5.2 The Reactivity with Nitrogen
77
applicable to each other. It is worth mentioning that the dissociative chemisorption
behavior of H 2 or N 2 on metal surfaces has long been a unique scientific topic in the
chemistry of transition metals where the surface d-orbitals play an important role.
Through theoretical calculations, researchers have modeled the H 2 chemisorption
on surface sites of small metal clusters, and found that the 3d orbital participation is
crucial in lowering the activation barrier for the dissociative chemisorption of such
diatomic gas molecules [63]. It is still interesting to note that the aforementioned
studies of small metal clusters in reacting with hydrogen and nitrogen both display
no conspicuous odd–even effect, which largely differs from the metal cluster reactivity with oxygen. As mentioned earlier, oxygen is spin-triplet in its ground state
and the two half-filled molecular orbitals are anti-bonding in nature. Therefore, the
activation of an oxygen molecule (
3 O 2 ) requires to fill its half-filled anti-bonding
orbitals and to reduce the multiplicity from triplet to singlet. For clusters with odd
number of electrons, the reaction can proceed without changes in the multiplicity of
the cluster; while for clusters with even number of electrons, the spin conservation
requires a spin excitation of the cluster leading to a barrier resulting in a dominant
odd–even effect. Nitrogen and hydrogen tend to chemisorb on metal clusters, where
the N–N bond and H–H bond do not have to be broken; and even with dissociative chemisorption, there is no necessity to change the spin multiplicity of the metal
clusters.
Nonpolar gas molecules have also been found to be reactive with other metal
clusters [45–48, 64] For instance, utilizing a fast flow tube method, Kaya et al. [46]
investigated the reactivity of cobalt cluster cations Co n
+ (n = 2–22) with a few
molecules including CH 4 , C 2 H 4 and C 2 H 2 (also N 2 , H 2 ). Their results indicated an
interesting cluster size dependence for the reactants CH 4 and C 2 H 4 (also N 2 , H 2 ),
where Co 4,5
+ and Co 10–15
+ displayed much higher reactivity than their neighboring
clusters; however, all these Co n
+ clusters were found to be highly reactive with
C 2 H 2 regardless of the cluster sizes. This interesting difference was explained by the
activation energies for their chemisorption reactions. Cationic Co n
+ clusters showed
similar size dependence as neutral Co n clusters, except for Co 4
+ and Co 5
+ that display
enhanced reactivity due to active sites induced by the influence of positive charge. On
the other hand, the adsorption rate of C 2 H 2 to the cobalt cluster surfaces was found
to be large enough to attain a stable chemisorption state instead of an activation of
the C−H or C−C bond. Therefore, gas-phase reactivity does not necessarily depend
on cluster sizes when the chemisorption state is not affected by the different frontier
orbital energies.
References
1. S.N. Khanna, P. Jena, Chem. Phys. Lett. 218, 383–386 (1994)
2. C. Bois, J.A. Cabeza, R.J. Franco, V. Riera, E. Saborit, J. Organomet. Chem. 564, 201–207
(1998)
3. J.A. Cabeza, I. del Rio, V. Riera, Inorg. Chim. Acta 268, 131–133 (1998)
4. B. Bergman, E. Rosenberg, Organometallics 21, 1508–1511 (2002)
77
applicable to each other. It is worth mentioning that the dissociative chemisorption
behavior of H 2 or N 2 on metal surfaces has long been a unique scientific topic in the
chemistry of transition metals where the surface d-orbitals play an important role.
Through theoretical calculations, researchers have modeled the H 2 chemisorption
on surface sites of small metal clusters, and found that the 3d orbital participation is
crucial in lowering the activation barrier for the dissociative chemisorption of such
diatomic gas molecules [63]. It is still interesting to note that the aforementioned
studies of small metal clusters in reacting with hydrogen and nitrogen both display
no conspicuous odd–even effect, which largely differs from the metal cluster reactivity with oxygen. As mentioned earlier, oxygen is spin-triplet in its ground state
and the two half-filled molecular orbitals are anti-bonding in nature. Therefore, the
activation of an oxygen molecule (
3 O 2 ) requires to fill its half-filled anti-bonding
orbitals and to reduce the multiplicity from triplet to singlet. For clusters with odd
number of electrons, the reaction can proceed without changes in the multiplicity of
the cluster; while for clusters with even number of electrons, the spin conservation
requires a spin excitation of the cluster leading to a barrier resulting in a dominant
odd–even effect. Nitrogen and hydrogen tend to chemisorb on metal clusters, where
the N–N bond and H–H bond do not have to be broken; and even with dissociative chemisorption, there is no necessity to change the spin multiplicity of the metal
clusters.
Nonpolar gas molecules have also been found to be reactive with other metal
clusters [45–48, 64] For instance, utilizing a fast flow tube method, Kaya et al. [46]
investigated the reactivity of cobalt cluster cations Co n
+ (n = 2–22) with a few
molecules including CH 4 , C 2 H 4 and C 2 H 2 (also N 2 , H 2 ). Their results indicated an
interesting cluster size dependence for the reactants CH 4 and C 2 H 4 (also N 2 , H 2 ),
where Co 4,5
+ and Co 10–15
+ displayed much higher reactivity than their neighboring
clusters; however, all these Co n
+ clusters were found to be highly reactive with
C 2 H 2 regardless of the cluster sizes. This interesting difference was explained by the
activation energies for their chemisorption reactions. Cationic Co n
+ clusters showed
similar size dependence as neutral Co n clusters, except for Co 4
+ and Co 5
+ that display
enhanced reactivity due to active sites induced by the influence of positive charge. On
the other hand, the adsorption rate of C 2 H 2 to the cobalt cluster surfaces was found
to be large enough to attain a stable chemisorption state instead of an activation of
the C−H or C−C bond. Therefore, gas-phase reactivity does not necessarily depend
on cluster sizes when the chemisorption state is not affected by the different frontier
orbital energies.
References
1. S.N. Khanna, P. Jena, Chem. Phys. Lett. 218, 383–386 (1994)
2. C. Bois, J.A. Cabeza, R.J. Franco, V. Riera, E. Saborit, J. Organomet. Chem. 564, 201–207
(1998)
3. J.A. Cabeza, I. del Rio, V. Riera, Inorg. Chim. Acta 268, 131–133 (1998)
4. B. Bergman, E. Rosenberg, Organometallics 21, 1508–1511 (2002)