Page 116 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

464 Catalysis, Homogeneous

FIGURE 20 σ-Bond metathesis.
FIGURE 18 Nucleophilic anti-attack.
−3
occurring, such as in Co(CN) . Indeed in the subsequent
5
10
0
8
2
involving d → d ,d → d , etc. The oxidative addition reactions the proton is transferred as a hydrogen radical
of dihydrogen to low-valent metal complexes is a com- to the activated alkenes (e.g., acrylates, styrene) and the
mon reaction in many catalytic cycles. In spite of the high reference to the process as a homolytic dissociation is cer-
strength of the dihydrogen bond the reaction proceeds tainly applicable to the reactions with alkenes. Formally,
smoothly to afford cis dihydrido complexes. The bond en- dihydrogen is transformed into two hydride anions and the
ergy of a metal hydrogen bond is in the order of 240 +/− reaction is again an oxidative addition.
40 kJmol −1 which is sufficient to compensate for the loss 2 Co(CN) −3 + H 2 → 2 HCo(CN) −3
−1
of the H-H bond (436 kJmol ). The hydride is formally 5 5
charged with a minus one charge and this electron count
gives dihydrogen the role of an oxidizing agent. The clas- 2. Heterolytic Splitting
8
sic example of oxidative addition to a d metal complex
is the reaction discovered by Vaska and Diluzio: Heterolytic splitting of dihydrogen refers to the splitting
of dihydrogen into a proton and a metal-bonded hydride. It
trans-IrCl(CO)(PPh 3 ) 2 + H 2 → IrH 2 Cl(CO)(PPh 3 ) 2
is a common reaction, probably for many transition metal
In rhodium complexes the reaction has found widespread cations, but there are only a few cases for which there is
application in hydrogenation. In model compounds the clear proof for its occurrence. In the ideal case the het-
reaction reads: erolytic splitting is catalyzed by the metal ion and a base
which assists in the abstraction of the proton. In this re-
+
Rh(diphosphine) + H 2 → RhH 2 (diphosphine) + action there is no formal change in the oxidation state of
2
2
The second reaction, bimetallic oxidative addition— the metal. The mechanism has been put forward for Ru(II)
also referred to as homolytic splitting—involves a reac- complexes which can react with dihydrogen according to:
tion of a dimeric complex with H 2 in which two metal RuCl 2 (PPh 3 ) 3 + H 2 → RuHCl(PPh 3 ) 3 + HCl
7
centers participate. For instance, a dimer of a d metal
6
complex reacts with dihydrogen to give two d species. Ruthenium has a sufficient number of d-electrons to al-
In this process dihydrogen also turns formally into two low for oxidative addition of dihydrogen, which could
hydride anions. A well-known example is the conversion then be quickly followed by reductive elimination of
of dicobaltoctacarbonyl into hydridocobalttetracarbonyl: HCl.Observationsondihydrogencomplexesofruthenium
have thrown light on the mechanism heterolytic splitting.
Co 2 (CO) 8 + H 2 → 2 HCo(CO) 4 2 +
CpRu(L)((η -H 2 ) reacts rapidly with NEt 3 as can be de-
1
Another example implies two molecules of a Co(II), a d 7 duced from the dynamic H-NMR spectra which indicate
complex, which does suggest that radical reactions may be a rapid exchange of the dihydrogen complex with its con-
jugate base, CpRu(L)(H) (Fig. 21).

K. Activation by Lewis Acids
In Section H we have discussed the activation of carbon
containing fragments toward nucleophilic attack by coor-
dination of the fragment to a transition metal. Here we will

FIGURE 19 Nucleophilic attack at coordinated CO followed by a
2
“shift” reaction. FIGURE 21 η -H 2 complex and heterolytic splitting.