Hgands bond numbers

The number of valence electrons in niobium oxychloride clusters decreases as the number of oxide hgands increases (Table 6.4). The compound Cs2LuNbi Cli70 has the VEC of 16 as found in most chloride clusters, suggesting that the presence of one oxide ligand per cluster does not tip the balance between the Nb-Nb bonding and Nb-L anti-bonding contributions to the a2u state. The VEC of most clusters... 

The Lu—C cr-bonding distances range from 2.425(15) to 2.501(17) A. These distances are approximately 0.2 A shorter than the corresponding distance for a pentahapto cyclopentadienide lutetium bond as predicted from ionic radii. Coordination about the lutetium atom is a slightly distorted tetrahedron. The formal coordination number of four is extremely low for the lanthanides. The only other lanthanide complex with such a low coordination number is the 3-coordinate compound [Lu N(SiMes)2 3] 131). In both cases, the low coordination number is stabilized by the use of bulky hgands. 

For the larger chloride ion, the final member in the series is the tetrachloride, An02CLi A The uranium, neptunium, and plutonium tetrachloride dianions have been isolated with a number of different cations, for example, Na+, NH4+, K+, Cs+, K+18-crown-6, and so on. The An=0 distances were found to be 1.768, 1.751, and 1.737A for An = U, Np, and Pu, respectively. The An-Cl distances in these complexes is virtually unchanged 2.675 A (U), 2.659 A (Np), and 2.656 A (Pu). These stmctural data suggest that the chloride hgands are bound in essentially a purely ionic manner while the covalent nature of the An=0 bond is preserved. 

Coordination numbers are so dependent on hgand geometry that the concept of preferred coordination number is of limited use. For example, hthium may be said to prefer 4 coordination, but examples of 1 to 12 coordination are known. Low formal coordination numbers are often accompanied by secondary interactions with other sources of electron density, such as C H bonds and arene 7r-systems. There are even examples of C H activation caused by a low-coordinate sodium cation. Of course, there is a trend towards higher preferred coordination for the heavier group members (6 for Na, K 8 for Rb, Cs). 

There are broadly two rontes to coordination complexes of alkali inorganics. The first is dissolution of the salt in an excess of the coordinating hgand. This is often kinetically unfavorable, since it requires rupture of a very large number of bonds. The second is to form the desired salt in situ in the presence of a stoichiometric amount of donor from a reactive metal source, either a metal hydride, organyl, or the metal itself, for example, equations (15) and (16). The second of these routes has found more general success. ... 

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