Axial central atom

C09-0133. Among the halogens, only one known molecule has the formula X 7. It has pentagonal bipyramidal geometry, with five Y atoms in a pentagon around the central atom X. The other two Y atoms are in axial positions. Draw a ball-and-stick model of this compound. Based on electron-electron repulsion and atomic size, determine the identities of atoms X and Y. Explain your reasoning. (Astatine is not involved. This element is radioactive and highly unstable.)... 

LiaP and NaaP have the same structure as NaaAs. Each phosphorus atom is surrounded by 5 alkali metal ions at the comers of a trigonal bipyramid. The lattice contains two types of alkali metal atoms. One sort is surrounded by a trigonal prism of other alkali metal ions, in which the centres of the three vertical faces of the prism are occupied by three phosphorus atoms. The other type of alkali metal ion is surrounded by 4 phosphorus atoms in a distorted tetrahedron. Seven alkali metal ions are found at larger distances. In sodium phosphide the distance between the central phosphorus atom of the trigonal bipyramid and an axial sodium atom is 2.93 A and that between the phosphorus atom and an equatorial sodium atom 2.88 A... 

Figure 9.3. Interaction diagram which yields the nature of bonding in the axial bonds of a trigonal bipyramid. The HOMO is nonbonding. If enough stabilization results from the bonding combination, a stable intermediate may be formed. There are no known examples where this is the case when C is the central 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. 

For the rTu and rT, combinations there are no central atom counterparts and consequently, as they therefore do not mix with the central atom orbitals, they do not take part in the bonding. Nonetheless, appropriate rTl combinations can be found. The axial vectors R9, wid Ra (see 9-6) form a basis for rTl and using this piece of information, we find the following T3 1 combinations which mirror Rm, R and Rt ... 

As one centre of repulsion is withdrawn from the central atom (Figure 9), the trigonal bipyramids T[ and T3 are stabilized, and the unique ligand occupies one of the axial sites of a trigonal bipyramid. Examples of this stereochemistry include complexes containing one uncharged ligand... 

Almost in all cases (Table 3) photosubstitution reactive excited states are longer-lived spin-forbidden states involving an antibonding d-orbital of the central atom (usually dzi). The ability of complexes to undergo a photoejection of one axial ligand stems from the dissociative character of such a state. A special attention should be paid to complexes in metal-to-ligand charge-transfer (MLCT) reactive excited states (d, n ). On one side, the increase of positive... 

A light-induced redox change of the central atom and an axial ligand, followed by escaping of the redox changed ligand, can be expressed as follows... 

A photoreduction of an axial ligand associated with the central atom photooxidation is possible in principle (MLCT states might be responsible for such a process) it is not, however, a common photochemical process. 

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