Tetranuclear Metal Clusters

Selected examples of tetranuclear clusters are described in Table 2.3. 

Among tetrahedral metal clusters the carbonyl complexes are the more representative. Neutral carbonyl clusters of cobalt and rhodium have similar structures (Fig. 2.16) with nine terminal and three edge-bridging carbonyl 

Cubane cluster Interpenetrated tetrahedra of 2 S and Ti atoms where the latter are bonded to terminal ligands. Two sets of d(Ti-Ti), 3.00 (four) and 2.93 A (two). 

Interpenetrated tetrahedra of S and V atoms 2, 3 (cubane cluster). d(V-V) = 2.873 A (av. two slightly different sets of distances). 

Cr atoms form an approximate tetrahedron 3, 5 capped by the Cp rings and by the O-atoms above each face. d(Cr-Cr) = 1.937 A. 

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The bonding capabilities of transition metal clusters (no nonmetals in the framework), based on molecular orbital calculations, has been nicely summarized by Lauher14 (Table 16.3). Within this table we see three structures (tetrahedron, butterfly, and square plane) for tetranuclear metal clusters. The tetrahedron is a 60-electron cluster, while the butterfly and square plane clusters have 62 and 64 electrons. respectively. When we go from a tetrahedron to a butterfly, one of the edges of the tetrahedron is lengthened corresponding to bond breaking. 

In both [Fe4X4(N0)4](X = S or Se) and [Fe4S2(NO)4(NCMe3)2] the total valence electron count is 60. This is the number characteristic of tetrahedral tetranuclear metal clusters, such as [Ir4(CO)12], in the Wade and Mingos skeletal-electron counting schemes (76, 77) and, furthermore, each iron atom in these clusters obeys the 18-electron rule, provided that it forms single Fe-Fe bonds to each of the other iron atoms in the tetrahedron. 

In the sections below we will describe in detail the known photochemistry of di-, tri-, and tetranuclear metal clusters. Results will be discussed in simple electronic structural terms. 

This fact obviously reflects the electron sink nature of the metal cage. In particular the studies of Dahl and co-workers [92] have established the amphoteric behaviour of a variety of trinuclear and tetranuclear metal clusters extensive structural investigations on the variation of metal-metal bond distances (and indirectly also... 

Table 6. Bond distances (A) in tetranuclear metal carbonyl closed clusters... 

In mixed metal clusters, redistributions can occur in the mass spectrometer so that the highest observed peak arises from such a product. This is clearly exemplified by the heterometallic dodecacarbonyls containing metals of the cobalt triad. The thermal stability of this series of clusters decreases as the rhodium content increases and facile redistributions result in the formation of tetranuclear species containing less rhodium the very thermally stable Q I CO) does not rearrange184. ... 

In practice, however, already with a comparatively small number of metal atoms it is no longer feasible to investigate all possible spin states with all potential realizations by various local spin distributions. Assumptions on the interaction of the metal centers on the basis of their structural arrangement and experimental susceptibility measurements have to be made. For example, for the BS state of a tetranuclear transition-metal cluster, one has to decide which of the four metal atoms couple in an antiferromagnetic fashion with each other. Prominent coupling schemes are, e.g., the dimer-of-dimers 2-plus-2-type or the 3-plus-... 

TRI- AND TETRANUCLEAR CARBONYL-RUTHENIUM CLUSTER COMPLEXES CONTAINING ISOCYANIDE, TERTIARY PHOSPHINE, AND PHOSPHITE LIGANDS. RADICAL ION-INITIATED SUBSTITUTION OF METAL CLUSTER CARBONYL COMPLEXES UNDER MILD CONDITIONS... 

CYCLOPENTADIENYLSODIUM AND SOME MONO-, TRI-, AND TETRANUCLEAR METAL CARBONYL DERIVATIVES AND CLUSTER COMPLEXES... 

A theoretical treatment of tetranuclear metal hydride clusters has been published by Hoffmann and co-workers233). In that article, clusters with various electron configurations (56, 60, 63 electrons) are discussed, and a rationalization is given for... 

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