30 GENERAL PROPERTIES OF ORGANOMETALLIC COMPLEXES

2.1 THE 18-ELECTRON RULE

The 18e rule1 is a way to help us decide whether a given d-block transition metal organometallic complex is likely to be stable. Not all the organic formulas we can write down correspond to stable species. For example, CH5 requires a 5-valent carbon and is therefore not stable. Stable compounds, such as CH4,have the noble gas octet, and so carbon can be thought of as following an 8e rule. This corresponds to carbon using its s and three p orbitals to form four filled bonding orbitals and four unfilled antibonding orbitals. On the covalent model, we can consider that of the eight electrons required to fill the bonding orbitals, four come from carbon and one each comes from the four H substituents. We can therefore think of each H atom as being a 1e ligand to carbon. To assign a formal oxidation state to carbon in an organic molecule, we impose an ionic model by artificially dissecting it into ions. Each electron pair in any bond is assigned to the most electronegative of the two atoms or groups that constitute the bond. For methane, this dissection gives C4− + 4H+, with carbon as the more electronegative element. This makes methane an 8e compound with an oxidation state of −4, usually written C(-IV). Note that the net electron count always remains the same, whether we adopt the covalent (4e {Catom}+4 × 1e {4H atoms}=8e) or ionic (8e{C4−ion}+4 × 0e{4H+ions}=8e) model. The 18e rule, which applies to many low-valent transition metal complexes, follows a similar line of reasoning. The metal now has one s,andthreep orbitals, as before, but now also five d orbitals. We need 18e to fill all nine orbitals; some come from the metal, the rest from the ligands. Only a limited number of combinations of metal and ligand give an 18e count. Figure 1.5 shows that 18e fills the molecular orbital (MO) diagram of the complex ML6 up to the dπ level, − ∗ and leaves the M L antibonding dσ orbitals empty. The resulting configuration is analogous to the closed shell present in the group 18 elements and is therefore called the noble gas configuration. Each atomic orbital (AO) on the metal that remains nonbonding will clearly give rise to one MO in the complex; each AO that interacts with a ligand orbital will give rise to one bonding MO, which will be filled in the complex, and one antibonding MO, which will normally be empty. Our nine metal orbitals therefore give rise to nine low-lying orbitals in the complex, and to fill these we need 18 electrons. Table 2.1 shows 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 from its lone pair; π back bonding makes no difference to the electron count for the metal. In the free atom, it had pairs of dπ electrons for back bonding; in the complex it still has them, now delocalized over metal and ligands. 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,theMn(CO)5 fragment, also 17e, does dimerize, probably because, as THE 18-ELECTRON RULE 31

TABLE 2.1 First-Row Carbonyls − V(CO)6 17e; 18e V(CO)6 also stable Cr(CO)6 Octahedral − − (CO)5Mn Mn(CO)5 M M bond contributes 1e to each metal; all the CO groups are terminal Fe(CO)5 Trigonal bipyramidal (CO)3Co(µ-CO)2Co(CO)3 µ-CO contributes 1e to each metal, and there is also an M−M bond Ni(CO)4 Tetrahedral a 5-coordinate species, there is more space available to make the M−M bond. This completes the noble gas configuration for each metal because the unpaired electron in each fragment is shared with the other in forming the bond, much as the 7e methyl radical dimerizes to give the 8e compound, ethane. In the 17e fragment Co(CO)4, dimerization also takes place via a metal–metal bond, but a pair of COs also move into bridging positions. This makes no difference in the electron count because the bridging CO is a 1e ligand to each metal, so an M−M bond is still required to attain 18e. The even-electron metals are able to achieve 18e without M−M bond formation, and in each case they do so by binding the appropriate number of COs; the odd-electron metals need to form M−M bonds.

