Cluster chemistry basic properties

Early transition metal oxygen anion clusters, or polyoxometalates for short, are a large and rapidly growing class of inorganic complexes [1-15]. These compounds attracted us as it was apparent they simultaneously exhibited a unique set of properties we felt could be utilized to address new catalytic transformations, photoredox chemistry, and, ultimately construction of sophisticated single-molecule multifunctional devices capable of several temporally linked functions. These prognostications have already been borne out to a considerable extent. This article reviews the title subject, an enterprise still in its infancy. Presented sequentially in this chapter are the basic properties of polyoxometalates, general features of the photochemistry, the mechanisms elucidated in polyoxometalate photochemistry thus far, and an overview of the photochemistry of two representative complexes in tabular form (Table I). 

Abstract Amino acids are the basic building blocks in the chemistry of life. This chapter describes the controllable assembly, structures and properties of lathanide(III)-transition metal-amino acid clusters developed recently by our group. The effects on the assembly of several factors of influence, such as presence of a secondary ligand, lanthanides, crystallization conditions, the ratio of metal ions to amino acids, and transition metal ions have been expounded. The dynamic balance of metalloligands and the substitution of weak coordination bonds account for the occurrence of diverse structures in this series of compounds. 

The structural relationship between the molecular and solid-state compounds has been a hot issue in inorganic chemistry for some time (25-27). The extrusion (or excision) from preformed solid-state cluster compounds is one of the major synthetic methods of the preparation of cluster complexes (26). Use of cluster complexes as precursors to solid-state cluster compounds is the reverse reaction of excision. Both reactions utilize the structural similarity of the metal cluster units. The basic cluster units of polyhedra (deltahedra) or raft structures are triangles, and both molecular and solid-state clusters with octahedral, tetrahedral, and rhomboidal cores have been reported. Similarity of other properties such as electronic structures based on the cluster units is also important. The present review is concerned with the syntheses and structures of the cluster complexes of the group 6 metals and with their relationships to solid-state chemistry. 

Substitution at one or more metal sites will generally break the symmetry of the cluster core, and can greatly influence its electronic properties and reactivity. Consider, for example, the possible substitutions of a metal M into an octahedral core of composition MfiX v (x = 8, 12). The first substitution will afford an MsM X core, for which the symmetry has been lowered to C4v. A second substitution generates an M4M 2Xx core with two possible isomers One in which the M atoms are positioned at trans vertices (D4/,) and another where they are positioned at cis vertices (C2v). With a third substitution to give an M3M 3Xx core, fac and mer isomers become possible, while further substitutions simply repeat the pattern with M and M interchanged. Here again, the substitutions can be anticipated to alter the basic electronic properties of the cluster. Moreover, the outer-ligand substitution chemistry could potentially be quite different... 

The reliability of method for electronic structure calculations and models used in zeolite modeling is briefly reviewed. Only traditional ab initio methods and methods based on the density functional theory are discussed. Periodic, cluster, and combined models are described and their suitability for investigation of various properties is discussed. This contribution is written for non-experts in computational chemistry. The author hopes that it will help them to gain a basic orientation in the field. 

Figure 4. Synthesis of platinum cluster anions, [Pt3(CO)3(/i-CO)3] , in solution, on the basic surface of MgO and in the Micro/mesoporous cages of NaY(12 A) and FSM-16 (28, 48 The chemistry on MgO and in micro/mesoporous cages is quite similar to that occurring in the basic solution. However, the cavities afford the templating micro reactor for the synthesize of the selected clusters owing to their chemical properties and confinement of the micro cavities. By contrast, a wide distribution of Pt cluster anions having different sizes was formed on the MgO surface by the surface-mediated carbonylation reaction.

That there are such differences between the structural chemistry of reduced Nb and Ta oxides is slightly surprising and two basic questions arise. Firstly, is there also a chemistry of condensed TaeO clusters as found for reduced oxoniobates and, secondly, how do the physical properties of such tantalum compounds compare with those of the niobates Future studies are required to answer this question. 

Today we know that the HF method gives a very precise description of the electronic structure for most closed-shell molecules in their ground electronic state. The molecular structure and physical properties can be computed with only small errors. The electron density is well described. The HF wave function is also used as a reference in treatments of electron correlation, such as perturbation theory (MP2), configuration interaction (Cl), coupled-cluster (CC) theory, etc. Many semi-empirical procedures, such as CNDO, INDO, the Pariser-Parr-Pople method for rr-eleetron systems, ete. are based on the HF method. Density functional theory (DFT) can be considered as HF theory that includes a semiempirical estimate of the correlation error. The HF theory is the basie building block in modern quantum chemistry, and the basic entity in HF theory is the moleeular orbital. 

Gas-phase chemistry studies of atomic and molecular rare-earth and actinide ions have a deep-rooted history of more than three decades. In gas phase, physical and chemical properties of elementary and molecular species can be studied in absence of external perturbations. Due to the relative simplicity of gas-phase systems compared to condensed-phase systems, solutions or solids, it is possible to probe in detail the relationships between electronic structure, reactivity, and energetics. Most of this research involves the use of a variety of mass spectrometry techniques, which allows one exerting precise control over reactants and products. Many new rare earth and actinide molecular and cluster species have been identified that have expanded knowledge of the basic chemistry of these elements and provided clues for understanding condensed-phase processes. Key thermodynamic parameters have been obtained for numerous atomic and molecular ions. Such fundamental physicochemical studies have provided opportunities for the refinement and validation of computational methods as applied to the particularly challenging lanthanide and actinide elements. Among other applications, the roles of... 

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