Electronic properties of the M6L14 unit

In the MfiLu unit, the highest levels occupied by the electrons (aig, tiu, t2g, t2u, Cg) have M-M bonding character. These levels are fully occupied by 24 valence electrons per Me cluster (VEC = 24), which corresponds schematically to twelve 2-electrons-2-center bonds. The metal-metal bond is stronger than in the MeLig compounds and the corresponding M-M distances are considerably shorter ca. 2.6 A compared with 2.9 A for MeLig). Compounds based on such discrete units, obtained by solid-state chemistry syntheses, usually have VEC = 24 and the HOMO level of eg symmetry is fully filled. When the VEC = 23, as in a recent example, the eg HOMO level accommodates three electrons. 

In these discrete MeL materials, no direct interactions between the clusters can occur and they are insulating. The valence electrons remain localized on the cluster and, when the VEC = 23 the cluster is paramagnetic owing to one unpaired electron, as for the MeLig-based compounds when the VEC is 15. 

The different M MeX structures are Usted in Table 2. When possible they are described in terms of classical unit packings. The following sections cover only the Pn3 structure type for which a number of structural determinations have been performed on anionic- or cationic-substituted compounds these have revealed a wide variety of interatomic distances that are useful for comparative purposes. 

In these compounds the MeXi4 units form a f.c.c. packing (see, for instance, the structure of CdWeBr in Fig. 5), as in the series (R3), but in con- 

This series is again of the structural type M M X, with the same f.c.c. unit packing, hut now is replaced hy an ME pair with short M-M distances, which is centered on the origin of the unit-cell (see the copper environment in Cu2W6Bri4 in Fig. 6). The octahedral M site formed hy the six apical halogens in the 

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