Metal clusters, transition

Many cluster molecules are formed by transition metal atoms which are coordinated to 7r-acid ligands, in particular carbon monoxide, cyclopentadienyl and phosphines. Under these circumstances the 18-electron rule should be obeyed so that a transition metal fragment has ten more valence electrons than the corresponding isolobal Main Group fragment. This relationship is exemplified in Table 11.3. 

The structures of large transition metal clusters can be derived by bringing together smaller triangular, tetrahedral and octahedral fragments. These con- 

Cyclopentadienyls Mixed CoCp CpMn(CO) NiCp CpFe(CO) 

The latter can be considered as arising from two binary groups (p.e.c. 34) linked through one atom (p.e.c. 18). 

When 0s3(C0)j2 is heated in a sealed tube to 190 C, Os (CO)ig is formed, together with OSg, Os and OSg species (Fig. 11.11). The pentaosmium cluster 0s5(C0)jg and the anion Os5(CO)j which is derived from it by reduction possess trigonal bipyramidal structures (p.e.c. 72). This figure can be viewed either as a doso-deltahedron (14t -h 2) or as a capped tetrahedron (2 tetrahedra sharing one face p.e.c. 60 + 60 — 48). 085(00)3 reacts with carbon monoxide to form 085(00)35 which has the fascinating bow-tie arrangement of five metal atoms all lying in a plane. 

Another important class of clusters are those of transition metals with carbonyl and other ligands, of which osmium-carbonyl clusters are the best understood [247]. 

Very important from the chemical viewpoint are cluster halides of lanthanide metals (Ln) with the formal valence v 3 (Table 3.17). Thus, the structure and physical properties of LnE type compounds correspond in fact to the formula Ln + (I )2e, while the halides with v 2 have typically cluster structures based on chains (single or double) of Me octahedra, surrounded by halogen atoms. The LnX type compounds constitute a peculiar class of 2-dimensional metals the Ln-Ln bond distance in them depends on the size of the anion (compare Tb-Tb distances of 3.79 

Compound Type Structure Examples Compound Type Structure Examples 

M7X12 MsXi2 discrete Mg SC7CI12 M4X5 MgXi2 IC Er4l5 

A in TbCl and 3.84 A in TbBr). Note that mono-iodides of lanthanides do not exist at all, probably because Ln—Ln bond would be too long to stabilize the structure. 

The scaling of the metal-metal bond length of Pd species with cluster size was recently studied in detail [168]. There is considerable evidence that the metal-metal bonds contract for smaller clusters [174-177]. In a liquid droplet model, this phenomenon is easily rationalized by the increasing pressure with decreasing particle radius a more chemical argument refers to the fact that the fraction of low-coordinated atoms increases with decreasing cluster size. 

Just as bond lengths, also binding energies (per atom) of Au and Pd clusters exhibit a linear scaling [174,175]. Scaling with 1/R can be rationalized by the assumption that surface atoms are bound weaker than fully coordinated atoms inside a cluster. Thus, the size dependence of the binding energy per atom is traced back to the surface-to-volume ratio which is proportional to 1/R. An experimental confirmation for this phenomenon was provided for Ni clusters [180] and confirmed by calculations [181]. 

Surprisingly, the optimized clusters NieAu32 and Nii3Au42 showed almost perfect truncated cuboctahedral and cuboctahedral shapes [182], respectively, although the metal bulk atomic radii of Ni (249 pm) and Au (288 pm) are quite different. For a single cluster, a structural isomer of similar stability was found. This finding resulted from a mutual adaption of bond lengths of both metals. In 

The reactivity of the smallest clusters towards hydrogen molecules is directly related to the stability fluctuations discussed in Section 10.2.1. For metal clusters with opend-shells (most of the transition metals) such as Ni, Co and Fe, large variations in 

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The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Figure Cl. 1.3. Relative reactivity of transition-metal clusters with H2 (full curves, log scale) and tire promotion...