Ionic Versus Covalent Model Unfortunately, there are two conventions for counting electrons: the ionic and covalent models, both of which have roughly equal numbers of supporters. Both methods lead to exactly the same net result; they differ only in the way that the electrons are considered as “coming from” the metal or from the ligands. Take HMn(CO)5: We can adopt the covalent model and argue that the H atom, a 1e ligand, is coordinated to a 17e Mn(CO)5 fragment. On the other hand, on the ionic model, we can consider the complex as being an anionic 2e H− ligand + coordinated to a cationic 16e Mn(CO)5 fragment. The reason is that H is more electronegative than Mn and so is formally assigned the bonding electron pair when we dissect the complex. Fortunately, no one has yet suggested counting the + − molecule as arising from a 0e H ligand and an 18e Mn(CO)5 anion; ironically, protonation of the anion is the most common preparative method for this hydride. These different ways of assigning electrons are simply models. Since all bonds between dissimilar elements have at least some ionic and some covalent charac- ter, 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 the 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 or in the described in Chapter 15. In classical coordination chemistry, the oxidation state 5 COMPLEXES OF π-BOUND LIGANDS

In this chapter we continue our survey of the different types of ligand by looking at cases in which the π electrons of an unsaturated organic fragment, rather than a lone pair, are donated to the metal to form the M−L bond.

5.1 AND ALKYNE COMPLEXES

In 1827, the Danish chemist Zeise obtained a new compound he took to be KCl·PtCl2·EtOH from the reaction of K2PtCl4 with EtOH. Only in the 1950s was it established that Zeise’s salt, 5.1, is really K[PtCl3(C2H4)]·H2O, containing a coordinated ethylene, formed by dehydration of the ethanol, and a water of crystallization. The metal is bonded to both carbons of the ethylene, but the four C−H bonds bend slightly away from the metal, as shown in 5.4; this allows the metal to bind efficiently to the π electrons of the alkene. For Zeise’s salt, the best bonding picture1 is given by the Dewar–Chatt model. This involves donation of = the C C π electrons to an empty dσ orbital on the metal, so this electron pair is now delocalized over three centers: M, C, and C. This is accompanied by back = ∗ donation from a metal dπ orbital into the ligand LUMO, the C C π level, as shown in 5.2 (occupied orbitals shaded). By analogy with the bonding in CO, we will refer to the former as the “σ bond” and the latter as the “π bond.” As is the case for CO, a σ bond is insufficient for tight binding, and so only metals capable of back donation, and not d0 metals such as Ti(IV), bind well.

The Organometallic Chemistry of the Transition Metals, Fourth Edition, by Robert H. Crabtree Copyright  2005 John Wiley & Sons, Inc., ISBN 0-471-66256-9 125 126 COMPLEXES OF π-BOUND LIGANDS

+ − C + Cl M Cl Pt − C Cl −

5.1 5.2

The C=C bond of the alkene lengthens on binding. The M−alkene σ bond depletes the C=C π bond by partial transfer of these electrons to the metal and so slightly weakens and, therefore, lengthens it. The major factor in lengthening the C=C bond, however, is the strength of back donation from the metal. By filling the π ∗ orbital of the C=C group, this back donation can sharply lower the C−C bond order of the coordinated alkene. For a weakly π-basic metal this reduction is slight, but for a good π base it can reduce it almost to a single bond. For Zeise’s salt itself, M−L σ bonding predominates because the Pt(II) is weakly π basic, and the ligand (C−C: 1.375 A)˚ more nearly resembles the free alkene (1.337 A).˚ The substituents are only slightly bent back away from the metal, and the C−C distance is not greatly lengthened compared to free ethylene. Pt(0), in contrast, is − much more strongly π basic,andinPt(PPh3)2C2H4,theC C distance becomes much longer (1.43 A).˚ In such a case the metal alkene system approaches the metalacyclopropane extreme, 5.3, as contrasted with the Dewar–Chatt model, 5.4, involving minimal π back donation; both are considered η2 structures.

