Four coordinated

Glass-forming systems other than siUca have been examined. The fraction of three- and four-coordinated boron in borate glasses can be determined by nmr (29). Both nmr and x-ray diffraction (30) results led to the suggestion that the boroxyl ring is the stmctural unit of vitreous The... 

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. 

Iron hahdes react with haHde salts to afford anionic haHde complexes. Because kon(III) is a hard acid, the complexes that it forms are most stable with F and decrease ki both coordination number and stabiHty with heavier haHdes. No stable F complexes are known. [FeF (H20)] is the predominant kon fluoride species ki aqueous solution. The [FeF ] ion can be prepared ki fused salts. Whereas six-coordinate [FeCy is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeCfy] can be isolated if large cations such as tetraphenfyarsonium or tetra alkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to kon(II) and bromine. Complex anions of kon(II) hahdes are less common. [FeCfy] has been obtained from FeCfy by reaction with alkaH metal chlorides ki the melt or with tetraethyl ammonium chloride ki deoxygenated ethanol. 

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). 

A large number of haUde complexes of thaUium(III) have been prepared by precipitation of the complexes from solution with a suitable cation, eg, H", (C2H )4N", (CgH )4As", and K". Both four-coordinated [HXJ and six-coordinated [HX ] ions exist in solutions and in soUd states. 

Reactions with Alcohols. The tendency of titanium(IV) to reach coordination number six accounts for the rapid exchange of alkoxy groups with alcohols. Departure of an alkoxy group with the proton is the first step in the ultimate exchange of all four alkoxyls. The four-coordinated monomer is expected to react... 

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. 

Mononuclear Carbonyls. The lowest coordination number adopted by an isolable metal carbonyl is four. The only representative of this class is nickel carbonyl [13463-39-3] the first metal carbonyl isolated (15). The molecule possesses tetrahedral geometry as shown in stmcture (1). A few transient four-coordinate carbonyls, such as Fe(CO)4, have also been detected (16). 

For halides the cation should have a charge of 2+ rather than 4+ for tetrahedral coordination. The only fluoride compound capable of containing two-coordinate F and four-coordinate cations is Bep2. For ZrF, the radius ratio rule predicts that Zr" " is eight-coordinate if all fluorine atoms are two - c o o rdinate. 

Discernible associative character is operative for divalent 3t5 ions through manganese and the trivalent ions through iron, as is evident from the volumes of activation in Table 4. However, deprotonation of a water molecule enhances the reaction rates by utilising a conjugate base 7T- donation dissociative pathway. As can be seen from Table 4, there is a change in sign of the volume of activation AH. Four-coordinate square-planar molecules also show associative behavior in their reactions. 

R. van Eldik, ed.. Inorganic High Pressure Chemistry, Elsevier, Amsterdam, The Netherlands, 1986. High pressure coordination kinetics including solvent exchange, octahedral and four-coordinate substitution, electron transfer, photochemical, and bioinorganics are discussed. 

Examples of the C2N3S ring system containing three- or four-coordinate sulfur are also well known. The monohalogenated derivatives 12.2 (E = S, Se), are best prepared by the condensation of imidoyl amidines with SCI2 or SeCU, respectively (Scheme 12.1)." In the case of the selenium derivative, the initial product is heated at 60°C and then at 120°C in order to convert it to 12.2 (E = Se) via (PhC)2(NH)N2SeCl2. In the solid state this intermediate is a weakly associated, centrosymmetric dimer with Se-Cl and Se Cl distances of 2.42 and 3.39 A, respectively. ... 

Ring Systems Containing Four-coordinate Chalcogen... 

Sulfur-Nitrogen Polymers Containing Four-Coordinate Sulfur(Vl)... 

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

Compound [PtCl( Bu2PCMe2CH2)2l reacts with pyrazole or 3,5-dimethyl-pyrazole in the presence of sodium hydroxide to form 242 (R = H, Me) [84ICA (82)L9]. The chelate ring is not planar in this case, and the trans strucmre of the pyrazolate derivative was demonstrated. Tlie four-coordinated platinum atoms are characterized by a distorted square-planar coordination. 

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