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

One of tire interesting aspects of transition-metal clusters is tlieir novel magnetic properties [91, 92, 93 and 94l]. ... 

Concelcao J, Laaksonen R T, Wang L S, Guo T, Nordlander P and Smalley R E 1995 Photoelectron spectroscopy of transition metal clusters correlation of valence electronic structure to reactivity Rhys. Rev. B 51 4668... 

Yang S and Knickelbein M B 1990 Photoionization studies of transition metal clusters ionization potentials for Fe... 

Wang L S, Cheng H S and Fan J 1995 Photoeieotron spectroscopy of size-selected transition metal clusters Kc , n = 3-24 J. Chem. Phys. 102 9480... 

Wang L S and Wu H 1998 Probing the electronic structure of transition metal clusters from molecular to bulk-like using photoeieotron spectroscopy Cluster Materials, Advances In Metal and Semiconductor Clusters vo 4, ed M A Duncan (Greenwich JAI Press) p 299... 

Pastor G M, Dorantes-Davila J and Bennemann K H 1989 Size and structural dependence of the magnetic properties of small 3d-transition metal clusters Phys. Rev. B 40 7642... 

Transition metal clusters of jr-acid ligands. R. D. Johnston, Adv. Inorg. Chem. Radiochem., 1970, 13,471-533 (412). 

Transition metal cluster compounds. R. B. King, Prog. Inorg. Chem., 1972,15, 287-473 (383). 

The hydrido-transition metal cluster complexes. A. P. Humphries and H. D. Kaesz, Prog. Inorg. Chem, 1979, 25,145-222 (244). 

Recent results in the chemistry of transition metal clusters with organic ligands. H. Vahrenkamp, Struct. Bonding (Berlin), 1977, 32, 2-56 (408). 

Electronic structures of transition metal cluster complexes. M. C. Manning and W. C. Trogler, Coord. Chem. Rev., 1981, 38, 89-138 (406). 

Selective metal-ligand interactions in heterometallic transition metal clusters. E. Sappa, A. Tiripio-chio and P. Braunstein, Coord. Chem. Rev., 1985, 65, 219 (218). 

The electronic structure of transition metal cluster complexes with weak- and strong-field ligands. G. P. Kostikova and D. V. Korol kov, Russ. Chem. Rev. (Engl. Transl.), 1985,54, 344 (137). 

Developments in transition metal cluster chemistry — the way to large clusters. G, Schmid, Struct. Bonding (Berlin), 1985,62, 51 (132). 

Pauling, L. Structure of Transition-Metal Cluster Compounds Use of an Additional Orbital Resulting from the f,g Character of spd Bond Orbitals Proc. Natl. Acad. Sci. (USA) 1977, 74, 5235-5238. 

Transition Metal Cluster with it-Acid Ligands R. D. Johnston... 

Licoccia S, Paolesse R (1995) Metal Complexes of Corroles and other Corrinoids. 84 71-134 Lin Z, Fan M-F (1997) Metal-Metal Interactions in Transition Metal Clusters with n-Donor Ligands. 87 35-80... 

Schmelcher PS, Cederbaum LS (1996) Two Interacting Charged Particles in Strong Static Fields A Variety of Two-Body Phenomena. 86 27-62 Schmid G (1985) Developments in Transition Metal Cluster Chemistry. The Way to Large Clusters. 62 51-85... 

Vahrenkamp H (1977) Recent Results in the Chemistry of Transition Metal Clusters with Organic Ligands. 32 1-56... 

Homogeneous catalysis by transition metal clusters has been reviewed from the perspective of the specific transformations.Examples of very mixed-metal clusters catalyzing processes homogeneously are collected in Table IX. As is generally the case with homogeneous catalysis, the catalytic precursor is well defined, but the nature of the active catalyst is unclear. 

Transition metal clusters with n- 63 acid ligands (386)... 

A broader and more important implication of the oxychlorides is the potential of expanding the ligand combination to other transition-metal cluster systems. The advances in soft-chemistry techniques open up new possibilities for the sta-... 

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