Pentagonal planar ions

Pentagonal planar ion with two nonbonding electron pairs above and below the plane of the pentagon. 

Pentagonal planar ion wiih Iwo nonbunding clcciron pairs above and below ihe plane of Ihe pentagon. 

IS the first example of a pentagonal planar inorganic ion (Fig. I7.3g)., It can be rationalized in terms of five bonding pairs to fiuorine atoms in a plane with a lone pair above and below the plane. The lone pairs appear to be "locked in to the axial positions as the molecule is rot fluxiona). as is the isoelectronic XcF6. 

Compounds that appear to contain the [XeF]+ (1) and bent [Xe2F3]+ ions are known although the former is always strongly coordinated to a counterions such as SbF-6. Complex anions include XeF-5, XeF-7 and XeF2-g, the first of which has a unique pentagonal planar structure (3), as expected from VSEPR. 

Perhaps the only known example of pentagonal planar symmetry is the ion XePs. On the basis of the symmetry of this ion, predict the number of infrared-active xenon-fluorine stretching vibrations. 

The only simple examples are the pentagonal bipyramidal IF7 (see Fig. 1) and the ion j which is pentagonal planar, having two lone-pairs occupying the axial positions (for XeF6 see below). 

Another example is given by the XeF " ion, which has the three possible structures shown in Fig. 1.21. The results of vibrational analysis for each are summarized in Table 1.12. It is seen that the numbers of IR-active vibrations are 5,6, and 3 and those of Raman-active vibrations are 6, 9, and 3, respectively, for the D3 , C41 , and D5/ structures. As discussed in Sec. 2.7.3, the XeF ion exhibits three IR bands (550-400, 290, and 274 cm ) and three Raman bands (502, 423, and 377 cm ). Thus, a pentagonal planar structure is preferable to the other two structures. The somewhat unusual structures thus obtained for XeF4 and XeF5 can be rationalized by the use of the valence shell electron-pair repulsion (VSEPR) theory (Sec. 2.6.3). 

The second example of a pentagonal planar XYs-type molecule is the IFs " ion, which is isoelectronic with the XeFs ion. It was obtained as the N(CH3)4 salt from the reaction of N(CH3)4lF4 and N(CH3)4F in CH3CN solution, and the IR and Raman spectra were assigned under symmetry [1176]. 

Similarly, the macrocyclic ligand 9.2 is expected to impose a planar pentagonal N5 donor set onto a metal ion. Although metal ions such as lithium form pentagonal... 

The complexes Mnr(dtc)3 (Mni = Np, Pu) are obtained by treating the metal tribromide with Na(dtc) in anhydrous ethanol. Pu(dtc)3 is fairly stable to oxidation, but Np(dtc)3 and the even less stable U(dtc)3 are rapidly oxidized to M(dtc)4, so that neither can be isolated. However, the anionic complexes, (NEt4)[Mm(dtc)4] (M11 = Np, Pu), have been prepared and the geometry about the metal atom is a distorted dodecahedron, best regarded as a planar pentagon of five S atoms with one S atom above and two S atoms below the pentagon.23 These salts of the [M(dtc)4] ion are isostructural with the analogous lanthanide complexes, whereas Pu(dtc)3 is not isostructural with any of the lanthanide Ln(dtc)3. 

The first pentagonal bipyramidal complex of zinc, ZnLCl2 3H20, was reported in 1973, where L is the planar five-coordinate 2,6-diacetylpyridinebis(semicarbazone).346 The crystal structure determination shows the complex to contain the [LZnG(H2Q)]+ cation, with three bland two O-donor atoms forming a slightly distorted planar pentagon around the metal ion and the chlorine and water in axial positions. 

The structures of these ions normally conform to those predicted by the VSEPR theory, as shown in Fig. 17.2.2. Since the anion XY has two more electrons than the cation XY+, they have very different shapes. The anion IFj- is planar with lone pairs occupying the axial positions of a pentagonal bipyramid. In [Me4N](IF6), IFg is a distorted octahedron (C3V symmetry) with a sterically active lone pair, whereas both BrFg and ClFg are octahedral. The anion IF " has the expected square antiprismatic structure. 

The nickel ) complex of 92 cannot be prepared directly via the template method, but can be prepared by a transmetallation procedure. Synthesis of the macrocycle in the presence of one of the metal ions known to be effective as a template is followed by a metal exchange process in solution to insert the nickel ) ion. This cation exhibits a strong preference for the square planar, square pyramidal, and octahedral geometries 79). Thus the failure of the nickel ) cation to behave as a template ion in the synthesis of 92 is probably due to the disinclination of the metal to accommodate the pentagonal array of donor nitrogen atoms necessary for reaction to occur. 

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