M M 5.3 5.4 X2 L

In the metalacyclopropane extreme (i.e., cyclopropane with M replacing one CH2 group), the substituents on carbon are strongly folded back away from the metal as the carbons rehybridize from sp2 to something more closely approaching sp3. The presence of electron-withdrawing groups on the alkene also encourages back donation and makes the alkene bind more strongly to the metal; for example, − Pt(PPh3)2(C2CN4) has an even longer C C distance (1.49 A)˚ than the C2H4 complex. In the Dewar–Chatt extreme, we can think of the ligand acting largely as a simple L ligand such as PPh3, but in the metalacyclopropane extreme, we have what is effectively a cyclic dialkyl, and so we can think of it as an X2 (diyl or σ2) ligand. In both cases we have a 2e ligand on the covalent model, but while the L (ene or π) formulation, 5.4, leaves the oxidation state unchanged, the X2 picture, 5.3, makes the oxidation state more positive by two units. By 6 AND REDUCTIVE ELIMINATION

We have seen how neutral ligands such as C2H4 or CO can enter the coordi- nation sphere of a metal by substitution. We now look at a general method for simultaneously introducing pairs of anionic ligands, A and B, by the oxidative addition of an A−B molecule such as H2 or CH3-I (Eq. 6.1), a reaction of great importance in both synthesis and (Chapter 9). The reverse reaction, reductive elimination, leads to the extrusion of A−B from an M(A)(B) complex and is often the product-forming step in a catalytic reaction. In the oxidative direction, we break the A−B bond and form an M−AandanM−B bond. Since A and B are X-type ligands, the oxidation state, electron count, and coordination number all increase by two units during the reaction. It is the change in formal oxidation state (OS) that gives rise to the oxidative and reductive part of the reaction names.

A oxidative addition L M + AB L M n reductive elimination n B (6.1) 16e 18e ∆OS = +2 ∆CN = +2

Oxidative additions proceed by a great variety of mechanisms, but the fact that the electron count increases by two units in Eq. 6.1 means that a vacant 2e site is always required on the metal. We can either start with a 16e complex or a

The Organometallic Chemistry of the Transition Metals, Fourth Edition, by Robert H. Crabtree Copyright  2005 John Wiley & Sons, Inc., ISBN 0-471-66256-9 159 160 OXIDATIVE ADDITION AND REDUCTIVE ELIMINATION

2e site must be opened up in an 18e complex by the loss of a ligand. The change in oxidation state means that a metal complex of a given oxidation state must also have a stable oxidation state two units higher to undergo oxidative addition (and vice versa for reductive elimination). Equation 6.2 shows binuclear oxidative addition, in which each of two metals changes its oxidation state, electron count, and coordination number by one unit instead of two. This typically occurs in the case of a 17e complex or a binuclear 18e complex with an M−M bond where the metal has a stable oxidation state more positive by one unit. Table 6.1 systematizes the more common types of oxidative addition reactions by dn configuration and position in the periodic table. Whatever the mechanism, there is a net transfer of a pair of electrons from the metal into the σ ∗ orbital of the A−B bond, and of the A−B σ electrons to the

TABLE 6.1 Common Types of Oxidative Addition Reactiona Change in Change in Coordination dn Configuration Geometry Examples Group Remarks

X2 d10 → d8 Lin. −−−→ Sq. Pl. Au(I) → (III) 11

−2L, X2 Tet. −−−−→ Sq. Pl. Pt, Pd(0) → (II) 10

X2 d8 → d6 Sq. Pl. −−−→ Oct. M(II) → (IV) 10 M = Pd, Pt Rh, Ir(I) → (III) 9 Very common M(0) → (II) 8 Rare −L, X2 TBP. −−−→ Oct. M(I) → (III) 9 M(0) → (II) 8 X2 d7 → d6 2Sq. Pyr. −−−→ 2Oct. 2Co(II) → (III) 8 Binuclear

−L, X2 2Oct. −−−→ 2Oct. 2Co(II) → (III) 8 Binuclear

X2 d6 → d4 Oct. −−−→ 7-c Re(I) → (III) 7 M(0) → (II) 6 V(−I) → (I) 5 X2 d4 → d3 2Sq. Pyr. −−−→ 2Oct. 2Cr(II) → (III) 6 Binuclear

