Rhomboidal Clusters

A few cluster complexes with fused triangles of molybdenum or tungsten have been reported. They are tetranuclear and hexanuclear cluster complexes, which may be viewed also as having fused incomplete cu-banes of M3E4. 

The cluster [Mo4(pt3-S)2(M2-S)4(SH)2(PMe3)6] has been prepared by the reaction of (NH4)2[Mo3S13] with trimethylphosphine in a mixed solvent of butylamine and thf at room temperature followed by reflux in 14% yield (99). The SH ligands can be replaced by halogens in reac- 

The average oxidation state of the metals in both molybdenum and tungsten clusters is +3.5, and the number of cluster valence electrons is 10. The number is consistent with five M—M single bonds. 

A solution of [Mo302S2(H20)9]4+ in 4M Hpts (pts = para-toluene sulfonate) was reacted with indium metal for 24 hr at room temperature and stored in a refrigerator for a few days to form [Mo4(p3-S)2(p2-0)4 (H2O)10](pts)4 10H2O (101). The cluster core of Mo4S204 is similar to those of Mo4S6 cluster complexes. 

The electronic levels of a model compound [Mo4S6Cl2(PH3)6] have been calculated by the DV-Xa method (100). In a cluster of C2A symmetry, five metal—metal bonding orbitals (two ag, one au, one bg, and one bu) are expected. These orbitals are strongly mixed with the ligand orbitals and show large HOMO-LUMO gaps (Fig. 24), which may be the characteristics of the electron-precise clusters with 10 electrons. 

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The interesting compound Ba MogOig (17,22) contains tetranuclear rhomboidal cluster units, tied into infinite chains by Mo-O-Mo bonding as represented in the connectivity ] o ed in sites along tunnels... 

In one pair of chains the rhomboidal cluster units have five Mo-Mo bonds of nearly equal length (regular units), and in the other pair the (distorted) rhomboidal units have three short and two long Mo-Mo bonds. Figure 3 shows how these rhomboidal units are bound together within the infinite chains. A comparison of Mo-Mo and selected Mo-0 bond distances for the regular and distorted cluster units is given in Table III. 

The compound Bai 14Mo8016 contains similar rhomboidal clusters of Mo4 in the infinite chains (114). One is more regular, with five Mo—Mo bonds in the edge-shared bitriangle (2.578(1)—2.616(1) A), and another is rather distorted with two long and three short Mo—Mo bonds (2.546 -2.847 A). The CVE is 18.26 this can be divided into 10 for the more regular cluster and 8.26 for the distorted cluster, and the distortion can be understood from the viewpoint of electron deficiency. 

The preceding three reduced molybdenum clusters indicate that rhomboidal cluster units are common among this kind of oxide, and the Mo-Mo distances in the cluster units are dependent on CVE assignable to the Mo4 units. MO calculations on Mo4 molecular cluster compounds containing 10 CVE and 8 CVE have shown that the 8-electron clusters distort from a second-order Jahn-Teller effect (116). Such a distortion has been observed in the 8-electron W4(OEt)16 (117). 

We recently reported on the cluster Gaf, Rx2 (R = SiPh2Me) 67 in which the mean oxidation state of the gallium atoms is +1 [105] (Figure 2.3-18). After cleavage of two R groups the formation of an octahedral Ga<,R<, precloso unit analogue to the tetrahedral Ga4R4 cluster (cf. Section 2.3.3.1) could be assumed. Model calculations have shown however that a rhomboid Ga6 unit with two naked Ga atoms would be favored over the precloso cluster [105],... 

X-ray crystallography, 40 20-21 synthetic models, 40 23-48 xanthane oxidase, 40 21-23 chalcogenide halides, 23 370-377, 413 Chevrel phases, 23 376-377 metal-metal bonding, 23 330, 373 structural data, 23 373-376 as superconductors, 23 376 synthesis, 23 371-372 chloride, 46 4-24, 35-44 heterocations of, 9 290, 291 cluster compounds, 44 45-46 octahedral, 44 47-49, 53-63 electronic structure, 44 55-63 molecular structure, 44 53-54 synthesis, 44 47-49 rhomboidal, 44 75-82 solid-state clusters and, 44 66-72, 74-75, 80-82, 85-87 tetrahedral, 44 72-75 triangular, 44 82-87 cofactor, 40 2, 4-12 anaerobic isolation, 40 5 molybdopterin and, 40 4-8 reduced form, 40 12 synthesis, 40 8-12 xanthine oxidase, 45 60-63 complexes... 

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. 

The new cluster compound contains a planar Os4Sn4 skeleton consisting of a central rhomboidal Os2Sn2 unit fused to two outer Os2Sn triangles331. 

These novel hexanuclear species display an unprecedented planar rhomboidal metal array consisting of two intact Re2 units and two Cu+ cations. These clusters can be considered as raft-like aggregates and therefore represent a geometric analogue of a metal surface.307... 

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