in our approach, which we believe to be different from those of others, is to focus on the changing chemistry associated with higher, middle and lower oxidation state compounds. The chemical stability of radical species and open-shell Werner-type complexes, on the one hand, and the governance of the 18-electron rule, on the other, are presented as consequences of the changing nature of the valence shell in transition-metal species of different oxidation state. 

Pyykkd, P. and Runeberg, N. (2002) Icosahedral WAU]2 A Predicted Closed-Shell Species, Stabilized by Aurophilic Attraction and Relativity and in Accord with the 18-Electron Rule. Angewandte... 

This mode of calculation has been called the EAN rule (effective atomic number rule). It is valid for arbitrary metal clusters (closo and others) if the number of electrons is sufficient to assign one electron pair for every M-M connecting line between adjacent atoms, and if the octet rule or the 18-electron rule is fulfilled for main group elements or for transition group elements, respectively. The number of bonds b calculated in this way is a limiting value the number of polyhedron edges in the cluster can be greater than or equal to b, but never smaller. If it is equal, the cluster is electron precise. 

Another example of a complex that obeys the 18-electron rule is ferrocene or bis(cyclopentadienyl) iron. The cyclopentadienyl anion is generated by the reaction of cydopentadiene with sodium, and ferrocene is obtained by the subsequent reaction with ferrous chloride,... 

With its unusual coordination mode, NO forms complexes with a wide variety of metals, especially in cases where the metal can accept the transfer of an electron from the itg orbital. With cobalt having 27 electrons, it is evident that the addition of no integral number of ligands that function as two-electron donors can bring the total to 36. However, when one ligand is an NO molecule, the cobalt has a total of 30 electrons, so three CO ligands can raise the total to 36. Therefore, the stable complex that obeys the 18-electron rule is [Co(CO)3NO]. It should be apparent that complexes such as Mn(CO)4(NO), Fe(CO)2(NO)2, and Mn(CO)(NO)3 also obey the 18-electron rule. 

Comparison of the C-O stretching frequencies for a series of metal carbonyl complexes can reveal interesting trends. The complexes listed below all obey the 18-electron rule, but with different numbers of CO ligands attached, the metal atoms do not have the same increase in electron density on them because the coordination numbers are different. 

For these complexes, the extent of back donation increases as the number of CO ligands increases, which causes the stretching frequencies to be found at lower wave numbers. A similar trend is seen for the following complexes (all of which obey the 18-electron rule), showing the effect of the charge on the metal ion ... 

Does the compound Co(CO)2(r 2-H2)(NO) obey the 18-electron rule Explain your answer. 

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