Oxidation state VII d

For oxidation state VII there are no complexes other than oxo complexes or the organometallics (e.g., RM03) which are covered in Section 18-D-12. 

All the remaining halogens have unfilled d orbitals available and the covalency of the element can be expanded. Compounds and complex ions are formed both with other halogens and with oxygen in which the halogen can achieve a formal oxidation state as high as + 7. for example chlorine has formal oxidation states of +1 in the chlorate(I) anion CIO + 5 in the chlorate(V) anion CIO 3, and + 7 in the chlorate(VII) anion CIO4. ... 

The series of 3d elements from scandium to iron as well as nickel preferably form octahedral complexes in the oxidation states I, II, III, and IV. Octahedra and tetrahe-dra are known for cobalt, and tetrahedra for zinc and copper(I). Copper(II) (d ) forms Jahn-Teller distorted octahedra and tetrahedra. With higher oxidation states (= smaller ionic radii) and larger ligands the tendency to form tetrahedra increases. For vanadium(V), chromium(VI) and manganese(VII) almost only tetrahedral coordination is known ( VI j is an exception). Nickel(II) low-spin complexes (d ) can be either octahedral or square. 

For both elements, complexes are formed in oxidation states from -I to VII. There are several broad classes of complex that cut across oxidation states and these, namely, oxo complexes (Section 18-D-6) and nitrido (and related) complexes (Section 18-D-7) will be discussed first. After a tour through the oxidation states (Sections 18-D-8 and 18-D-9) other classes of compounds (that could as well be called complexes) will be taken up. 

As Beck and Nitzmann(74) observed, the specific intensities of the species V(CO)g , Cr(CO)g, and Mn(CO)6, which are, respectively, 947, 610 and 252 X 10 liter mole" cm as measured in tetrahydrofuran, show a rather striking linear dependence upon the formal oxidation state of the control metal atom. The same trend also occurs for the species W(CO)6 and Re(CO)6" , and it is taken to indicate the availability of the d metal electron density. In fact, there is now a considerable body of data available to support the general conclusion that the intensities of the carbonyl stretching vibrations of an isoelectronic and isostructural series of derivatives increase steadily from cations through neutral molecules to anions. For examples, compare the following pairs of species Ni(CO)4, Co(CO)4- (Table I) M(CO)5X, M (CO)sX- (Table II) w-M(CO)4X2, cw-M (CO)4X2 (Table III) and less rigorously M2(CO),o, HM2 (CO),o (Table VII). Beck and Nitzmann also concluded that the effect on intensities on going from a less polar to a more polar solvent increased from cation... 

Manganese shows a number of oxidation states due to negligible energy difference between ns and (n 1 )d orbitals, so electrons from both the orbitals can participate in chemical reaction. The outer electronic configuration of Mn is 3d5 As2. The most stable oxidation states of Mn are + II, + IV and +VII. The unusual positive oxidation states shown by Mn are + I, + III, + IV and + VI. Other than this Mn shows oxidation states 0, 1, II and III. 

Nickel is an analogue of platinum and its oxidation state at which methane is produced is Ni(II) with a d electronic configuration, that is the same as that of Pt(II) which activates methane and other alkanes (see Chapter VII). Evidently the activation of methane on platinum complexes may be considered a conditional model for biological anaerobic oxidation of alkanes. It is of importance that Ni(II) as well as Pt(II) is a so-called soft acid and could prefer to react with methane ( soft base) rather than with such strong hard base as water, therefore surrounding water does not prevent this reaction. 

There will be circumstances other than those I have described here in which "high oxidation state" organometallic chemistry of rhenium in a catalytic reaction will be viable, although it is becoming clear that the balance necessary to achieve this feat is more difficult to maintain as one moves to the right in the transition metal series, and that some of the d rhenium chemistry in fact may look like chemistry of dP osmium species. On this basis it would seem unlikely that the principles that have been used to prepare Re(VII) alkylidyne and alkylidene complexes (a hydrogen migration reactions) can be extended further (to technetium, or especially osmium or ruthenium), at least in a routine fashion. 

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