Seven-coordinate molecules pentagonal bipyramidal

Noteworthy are the complexes obtained with the pentadentate ligands derived from the condensation of 2,6-diacetylpyridine with various substituted hydrazines (XI Table 98). These complexes have been found to be seven-coordinate with pentagonal bipyramidal structure where the pentadentate ligand invariably has its five donor atoms in the equatorial plane. The two residual coordination sites in the axial positions are occupied by two water molecules (325).2361-2364... 

A new and facile synthetic method to prepare complexes with Ti-S bonds is based on the oxidation of the low-valent titanium sandwich complex CpT CyHy) with dithiocarboxylic acid to give the Ti(iv) dithioacetate CpTi(S2CMe)3 (Scheme 421). The compound has been fully characterized including X-ray crystallography. It consists of discrete seven-coordinate molecules with a slightly distorted pentagonal-bipyramidal coordination... 

P24 Z = 4 Dx = 1.51 R = 0.079 for 1,240 intensities (film measurements at —193°). The a-D-xylopyranose molecule is in the JC4 conformation, as in the crystal structure of a-D-xylopyranose. The calcium atom is seven-coordinated by a distorted, pentagonal bipyramid of oxygen atoms consisting of three from water molecules and two from a hydroxyl group of each of two sugar molecules. The ring-oxygen atom is excluded from this first coordination shell. 

Although coordination numbers higher than six were not explicitly considered above, certain species, such as the seven-coordinate 14-1-7 molecule IF7 in a pentagonal bipyramid geometry, contain a linear F-I-F 3c/4e hypervalent bond. 

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