Hydride, Alkyl, and Aryl Complexes

V-V bonding range. Complex 11 on reaction with ethylene under atmospheric pressure at room temperature generated dinudear mixed-valence V(11)/V(II1) trihydride derivative [ (Me3Si)N=P(Ph)2C(H)P(Ph)j=N(SiMe3) V]2(H)(p-H)3(p,tiiniCCHCHPh), 12. The molecular connectivity was elucidated by an X-ray crystal structure [102]. 

---

The latter is outside the scope of organometallic chemistry, but within the first two topics the work involved three main themes olefin and acetylene complexes, alkyl and aryl complexes, and hydride complexes. As continuous subsidiary themes throughout ran the complex chemistry of tertiary phosphines and such ligands, the nature of the trans effect, and the nature of the coordinate bond. All the work from 1947 to 1969 was carried out in the Butterwick Research Laboratories, later renamed Akers Research Laboratories, of Imperial Chemical Industries Ltd., and I am indebted to that Company and particularly to Mr. R. M. Winter, the Company s Controller of Research, and Sir Wallace Akers, its Director of Research, who in 1947, made available to me the opportunity to develop my research in my own way, in those laboratories. 

Lanthanide alkyl and aryl complexes can readily react with a range of substrates with acidic protons, such as alcohols, phenols, phenylacetylene, and amine, to be converted into the corresponding lanthanide derivatives. Lanthanide alkyl complexes react with H2 to form the corresponding hydride complex, which is the popular route to lanthanide hydride. Various unsaturated small molecules can insert into an Ln-alkyl bond to form the derivates containing Ln-heteroatom bonds. The reaction modes found are summarized in Figure 8.14 [59-62]. 

Lanthanide alkyl and aryl complexes react with organoelement hydride compounds, such as hydrides of silicon, germanium, and tin, and so on, resulting in a hydride transfer to the lanthanide metal atom. Among the organoelement hydrides, organosilanes are the most popular source of the hydride. 

Photochemical cleavage of metal-carbon bonds constitutes a brief part of a recent review on the photochemistry of metal carbonyls. Reviews of metal-alkyl and -aryl complexes have also appeared, one of which is concerned solely with metals in Groups IV— VII. The other deals with the formation, stability, and decomposition pathways of transition metal-carbon a-bonds, subjects of much interest in recent years. Consequently the recent isolation of the first stable methylene complex [Ta(Me)(CHa)(jj-C6H6)2] and its unequivocal characterization by a single-crystal JlT-ray diffraction study is of great interest in view of the probability that related derivatives are involved in the recently reported (see Vol, 4, p. 293) examples of a-hydride elimination in metal alkyls. 

Solvato complexes of platinum(II) of the type fraws-[PtY(solvent)L2]+ (Y = hydride, alkyl, or aryl solvent = alcohol or ketone L = tertiary phosphine or arsine) have been known since 1961.1 They are obtained by halogen abstraction from the corresponding halo complexes tran.s-[PtXYL2] in the presence of the desired solvent.2 The methanol complex is also rapidly and quantitatively formed when trans-[PtH(N03)(PEt3)2] is dissolved in this solvent.2... 

Most of the reactions listed in Table 6 involve prior activation of the substrate by coordination to palladium in the form of a v-, a 77-ally lie, a 77-benzylic, or an alkyl or aryl complex. Once coordinated to the metal, the substrate becomes an electron acceptor and can react with a variety of different nucleophiles. The addition of nucleophiles (Nu) to the coordinated substrate may occur in two different ways, as shown by Scheme 9 for 7r-alkene complexes 397"399 (a) external attack leading to trans addition of palladium and nucleophile across the 77-system (path A) or (b) internal addition of the coordinated nucleophile to the complexed alkene resulting in cis addition of palladium and nucleophile to the double bond. The cis and trans adducts (120) and (121) may then undergo /3-hydride elimination (/3-H), producing the vinylic oxidation product... 

C-H bond dissociation energies, 1, 298 C-N triple bond additions, 10, 427 C02, ketene, ketenimine complexes, 5, 84 CO and CNR reductive coupling, 5, 66 cyclopentadienyl alkyl and aryls, 5, 66 cyclopentadienyl carbonyl complexes, 5, 64 cyclopentadienyl hydrides, 5, 69 cyclopentadienyl isocyanide complexes, 5, 65 rf-acyl and rj2-iminoacyl complexes, 5, 82 fj3-complexes, 5, 87... 

