Face-bridging ligands

A much more subtle case of stabilisation due to the presence of face-bridging ligands is found in the two tetrahedral clusters Co4Cp4(p3-H)4 (dec. ca. 300 °C)134> and Ni4Cp4(p3-H)3 (dec. ca. 320 °C)133h The structure of the last compound is shown in Fig. 2. 

Face-bridging ligands hexahedro-fg3-Xjg-octahedro-A/p 4.6.83 Unsymmetrically bridged octahedro-A/g... 

Figure 4 10. Infrared spectra of iridium carbonyls in the cages of the NaY zeolite. (5] a) The precursor [Ir(CO)2(acac)), b) tIr4(CO) ], c) [Ir6(CO),d isomer with edge bridging ligands, and d) a mixture of the isomers of [Irfi(CO)i6] with edge bridging and face bridging ligand. Reproduced from the Journal of the American Chemical Society with permission of the American Chemical Society.

Edge and face refer respectively to the dusters with edge bridging and face bridging ligands. 

The used NaY zeolite catalyst containing iridium carbonyl clusters was yellow when removed from the flow reactor after three days of continuous operation at 250 °C and 20 bar, consistent with the presence of the [Ir((CO)i6] isomer with face bridging ligands. [5] The catalyst used under the same conditions but supported in the NaX zeolite was reddish brown after a day of operation, consistent with the presence of [Ir6(CO)is]. [4, 60]... 

Many non-carbonyl ligands such as NO, S, PR, O or RC can be edge or face-bridging ligands in triangular metal clusters (Fig. 2.12). Phosphines PR3 are typical donor terminal ligands in trinuclear triangular clusters. 

Although compounds with edge and face-bridging ligands may be formally considered as main group-transition metal mixed clusters (s. Chap. 3), in these section they will be referred to only as ligands. 

In the anions [Mo6Xi4] only 6 of the halide ligands may be substituted with relative facility, namely those bonded to the metal as terminal ligands. The other 8 face-bridging ligands are, however, very inert to exchange reactions. 

As analyzed in a previous Section (see Sect. 2.4.5), there are some clusters containing strong face-bridging ligands which have the property of holding to cluster structures in spite of cleavage of the metal-metal bonds arising for instance from an excess of cluster skeleton electrons. This property may also be useful for cluster catalyst precursors. 

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