Structural trends within transition metals

One property of a transition metal ion that is particularly sensitive to crystal field interactions is the ionic radius and its influence on interatomic distances in a crystal structure. Within a row of elements in the periodic table in which cations possess completely filled or efficiently screened inner orbitals, there should be a decrease of interatomic distances with increasing atomic number for cations possessing the same valence. The ionic radii of trivalent cations of the lanthanide series for example, plotted in fig. 6.1, show a relatively smooth contraction from lanthanum to lutecium. Such a trend is determined by the... 

An alternate way to examine these preferences is to look for trends within the vast collection of known four-coordinate metal complexes. Alvarez and coworkers analyzed the structures of more than 13,000 four-coordinate transition-metal complexes and reported these trends (1) (f, d, (f, (f, and d configurations prefer the tetrahedral geometry, (2) d and (f complexes show a strong preference for the square planar geometry, (3) d, ct, cfi, and metals appear in either tetrahedral or square planar structures, (4) a significant fraction of ions have structures intermediate between square planar and tetrahedral, and (5) a large number of structures that cannot be adequately described as tetrahedral, square planar, or intermediate are found for d, cfi, and d complexes. These trends build on the angular overlap-derived preferences. 

Abstract The concept of stable superatoms, molecular species which mimic the shell closures emphasised by Lewis and Kossel, has become an important paradigm of stability in cluster chemistry. In this review we discuss recent work, both experimental and theoretical, on the family of endohedral clusters M Ex, where M is a transition metal ion and E is a member of group 14 (Si, Ge, Sn, Pb). The structural chemistry within this family is very varied, ranging from deltahedral motifs for the heavier tetrels to open 3-connected stmctures such as the hexagonal prism in Cr Sii2- We explore the arguments that have been presented to rationalise these structural trends and their implications for chemical bmiding. 

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