−L,X2 2Oct. −−−→ 2Oct. 2Cr(II) → (III) 6 Binuclear

X2 d4 → d2 Oct. −−−→ 8-c Mo, W(II) → (IV) 6 d2 → d0 Various M(III) → (V) 5 M(II) → (IV) 4 a Abbreviations: Lin. = linear, Tet. = tetrahedral, Oct. = octahedral, Sq. Pl. = square planar, TBP = trigonal bipyramidal, Sq. Pyr. = square pyramidal; 7-c, 8-c = 7- and 8-coordinate. 170 OXIDATIVE ADDITION AND REDUCTIVE ELIMINATION

The rate of the second type (Eq. 6.25) usually follows the rate equation shown in Eq. 6.27, suggesting that Cl− addition is the slow step. This step can be carried out independently with LiCl alone, but no reaction is observed with HBF4 alone − because the cationic iridium complex is not basic enough to protonate and BF4 is a noncoordinating anion.

Rate = k[complex][Cl−] (6.27)

Other acids (or Lewis acids), which are ionized to some extent in solution, such as RCO2HandHgCl2 (Eqs. 6.28 and 6.29), may well react by the same mechanism, but this has not yet been studied in detail.

RCO H −−−→2 1 IrCl(CO)L2 IrH(κ -OCOR)Cl(CO)L2 (6.28)

HgCl −−−→2 IrCl(CO)L2 Ir(HgCl)Cl2(CO)L2 (6.29)

As we saw in Chapter 3, alkyls LnM(R) can often be cleaved with acid to give + the alkane. In some cases simple protonation of the metal to give LnM(R)H or + of the M−R bond to give the σ complex LnM(H−R) is the likely mechanism, but in others (e.g., Eq. 6.30) there is a dependence of the rate, and in some cases even of the products,21 on the counterion; in such cases, an oxidative addition–reductive elimination mechanism seems more likely:

HCl −RH −−−→ −−−→ 21 PtR2(PR3)2 PtHClR2(PR3)2 PtRCl(PR3)2 (6.30)

ž Oxidative addition increases both the oxidation state and the coordination number by two. ž Oxidative addition can go by many different mechanisms.

6.5 REDUCTIVE ELIMINATION

Reductive elimination, the reverse of oxidative addition, is most often seen in higher oxidation states because the formal oxidation state of the metal is reduced by two units in the reaction. The reaction is especially efficient for intermediate oxidation states, such as the d8 metals Ni(II), Pd(II), and Au(III) and the d6 metals Pt(IV), Pd(IV), Ir(III), and Rh(III). Reductive elimination can be stimulated by 22 oxidation or photolysis: The case of photoextrusion of H2 from dihydrides is the best known (Section 12.4). Certain groups are more easily eliminated than others, for example, Eqs. 6.31–6.35 often proceed to the right for thermodynamic reasons. Reactions that involve H, such as Eqs. 6.31 and 6.33, are particularly fast, probably because REDUCTIVE ELIMINATION 171 the transition state energy is lowered by the formation of a relatively stable σ-bond complex LnM(H−X) along the pathway; such complexes are known to be stable only where at least one H is eliminated.

LnMRH −−−→ LnM + R−H (6.31)

LnMR2 −−−→ LnM + R−R (6.32)

LnMH(COR) −−−→ LnM + RCHO (6.33)

LnMR(COR) −−−→ LnM + R2CO (6.34) −−−→ + − LnMR(SiR3) LnM R SiR3 (6.35)

In catalysis reactions (Chapter 9), a reductive elimination is often the last step in a catalytic cycle, and the resulting LnM fragment must be able to survive long enough to react with the substrates for the organic reaction and so reenter the catalytic cycle. The eliminations of Eqs. 6.31–6.35 are analogous to the concerted oxidative additions in that they are believed to go by a nonpolar, nonradical three- center transition state, such as 6.10. Retention of stereochemistry at carbon is a characteristic feature of this group of reactions.

R M X 6.10

Since there are several mechanisms for oxidative addition (Section 6.4) the principle of microscopic reversibility (which holds that a reversible reaction pro- ceeds by the same mechanism in both forward and reverse directions) suggests that reductive eliminations should show the same variety. We only discuss the concerted pathway here.

