Tetraadsorbed species

As shown in Table V, 2,3-dimethylbutane is the major reaction product in every case except for the cis isomer on platinum. Since both mechanisms A and B (nonselective and selective) compete on platinum films (86, 87), and probably as well on nickel and rhodium films, the excess of 2,3-dimethylbutane over the amounts expected by Mechanism A may be assigned to Mechanism B, involving the selective rupture of —CHj—CHj— bonds. a,a,, -Tetraadsorbed species were suggested to be associated with Mechanism B (89), whose contribution increases with different metals in the same order, Pt < Ni < Rh, as that for multiple exchange of methane (90). Since multiple exchange of methane is usually taken as a criterion for the ability of a metal to form adsorbed methylene, this result is in fair agreement with the proposed a,a,J ,)S-tetraadsorbed precursors (Scheme 34). The latter were... 

While the first process is likely in the case of iridium, nickel, and cobalt, it should not be so easy on platinum, because of its competition with carbene-olefin isomerization (see Section III, Scheme 29). We believe that the only way of explaining why 1,2-dicarbenes may account for the hydrogenolysis of cyclic hydrocarbons (Scheme 34), but only for a minor part for the hydrocracking of acyclic hydrocarbons, is the competition, for the latter, between carbene-dicarbene formation and carbene-olefin isomerization. Carbene-olefin interconversions are unlikely in the case of cyclic hydrocarbons, since a dicarbene species cannot transform into a 1,1,2,3-tetraadsorbed species (l-carbene-2,3-olefin) and further into a 1,1,3-triadsorbed species without C-C rupturing. 

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