Metal aryl hydrides

Chart 11.6 Equilibrium favoring metal aryl hydride and alkane over the alkyl hydride complexes and free aromatic substrates likely indicates that the difference of M—Ar and M—R BDEs is greater than difference in BDE for Ar H and R—H (assuming that AS 0 and that the two M—H BDEs are approximately equivalent). 

M—H bond energies are approximately constant and negligible difference in AS, the thermodynamic preference of metal aryl hydride complexes and free alkane over metal alkyl hydride complexes and free aromatic substrate suggests that the ABDE for M Ar versus M—R is greater than the ABDE for Ar—H versus R—H (Chart 11.6). 

Metal hydrides, which normally involve strongly electropositive metals such as sodium, lithium, boron, and aluminium, form hydrides like lithium hydride. Metal alkyl hydrides or metal aryl hydrides are elaborate molecules called complexes including HIr(CO)(CH3)-(P(C,H5)3)2C1. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

The use of metal alkyls, metal aryls, metal hydrides, and metal carbides for preparing alcoholates of carbohydrates in inert, aprotic solvents has not yet been reported. 

Alkyl-, aryl- and chlorosilanes react readily with metal carbonyl hydrides ... 

Hydrides, Grignard Reagents, Metal-alkyls and Metal-aryls, and Other Reductants... 

One of the intriguing attributes of many systems that initiate C—H oxidative addition is the commonly observed selectivity for stronger C—H bonds, which can be divided into kinetic and thermodynamic selectivity. For metal-mediated C—H activation, kinetic and thermodynamic selectivities are often identical. For example, arenes often undergo reaction more rapidly than alkanes that possess weaker C—H bonds, and aryl hydride complexes (plus free alkane) are commonly favored thermodynamically over alkyl hydride systems (plus free aromatic substrate). Assuming that... 

Me3SiPPh2 assists in the insertion of CO into the Mn-R bond or RMn(CO)5 (R = alkyl or aryl) followed by silyl migration to oxygen. With Mn(CO)5H, however, only cis-substitution by the phosphine occurs, while with the cyclopentadienyl metal tricarbonyl hydrides, silylation occurs at the metal (Scheme 23)92. 

When there is a single substituent on the benzene ring, there are six possible isomeric aryl hydride complexes the metal center can be cis or trans and the attack on the ring can... 

Rapid conversion between C-Ir/H-C 19 and C-H/lr-C 21 is observed, a reaction that involves oxidative addition of the agostic C-H bond and reductive elimination of the aryl hydride. This is a rare example of reversible metalation of an agostic group under mild conditions. The tautomeric equilibration was suggested to go via the doubly metalated species 20 having a non-classical hydride but an Ir(V) dihydride could not be excluded. 

Using TAfS° (68) rs 14 kJ mol, A,G (68) rs —49 kJ mol is finally obtained, that is, the energetics of reactions (66) and (68) are comparable. A possible reason for the different reactivity of arenes and alkanes is that an arene may have a kinetic advantage over an alkane by forming a strong T/ -bond with the metal. The electron backdonation from the metal to the antibonding tt orbitals of the arene weakens the C(sp )-H bond and favors the formation of the aryl hydride. While this kinetic explanation may account, by itself, for the preference of arene addition, it is observed in Table 1 that for late transition metal complexes the differences DH° (M-Ph) — DH°(M-Me) are substantially higher than Z)//°(Ph-H) — Z)/7 (Me-H). This trend will, of course, imply that benzene activation is thermodynamically favorable, relative to methane activation. 

They are little affected by the polarity of the solvent, but may be accelerated to some extent by electron-releasing ligands. The C—H and Si—H bonds of various hydrocarbons and silanes can also oxidatively add to metals. Among different type of C—H bonds, those of arenas are particularly prone to do this because of the high thermodynamic stability of the aryl hydride adduct. 

Transition-metal-alkyl bonds can be formed by a variety of reactions that include metathetical replacement of a halide ion, oxidative addition, and insertion of an alkene into a metal-hydride bond. " A similar set of reactions is available for the synthesis of transition-metal-aryl bonds, although the analogous insertion of a benzyne intermediate into a metal-hydride bond is not particularly viable as a synthetic route. For alkyl complexes that have longer chains than methyl, thermal decomposition to give the metal-hydride complex by a j5-hydrogen transfer reaction is frequently observed at ambient temperature. 

Mercury compounds 66,305,313 Metal alkene compounds 76, 304 Metal alkyl compounds 66, 296, 302 Metal alkyne compounds 84, 307 Metal ammine complexes 296. 309 Metal aryl compounds 158 Metal azides 92, 295. 321 Metal carbonyl compounds 121, 294, 295, 309, 314, 317 Metal cyano compounds 297, 303 Metal ethylene complexes 73, 296, 304 Metal halides 297, 303 Metal hexafluoro compounds 304 Metal hydrides 293, 323 Metal-ligand vibrations 292 Metal olefin compounds 73, 76, 296, 304... 

Apart from the metal-hydride system, metal-alkyl and metal-aryl systems have been shown to add across fluoro-olefins. 

It is a colourless gas which decomposes on heating above 420 K to give metallic tin, often deposited as a mirror, and hydrogen. It is a reducing agent and will reduce silver ions to silver and mercury(II) ions to mercury. SnSn bonding is unknown in hydrides but does exist in alkyl and aryl compounds, for example (CH3)3Sn-Sn(CH3)3. 

Uses. The largest use of lithium metal is in the production of organometaUic alkyl and aryl lithium compounds by reactions of lithium dispersions with the corresponding organohaHdes. Lithium metal is also used in organic syntheses for preparations of alkoxides and organosilanes, as weU as for reductions. Other uses for the metal include fabricated lithium battery components and manufacture of lithium alloys. It is also used for production of lithium hydride and lithium nitride. 

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