Octahedral Complexes Octahedral d6 complexes of Pt(IV), Pd(IV), Ir(III), and Rh(III) tend to undergo reductive elimination readily but often with initial loss of a ligand to gener- ate a 5-coordinate intermediate, a much more reactive species than the starting 6-coordinate complex. When ligand dissociation does not occur, reductive elim- ination can be slow, even when it would otherwise be expected to be very favorable. For example, complexes with an alkyl group cis to a hydride are rare because reductive elimination of an alkane (Eq. 6.31) is usually very thermody- namically favorable. A stable example of this type is mer-[IrH(Me)Cl(PMe3)3], ◦ with H and Me cis, which survives heating to 100 C. The Rh analog, 6.11, with 7 INSERTION AND ELIMINATION

Oxidative addition and substitution allow us to assemble 1e and 2e ligands on the metal. With insertion, and its reverse reaction, elimination, we can now combine and transform these ligands within the coordination sphere, and ultimately expel these transformed ligands to form free organic compounds. In insertion, a coordi- nated 2e ligand, A=B, can insert itself into an M−X bond to give M−(AB)−X, where ABX is a new 1e ligand in which a bond has been formed between AB and both M and X. There are two main types of insertion—1,1 and 1,2—as shown in Eqs. 7.1 and 7.2, in which the metal and the X ligand end up bound to the same (1,1) or adjacent (1,2) atoms of an L-type ligand. The type of insertion observed in any given case depends on the nature of the 2e inserting ligand. For example, CO gives only 1,1 insertion: that is, both the M and the X group end up attached to the CO carbon. On the other hand, ethylene gives only 1,2 insertion, in which the M and the X end up on adjacent atoms of what was the 2e ligand. In general, 1 2 η ligands tend to give 1,1 insertion and η ligands give 1,2 insertion. SO2 is the only common ligand that can give both types of insertion; as a ligand, SO2 can be η1 (S) or η2 (S, O).

X 1,1- X MCOM C (7.1) O 18e 16e

The Organometallic Chemistry of the Transition Metals, Fourth Edition, by Robert H. Crabtree Copyright  2005 John Wiley & Sons, Inc., ISBN 0-471-66256-9 183 184 INSERTION AND ELIMINATION

X X C C 1,2-migratory insertion (7.2) M MC C 18e 16e

In principle, insertion reactions are reversible, but just as we saw for oxidative addition and reductive elimination in Chapter 6, for many ligands only one of the two possible directions is observed in practice, probably because this direction is strongly favored thermodynamically. For example, SO2 commonly inserts into M−R bonds to give alkyl sulfinate complexes, but these rarely eliminate SO2. Conversely, diazoarene complexes readily eliminate N2, but N2 has not yet been observed to insert into a metal–aryl bond.

M−R + SO2 −−−→ M−SO2R (7.3) = M−N N−Ar −−−→ M−Ar + N2 (7.4)

The immediate precursor to the final insertion product usually has both the 1e and 2e ligands coordinated. This means that a net 3e set of ligands is converted to a 1e insertion product (ionic model: 4e → 2e), so that a 2e vacant site is generated by the insertion. This site can be occupied by an external 2e ligand and the insertion product trapped. Conversely, the elimination requires a vacant site, so that an 18e complex will not undergo the reaction unless a ligand first dissociates. The insertion also requires a cis arrangement of the 1e and 2e ligands, while the elimination generates a cis arrangement of these ligands. The formal oxidation state does not change during the reaction; Eq 7.5 shows the typical case of CO insertion.

X X MCO MC (7.5) O ∆OS = 0 ∆(e count) = −2

One way to picture insertion reactions is to consider that the X ligand migrates with its M−X bonding electrons (e.g., H− or Me−) to attack the π ∗ orbital of the A=B ligand.

ž 1,1-Insertion (Eq. 7.1) occurs with η1-bound ligands such as CO. 2 = ž 1,2-Insertion (Eq. 7.2) occurs with η -bound ligands such as H2C CH2.