Three transmetallation reactions are known. The reaction starts by the oxidative addition of halides to transition metal complexes to form 206. (In this scheme, all ligands are omitted.) (i) The C—C bonds 208 are formed by transmetallation of 206 with 207 and reductive elimination. Mainly Pd and Ni complexes are used as efficient catalysts. Aryl aryl, aryl alkenyl, alkenyl-alkenyl bonds, and some alkenyl alkyl and aryl-alkyl bonds, are formed by the cross-coupling, (ii) Metal hydrides 209 are another partner of the transmetallation, and hydrogenolysis of halides occurs to give 210. This reaction is discussed in Section 3.8. (iii) C—M bonds 212 are formed by the reaction of dimetallic compounds 211 with 206. These reactions are summarized in Schemes 3.3-3.6. 

Hydride complexes of palladium and platinum are almost invariably stabilized by phosphine ligands and play an important role in catalytic processes such as hydrogenation. Examples are Pt(H)ClL2 and Pt(H)2L2, as well as hydrido alkyls and aryls, trans,-Pt(H)(R)L2. There are cis and trans isomers. A typical reaction is the insertion of alkenes and alkynes into the Pt—H bond 33... 

There are quite a number of routes available for the production of iridium(ni) alkyl compounds. In addition to the halide displacement and olefin insertion pathways noted above for iridium(l) compounds, oxidative addition of C-H bonds to iridium(l) to form iridium(in) hydrido alkyl complexes is also a possibihty. This subject will be covered in detail in Section 9 and will not be discussed here. However, there are other oxidative addition routes that lead to the formation of iridium(lll) alkyls. First, oxidative addition of O2 or HCl to some alkyl and aryl iridium(l) complexes can produce iridium(lll) alkyl or aryl compounds. In some cases, HgCl2 can add, but this appears to lead to tractable products only for the very stable pentafluorophenyl complex. Of course, oxidative addition see Oxidative Addition) of alkyl halides such as H3CI will also yield alkyl iridium(lll) compounds. Addition of Mel to Vaska s compound yields a stable iridium(III) complex, but addition of Etl does not produce a stable compound, presumably due to subsequent /J-hydride elimination see fi-Hydride Elimination). A number of mechanistic studies have been done on the oxidative addition of alkyl halides to iridium(l), especially Vaska s complex see Vaska s Complex). 

Cleavage of Ln-C a-bonds of lanthanide alkyls and aryls by a hydrogen molecule at ambient pressure and room temperature is a popular method for the synthesis of neutral lanthanide hydride complexes (Equation 8.25). The first structurally characterized neutral lanthanide hydrides were prepared by hydrogenolysis of bi(cyclopentadienyl) lanthanide alkyl... 

A widely observed reaction of cationic arene complexes is the addition of hydride, alkyl, or aryl anions to the arene ligand giving cyclohexa-dienyl derivatives 51, 154, 185, 243, 251). Other nucleophiles such as CN , OMe , and Ng" have been employed, but the products are generally less stable 185, 432). The following order of reactivity of TT-complexes toward nucleophiles has been noted cycloheptatriene > arene > C Ph > C5H5 103). However, the order will depend to some extent on the system 17). 

Any pattern of nomenclature for the complex hydrides should be capable of extension to the complex alkyl and aryl derivatives. The last two examples indicate how this is accomplished. In strict analogy the alkyl groups should be designated methylo, ethylo, etc., but this may depart too much from custom and imply too definite a conviction as to the nature of the carbon-metal bond. 

Indeed, Mann s studies initiated an explosion of interest in transition metal phosphine chemistry. Chatt was Mann s student, and it is interesting to reflect that Chatt employed transition metal salts as a means of derivatising tertiary phosphines. Little did they realise how this area would expand Chatt s influence on metal-phosphine chemistry was profound, and out of it sprung the employment of phosphine complexes as tools for investigations into metal hydrides, nitrogen fixation, the stabilisation of unusual oxidation states, synthesis of metal alkyls and aryls, and the immense practical area of homogeneous and heterogen-... 

---