Oxidation states, less than

Zirconium forms anhydrous compounds in which its valence may be 1, 2, 3, or 4, but the chemistry of zirconium is characterized by the difficulty of reduction to oxidation states less than four. In aqueous systems, zirconium is always quadrivalent. It has high coordination numbers, and exhibits hydrolysis which is slow to come to equiUbrium, and as a consequence zirconium compounds in aqueous systems are polymerized. 

The serendipitous discovery of this compound has proven to be extremely important to our visions of possibilities for new metal-metal bonded structures in reduced oxide phases. In retrospect it is amazing that oxide phases containing molybdenum in oxidation states less than 4+ were essentially unknown and certainly structurally uncharacterized. The existence of the previously mentioned series M2 Mo30q and LiM Mo303 should have been a tip-off to an extensive chemistry for metal-metal bonded molybdenum oxide systems. Indeed, subsequent work has revealed a plethora of new compounds all of which (where structure has been determined) feature strong metal-metal bonding in either discrete cluster units or extended chain arrays. 

Only a small number of zirconium(III) and hafnium(III) complexes are known. Nearly all of these are metal trihalide adducts with simple Lewis bases, and few are well characterized. Just one zirconium(III) complex has been characterized structurally by X-ray diffraction, the chlorine-bridged dimer [ ZrCl PBu,) ]- Although a number of reduced halides and organometallic compounds are known in which zirconium or hafnium exhibits an oxidation state less than III, coordination compounds of these metals in the II, I or 0 oxidation states are unknown, except for a few rather poorly characterized Zr° and Hf° compounds, viz. [M(bipy)3], [M(phen)3] and M Zr(CN)5 (M = Zr or Hf M = K or Rb). 

FIGURE 20.6 Common oxidation states for first-series transition elements. The states encountered most frequently are shown in red. The highest oxidation state for the group 3B-7B metals is their periodic group number, but the group 8B transition metals have a maximum oxidation state less than their group number. Most transition elements have more than one common oxidation state. 

The donor acceptor properties of isonitrile are intermediate between those of CO and tertiary phosphines, and isontrile stabilizes the complexes in their higher oxidation state more and stabilizes the complexes in their lower oxidation state less than CO does. Thus, a direct reaction of zero-valent metal carbonyls with isonitrile usually results to the formation of the partially exchanged mixed-ligand complexes e.g., Cr(CO)g reacts with RNC at 120 130°C to give only the monosubstituted complex ... 

The single-crystal snrfaces of UO have been studied to a lesser extent, most likely because they are not readily available and the perceived irrelevance of their catalytic activity. Nnmerons investigations of polycrystalline and thin-fihn uranium surfaces have been stndied, due to their relevance in nuclear technology, [26-30]. It is however ironic that the first published LEED pattern of any metal oxide surface was that of a U02(1H) single crystal [28]. The reduced UOj surface, like TiO contains a distribution of oxidation states less than +4 [26]. Contrary to TiO, the UO strnctnre, Eig. 7.2, can also accommodate additional oxygen atoms making it a potentially good candidate for oxidation reactions [31]. 

From experimental evidence, the first row transition metals clearly prefer oxidation states +11 and -Fill. Higher and lower formal oxidation states, even formally negative ones (such as —I, —II), are known, although oxidation states less than +1 are almost exclusively... 

The oxidation states less than II, with the exception of Cu1, are found only with ra-acid type ligands or in organometallic compounds. 

The closed-shell nature of aluminosilicates renders them ineffective for certain reactions favoured by transition (d-block) elements. Haushalter has made efforts to prepare stable shape-selective microporous solids involving molybdenum phosphates [15]. These solids are prepared hydrothermally in aqueous HjPO in the presence of cationic templates along with anionic octahedral-tetrahedral frameworks containing Mo in oxidation state less than 5+ and possessing Mo-Mo bonds. Some of these contain around 40 vol% accessible internal void space. There is rich chemistry in these systems and there is considerable potential for applications. Based on this approach one may indeed discover novel microporous and catalytic oxide systems. Several open-framework metal phosphates [16] and carboxylates [17] with different connectivities have been prepared by hydrothermal synthesis. 

Reduced rare earth halides halides with the rare earfli element in an oxidation state less than - -3... 

Many engineering metals, such as iron, nickel, chromium, and titanium, produce metal ions of a variable valency. Uniquely, zirconium is predominantly quadrivalent in its oxides and many other compounds. It forms very few compounds in which its valence is other than 4. The chemistry of zirconium is characterized by the difficulty of achieving an oxidation state less than 4. This character, along with high oxygen affinity, allows zirconium to form protective oxide films even in highly reducing media, such as hydrochloric acid and dilute sulfuric acid. Under these conditions, common metals and alloys may form subordinate oxides or other compounds of low or no protective capability. 

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