Oxygen atoms, liganding

Figure 5.5 A diagram showing the electronic changes made in the formation of C02 the electrons shown in red are those from the ligand oxygen atoms...

To explain the existence of a conventional double bond in the phosphate ion requires five electrons to be supplied by the central phosphorus atom, which may enter into MO formation with suitable orbitals from the ligand oxygen atoms. To arrange this requires the use of one of the 3d orbitals of the phosphorus atom. This cannot occur in compounds of elements of the second period, where hypervalence is not observed. The availability of accessible d-orbitals has, in the past, been used as the justification of hypervalence in compounds of the subsequent periods. An alternative method of representing the structure of the PO4 ion is shown in Figure 6.6. 

To synthesize the 2 -ironcryptates [Mc Fe2(L3)3 ][X] (7-9, n = 1-3), 2,6-pyridinyl-spacered H2L3 (4) was treated with potassium hydride, strontium hydride, or lanthanum(III) chloride and subsequently with iron(III) chloride (Scheme 3) [71-75]. For instance, an X-ray analysis revealed that the cavity of 2 -metallocryptate [Kc Fe2(L3)3 ]PF6 (7) is occupied by a potassium ion, with ninefold coordination to six ligand oxygen atoms and to three pyridine nitrogen... 

The 24 electrons in the XO J ions are the lour sets ol 2p electrons ol ihe ligand oxygen atoms plus the valency electrons of the centra X atoms and those representing the negative charge. 

Antibiotic Cation Total coord. number Liganding oxygen atoms Number and lengths of "head-to tail" H-bonds Comments, if any Ref... 

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