Oxide four-coordinated

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

Much effort has been placed in the synthesis of compounds possessing a chiral center at the phosphoms atom, particularly three- and four-coordinate compounds such as tertiary phosphines, phosphine oxides, phosphonates, phosphinates, and phosphate esters (11). Some enantiomers are known to exhibit a variety of biological activities and are therefore of interest Oas agricultural chemicals, pharmaceuticals (qv), etc. Homochiral bisphosphines are commonly used in catalytic asymmetric syntheses providing good enantioselectivities (see also Nucleic acids). Excellent reviews of low coordinate (coordination numbers 1 and 2) phosphoms compounds are available (12). 

Boric oxide is an excellent Lewis acid. It coordinates even weak bases to form four-coordinate borate species. Reaction with sulfuric acid produces H[B(HSO 4] (18). At high (>1000° C) temperatures molten boric oxide dissolves most metal oxides and is thus very corrosive to metals in the presence of oxygen. 

Derived from the German word meaning devil s copper, nickel is found predominantly in two isotopic forms, Ni (68% natural abundance) and Ni (26%). Ni exists in four oxidation states, 0, I, II, III, and IV. Ni(II), which is the most common oxidation state, has an ionic radius of —65 pm in the four-coordinate state and —80 pm in the octahedral low-spin state. The Ni(II) aqua cation exhibits a pAa of 9.9. It forms tight complexes with histidine (log Af = 15.9) and, among the first-row transition metals, is second only to Cu(II) in its ability to complex with acidic amino acids (log K( = 6-7 (7). Although Ni(II) is most common, the paramagnetic Ni(I) and Ni(III) states are also attainable. Ni(I), a (P metal, can exist only in the S = state, whereas Ni(lll), a cT ion, can be either S = or S =. ... 

The acrylate complex 10 was suggested to be the major solution species during catalysis, since the equilibrium in Scheme 5-11, Eq. (2) lies to the right (fQq > 100)-Phosphine exchange at Pt was observed by NMR, but no evidence for four-coordinate PtL, was obtained. These observations help to explain why the excess of phosphine present (both products and starting materials) does not poison the catalyst. Pringle proposed a mechanism similar to that for formaldehyde and acrylonitrile hydrophosphination, involving P-H oxidative addition, insertion of olefin into the M-H bond, and P-C reductive elimination (as in Schemes 5-3 and 5-5) [11,12]. 

Unique combinations of properties continue to be discovered in inorganic and organometallic macromolecules and serve to continue a high level of interest with regard to potential applications. Thus, Allcock describes his collaborative work with Shriver (p. 250) that led to ionically conducting polyphosphazene/salt complexes with the highest ambient temperature ionic conductivities known for polymer/salt electrolytes. Electronic conductivity is found via the partial oxidation of unusual phthalocyanine siloxanes (Marks, p. 224) which contain six-coordinate rather than the usual four-coordinate Si. 

The interactions of dimethyl- and diethylzinc with bulky tris(hydroxyphenyl)methanes, Scheme 86, yielded, depending on the reaction conditions, a variety of alkylzinc alkoxides, featuring two-, three-, and four-coordinate zinc centers. These polynuclear compounds (Figure 63 shows the trinuclear ethylzinc derivative 136) are relatively poor catalysts for the co-polymerization of cyclohexene oxide and carbon dioxide.197... 

With a metal ion having four coordination centers, binding of four molecules (39) can occur. The reaction of four molecules of (39) with Hg(II) perchlorate in CH3CN produced complex (212) with a high yield [Eq. (150)]. Electrochemical oxidation of some phosphines on a mercury anode has been shown to lead to their complexes with Hg(II). Following this method, complex 212 was synthesized in high yield (92MI1). The M—P bonds were shown to be in an equatorial position. 

Holland and Peters earlier reported examples of three- and four-coordinate N2 complexes with iron in the formal +1 oxidation state, but in neither case were terminally bonded N2 species identified or isolated dinuclear end-on Fe-NN-Fe species were inevitably obtained (99,100). Therefore, Peters and coworkers recently began to turn their attention to new derivatives of the classic Sacconi-type tripodal ligands to achieve access to a terminally bound Fe-N2 complex. Indeed, by employing the mono-anionic ligand L8 — [f2-Ph2PC6H4)3Si] (Fig. 10) they were able to generate the desired complex [Fe(L8)(N2)] (10) (101). 

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