Ionization energies of cobalt-vanadium bimetallic clusters (ConVm)

A Density Functional Theory Investigation of the Bimetallic Clusters Nb2Rh and NbRh2 and the Complexes They Form with CO

The interaction of CO with the bimetallic clusters Nb2Rh and NbRh2 has been theoretically investigated using density functional theory. The lowest energy structure of Nb2Rh is found to be a doublet Cs scalene triangle and the global minimum of Nb2Rh–CO is a dissociative structure with C1 symmetry. The lowest energy minimum of NbRh2 is found to be a doublet C2v isosceles triangle and the global minimum of NbRh2–CO is a dissociative structure with Cs symmetry. In comparison with our previous work on Rh3 + CO (J. Comp. Chem., 2008, 29, 1497), these results show that substitution of a single Rh atom with Nb is sufficient to dissociate CO.

Nanoalloys: tuning properties and characteristics through size and composition

A brief sketch of the history of metals and alloys is followed by examples illustrating the current status of the field of nanoalloys and a discussion of our results on the characterization of structural and dynamical (thermal) properties of Ni-Al bimetallic clusters.

Reactivity of group 5 bielement clusters with H2

Bielement metal clusters composed of the group 5 elements, A(n)B(m) (A, B = V, Nb, Ta), were prepared in the gas phase over the wide n and m ranges. We measured reactivities of the attachment reaction of the neutral clusters to H(2), which were plotted as a function of a map with n and m. The clusters with n + m = 4 and 5 were found to be more reactive than the other clusters. The measurement of the ionization energies reveals that there is no strong correlation between the reactivity and the ionization energy. In addition, the reactivities of the cations, A(n)B(m)(+), were also highest at n + m = 4 and 5. These findings suggest that the electronic structure does not totally determine the reactivity of the neutral clusters. On the other hand, theoretical calculations for A(n)B(m) (n + m = 4) reported by Metha et al. showed that the optimized geometrical structures of the congener group 5 bielement clusters do not change significantly by changing n and m within n + m = 4. Hence, it is highly likely that the pyramidal (n + m = 4) and bipyramidal (n + m = 5) structures of A(n)B(m) determine the high reactivity of the clusters.

Structure and stability of binary transition-metal clusters (NbCo)n (n⩽5): A relativistic density-functional study

Equilibrium geometries and electronic properties of binary transition-metal clusters, (NbCo)n (n < or = 5), have been investigated by means of the relativistic density-functional approach. The metal-metal bonding and stability aspects of these clusters have been analyzed on the basis of calculations. Present results show that these clusters exhibit rich structural varieties on the potential-energy surfaces. The most stable structures have a compact conformation in relatively high symmetry, in which the Nb atoms prefer to form an inner core and Co atoms are capped to the facets of the core. Such building features in clustering of the Nb/Co system are related to the order of bond strength: Nb-Nb>Nb-Co>Co-Co. As the binary cluster size increases, the Nb-Co bond may become stronger than the Nb-Nb bond in the inner niobium core, which results in a remarkable increment of the Nb-Nb bond length. Amongst these binary transition-metal clusters, the singlet (NbCo)4 in T(d) symmetry has a striking high stability due to the presence of the spherical aromaticity and electronic shell closure. The size dependence of the bond length and stability of the cluster has been explored.

DFT Calculations on Group 5 Mixed Metal Tetramers: TaxNbyVz (x + y + z = 4)

We report Density Functional Theory (DFT) calculations on mixed-metal tetramers comprised of the Group 5 (Vb) elements V, Nb, and Ta. Our results show that the lowest energy structures for Nb4 and Ta4 are regular tetrahedra with Td symmetry and singlet multiplicity whereas V4 is a triplet state with C2v symmetry. The monosubstituted isomers, A3B, all have C3v symmetry but several higher energy Cs structures have been found that are approximately 100 kJ mol−1 higher in energy. The disubstituted isomers all posses arachno-butterfly structures; the A2B2 types with C2v symmetry and the A2BC types with Cs symmetry. However, the relative openness of the arachno structures is found to be specific to the composition of the mixed-metal cluster.

REACTIONS OFTRANSITIONMETALCLUSTERS WITHSMALLMOLECULES

Atoms and small molecules react with transition metal clusters in ways that are analogous to the physisorption and chemisorption reactions observed on the corresponding extended metal surface. However, often underlying these similarities are size-dependent variations in the reaction mechanisms and rates, the interpretation of which requires a detailed understanding of the structures of both the bare metal cluster substrates and the cluster-molecule complexes. Although polyatomic transition metal clusters cannot be characterized by the traditional methods of molecular spectroscopy, the combination of other physical and chemical probes can provide qualitative and semiquantitative structural information. These techniques, when combined with equilibrium geometries calculated using ab initio or semiempirical methods, provide a detailed picture of the structural origin of metal cluster reactivity and its variation with size.