Group 4 metals, hexanuclear clusters

For the mononuclear and dinuclear compounds, except V(CO)6, the mnnber of valence electrons per metal atom is 18. The EAN (effective atomic number) of the metal is the number of electrons of the d" configuration added to twice the number of CO groups, one electron being further added in the case of metal-metal bonded dinuclear systems. For trinuclear, tetranuclear, and hexanuclear compoimds (metal-carbonyl clusters), the cluster valence electron counting is 48, 60, and 86, respectively. As the [K(cryptand 2.2.2)] derivative. As the tetraalkylammonium derivative. As the tetrachloroaluminate. 

Formation of Heavy Transition Metal Group V Metal-Metal Bonds 9.2.8.2. In Niobium and Tantalum Halides Without Hexanuclear Cluster Units... 

The chemistry of Nb and Ta is predominantly that of the -1-5 oxidation state. The heavier group 5 metals differ from V (see Section 21.6) in the relative instability of their lower oxidation states, their failure to form simple ionic compounds, and the inertness of the M(V) oxides. In contrast to V, it is not meaningful to assign ionic radii to Nb and Ta in their lower oxidation states since they tend to form hexanuclear clusters with metal-metal bonding (see later). For M(V), radii of 64 pm are usually tabulated for Nb " and but these are unreal quantities since... 

Hexanuclear main-group metal clusters which are composed of s metal ions can be luminescent, too (Fig. 5) [93], e. g.,... 

Hexanuclear Organometallic Clusters. Although the octahedral arrangement of metal atoms is the most representative one for hexanuclear cluster compounds, a number of other geometries have also been observed. The structural characteristics of a selected group of hexanuclear compounds are briefly described in Table 2.6. The different types of structures found for these clusters are illustrated schematically in Fig. 2.25. 

Under similar conditions the use of the smaller and more Lewis acidic Yb(III) ion afforded a hexanuclear anionic [CpgYbgClia] cluster whose interstitial position is occupied by a presumably stabilizing /Eg-Cl atom (Figure 30, right). The regular octahedron of Ybg contains 12 additional /E-Cl groups, each bridging two adjacent metal atoms. The reduced size... 

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