Hafnium complexes stereochemistry

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

Coordination numbers (CN) of zirconium and hafnium range from 4 to 12, but because of the large values of their ionic and covalent radii (see Covalent Radii) (Table 1), their complexes typically have CNs of 6-8 (see Coordination Numbers Geometries). They have a varied stereochemistry... 

Product 5 was subjected to X-ray diffraction analysis (Fig. 2 and Table 2) [6]. The structure was found to be an octahedrally coordinated metal dichloro complex with the two bidentate ligands occupying the equatorial positions. The observed stereochemistry can be considered to be caused by the trans-effect of the tmen group [3], which activates the two chloro atoms of the hafnium tetrachloride in equatorial position and makes them susceptible to the nucleophilic attack by the silyl dianion. 

R. L. Davidovich, Stereochemistry of heteroligand fluoro-containing complex compounds of zirconium and hafnium, Russian J. Coord. Chem., 25, 225-236 (1999). 

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