Electron Poor Transition Metal Clusters

Trinuclear clusters play an important role in the chemistry of molybdenum and tungsten. Deep red, isostructural clusters containing both [M03O4F9]3- (148, 149) and [W304F9]5 (149-151) are later additions to this family of simplest types of cluster species. Their basic structures conform to the fii-type of trinuclear electron-poor transition-metal clusters where the metals are in a distorted octahedral environment. 

For Nb or Ta clusters MgLig units are formed when the ligands are Cl or Br for niobium, and Cl, Br or I for the tantalum. In contrast, the niobium iodides have the MeLi4 unit, owing to the important steric effect of the iodine, as discussed later. Such MeLis units can also be prepared from valence-electron-poor transition metals such as zirconium, but in this case an interstitial element located at the center of the octahedral cluster is necessary to stabilize the cluster. ... 

The availability of d orbitals on transition metal vertices leads to the possibility ofelectron-poor or hypoelectronic transition metal clusters with n vertices having less than 2 -I- 2 apparent, skeletal electrons. Such electron-poor clusters form deltahedra containing tetrahedral chambers, i.e., deltahedra with one or more vertices of degree 3 (Figure 9). The simplest examples of such deltahedra are the capped tetrahedra, of which the trigonal bipyramid (i.e., the monocapped tetrahedron) with five vertices is the smallest. The capped tetrahedra consist of a series of fused tetrahedral chambers with faces in common. An example of a cluster based on a bicapped tetrahedron is... 

Complexes of other transition metals have been reported to catalyze Pauson-Khand reactions. Buchwald reported intramolecular PKRs with 1.2 atm of CO at 90 °C in the presence of CpjTi(CO)2. " However, most other catalytic Pauson-Khand reactions have been conducted with late transition metal catalysts. Murai and Mitsudo simultaneously reported intramolecular PKRs catalyzed by ruthenium carbonyl clusters in dioxane or DMAc at 140-160 °C under 10-15 atm of CO. The first Rli-catalyzed PKR was reported by Narasaka. ° In this case, the reaction occurred with acceptable rates, even with CO pressures less than 1 atm. Shibata reported PKRs in refluxing xylenes under 1 atm of CO in the presence of catalytic amounts of PPli and [Ir(COD)Cl]2. Adrio and Carretero showed that the solvated molybdenum carbonyl complex Mo(DMF)3(CO)3 catalyzed intramolecular PKRs with monosubstituted olefins, as well as with disubstituted electron-poor olefins, and Hoye showed that W(CO)5(THF) catalyzes intramolecular PKRs. Iron and palladium complexes have also been reported to catalyze the PKR. 

Zintl also found that the most convenient way to produce the solutions of the anionic clusters in liquid ammonia is to extract alkali metal/post-transition element alloys in the solvent. However, detailed solid-state characterization of the clusters is very difficult using this technique, since poorly crystalline and often pyrophoric solids are obtained once the solvent is evaporated. These troublesome solids are alkali-metal ammoniates of cluster ions, " of which only [Li(NH3)4]3-[Li2(NH3)2Sb5] -2NH3 seem to have been completely structurally characterized, Furthermore, the ammoniates most often slowly revert back to the alloy upon further loss of ammonia. The last step involves transfer of electrons from the strongly reducing cluster anion back to the alkali-metal ion and thus represents a major synthetic obstacle. 

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