Trigonal bipyramidal arrangement VSEPR

According to the VSEPR model the T-shaped CIF3 molecule has a trigonal bipyramidal arrangement of three bonding electron pairs and two nonbonding electron pairs in the valence shell of the central Cl atom. 

These lowest oxidation states can be associated with a coordination number of two and a linear arrangement of the ligands about the central atom. On VSEPR theory these are 10-electron species, with a trigonal bipyramidal arrangement of three lone pairs of electrons in equatorial positions and two halogen ligands in axial positions. 

The Lewis structure reveals a VSEPR number of 5 for the central iodine atom, two bonded neighbors and three unshared pairs. To determine which corners of the trigonal bipyramid are occupied by the terminal iodine atoms, find the arrangement which maximizes the angles between the unshared pairs. The preferred arrangement, Fig. 9-45(a), must be the one in which the unshared pairs are all at 120° because any other alternative [Fig. 9-45(b) and (c)] would have two sets of pairs at 90°. Therefore, the two terminal iodine atoms must occupy the axial positions (180° to each other), making the molecule linear. 

The A1 atom is surrounded by three covalent bond electron pairs and two dative bond electron pairs. According to the VSEPR model the most stable arrangement of five electron pairs in the valence shell of the central atom is trigonal bipyramidal, and this is indeed the structure observed. The VSEPR model may also be used to rationalize the observation that the donor atoms occupy axial positions an axial bond electron pair is repelled by three (equatorial) bond pairs, while an equatorial bond electron is repelled by two axial bond pairs across the same angle. Since a covalent bond electron pair requires more space at the aluminum atom than a dative bond electron pair, the covalently bonded atoms occupy the equatorial positions. 

A tetravalent Group 16 atom is presumably surrounded by five electron pairs, viz. four bond pairs and one non-bonding electron pair. According to the VSEPR model the five electron pairs should arrange themselves in a trigonal bipyramidal manner. Since the nonbonding electron pair requires more space around the central atom than the bond pairs, it is expected to occupy an equatorial position, and deform the molecule in such a way that the bond electron pairs are pushed away. This is indeed the kind of distortion that is observed. The difference between axial and equatorial bond distances may be explained in the same manner as for PF5 and the other compounds of hypervalent Group 15 elements. 

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