_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Alkyls and aryls
The activation energy for the addition is very low; methane adds to (ry'­
C5H5)Ir(CO) in a frozen matrix even at 12 K. (See also reactions oftungstenocene,
p. 292). Another approach which involves a reversal of alkene hydrogenation is
mentioned on p. 186.
Benzene displaces cyclohexane from Cp*lr(PMe,)Cy(H) at 130"C. Kinetic
measurements suggest a two step mechanism:
k,
[Ir]Cy(H);:::::! [Iri+ CyH: [Ir] = (IJ'-C,Me,)Ir(PMe,)
L,
k
[Ir] + PhH ---=--. [Ir]Ph(H): Cy = cyclohexyl
From the observed position of equilibrium in this and similar reactions it has been
established that the relative Ir~C bond energies fali in the usual order Ir~aryl
> Ir~primary alkyl > Ir~secondary alkyl > Ir~tertiary alkyl. The Ir~Ph
bond must be at least 100 kJ mol-' stronger than Ir~Cy. For alkane activation,
M + RH--> M(R)H, to occur, E(M~R) + E(M~H) must be greater than E(R~H).
As E(R ~ H) is of the order of 3 50-4 50 kJ moi-', the values of E(Ir~ H) and
E(Ir~R) probably lie between 240 and 320 kJ moi-'.
7.1.6 Migratory insertion of carbon monoxide
A group of reactions which bear a formal resemblance to each other are the
'insertion' reactions
[M]-R + CO---> [M]-C(O)R
[M]-R + S0 1 ---> [M]-0-S(O)R
~ / 1 1
[M]-R + C=C ---> [M]-C-C -R
/ ~ 1 1
Methylpentacarbonylmanganese yields an acyl deriva ti ve not only when it reacts
with carbon monoxide, but also when treated with other ligands. This suggests
that it is one of the bound carbonyl groups which forms the acyl C=O, rather
than aCO molecule from outside. This has been confirmed by using ''C labelling.
Me O
'\,_~_f' + NaMn(C0) 5
o
--- H 3 CC-Mn(C0) 4 L
L 11
1
----
CL
225