Zirconium oxide coordination number

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 coordination chemistry of zirconium and hafnium is dominated by oxidation state IV and by higher coordination numbers, especially coordination number eight. Oxidation states and coordination geometries are summarized in Table 1. 

The chemistry of zirconium has some similarities to that of silicon and titanium, since it is in Group IV of the periodic table. Zirconium has a normal oxidation state of 4 with limited redox chemistry and a coordination number of up to 8. Zirconium compounds are normally colourless. 

The oxidation states and stereochemistries of zirconium and hafnium are summarized in Table 18-A-l. These elements, because of the larger atoms and ions, differ from Ti in having more basic oxides, having somewhat more extensive aqueous chemistry, and more commonly attaining higher coordination numbers, 7 and 8. They have a more limited chemistry of the III oxidation state. 

The results of fundamental investigations of the formation of hydrated zirconium dioxide are reported in [41]. The oxidation degree of zirconium ions is -t4 in nearly all its compounds. Possessing rather large radius (0.092 nm) and rather low ion potential, it forms sterically maximal number of bonds. Accordingly, in the major part of its compounds, Zt ions exhibits coordination number 8. 

The most common coordination number of titanium is six, although four-, five-, seven-, and eight-coordinate compounds are known (Table 2). Table 3 summarizes the common oxidation states of titanium with the associated coordination numbers and stereochemistries. Zirconium shows a similar range of oxidation states (see Zirconium Hafnium Inorganic Coordination Chemistry), however, Zr and Flfr are much less stable, relative to Zr and Hf, than is the case for titanium. 

Analytical Chemistry of the Transition Elements Coordination Numbers Geometries Coordination Organometallic Chemistry Principles Hydride Complexes of the Transition Metals Oxide Catalysts in Sohd-state Chemistry Periodic Table Trends in the Properties of the Elements Sol Gel Synthesis of Solids Structure Property Maps for Inorganic Solids Titanium Inorganic Coordination Chemistry Zirconium Hafnium Organometallic Chemistry. 

The reaction of zirconyl perchlorate with pyridine iV -oxide in ethanol (382) gave a product of composition Zr0(py0)e(C104)2. The infrared spectrum shows that bonding is through the oxygen. A similar product was obtained with quinoline A -oxide (327). Zirconium is assumed to have a coordination number of seven in these species. The nature of the zirconium-oxygen moiety is not known. 

Monomeric species M OR-tert)x have been characterized for titanium, vanadium, chromium, zirconium, and hafnium (x = 4) and for niobium and tantalum (x == 5). With chromium it was found that limiting Cr(III) to coordination number 4 in the dimeric Cr2(OBu )e caused instability and a remarkable facility toward valency disproportionation or oxidation to the stable quadricovalent Cr(OBu )4 (8, 9). In contrast, molybdenum formed a stable dimeric tri-tert-butoxide (Bu O)3Mo=Mo-(OBu )3 which is diamagnetic and presumably bound by a metal-metal triple bond (10, II). Yet another interesting feature of chromium is the synthesis of a stable diamagnetic nitrosyl Cr(NO) (OBu )3 in which the nitric oxide is believed to act as a three-electron donor with formation of a four-coordinated low spin chromium (II) compound (12). The insta-bihty of Cr2(OBu )e and the stability of both Cr(NO) (OBu )3 and Cr(OBu )4 must result from the steric effects of the tertiary butoxo groups since the less bulky normal alkoxo groups form very stable polymeric [Cr(OR)3]a. compounds in which the Cr(III) has its usual coordination number of 6 (octahedral). 

Figures 1.4(a) and 1.4(b) show parts of the baddeleyite-type structure of zirconium dioxide ZrO. This structure belongs to the monoclinic space group P2i/c and has a distorted fluorite-type structure. The coordination number (CN) of the Zr cations is seven, which is smaller than that of the fluorite-type oxide (CN = 8). This smaller value (CN = 7) is attributable to the smaller size of Zr cations ( (Zr + s) = 0.84 A) compared to the criterion deduced from Pauling s first rule (r( >8) 1.079 A) for A cations with a CN of eight...

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). 

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