Six coordination

The structural chemistry of six-coordinated fluoro complexes of the d-transition elements is the subject of an entire review paper by Babel (7). In contrast, six coordination is rare in actinide fluoro-complexes. The relatively large sizes of the actinide ions (Fig. 3) suggest that low coordination numbers should be found only in the fluoride complexes of the higher oxidation states. The radius ratio, r+/r, predicted for the lower hmit of stabihty of octahedral coordination is ]/2 — 1, which corresponds to a positive ion radius of 0.55 A in coordination with fluoride (1.33 A). 

Of the very few known structure types displaying six-coordinated actinides, only two, UFg and CsUFg, have been determined with sufficient accuracy to give trustworthy values for the effective ionic radii of the actinide ions. In UFg and CsUFe the uranium ions (U + and U +) have radius ratios with fluorine wed above the lower Hmit of stability for octahedral coordination but still lower than the lower limit predicted for the 8-coordinated square antiprism (0.645) or the 7-coordinated capped octahedron (0.592). However, both U + and U + are sufficiently larger than 0.55 A so that F—F distances in the octahedron cannot all be van der Waals contact distances, and thus, the MFg octahedra are susceptible to distortion. 

Isolated octahedral groups are found in orthorhombic XIFg (30). Within the accuracy of the single crystal X-ray experiment the coordination was not demonstrated to be significantly distorted from that of a regular octahedron. The average U—F distance of 2.05 A gives an effective U + radius of 0.72 A. 

The X-ray powder patterns of LiUFe (34) and of a-NaUFe (16) indicate that they are both isostractural with rhombohedral (R3) 

Underlined compounds are structure types, determined by single crystal techniques. 

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It is remarkable that chemists long resisted making the connection between boron and electron-deficient carbon, which, after all, are analogs. I was thus given the opportunity to be able to establish the general concept of five and six coordination of electron-deficient carbon and to open up the field of what I called hypercarbon chemistry. 

With Lammerstma and Simonetta in 1982, we studied the parent six-coordinate diprotonated methane (CH/ ), which has two 2e-3c bonding interactions in its minimum-energy structure (Cid- On the basis of ab initio calculations, with Rasul we more recently found that the seven-coordinate triprotonated methane (CHy ) is also an energy minimum and has three 2e-3c bonding interactions in its minimum-energy structure 3 ). These results indicate the general importance of 2e-3c bonding in protonated alkanes. 

Structures of (a) EDTA, and (b) a six-coordinate metal-EDTA complex. 

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. 

The participation of phosphoms d orbitals in the five- and six-coordinate compounds provides increased polarizabiUty, nucleophilicity, and ionic character. In fact, compounds such as phosphoms pentachloride [10026-13-8] 5) thought to have considerable ionic character. The CJ-bond orders between the phosphoms atom and its constituents in the higher coordinate compounds maybe less than one. 

PF3 forms complexes with amines, ethers, and other bases as well as F , with which phosphoms becomes six-coordinate. Dry phosphoms pentafluoride does not attack glass. The yellow crystalline phosphoms pentabromide forms from the reaction of PBr and excess bromine. 

Plutonium(III) in aqueous solution, Pu " ( 4)> is pale blue. Aqueous plutonium(IV) is tan or brown the nitrate complex is green. Pu(V) is pale red-violet or pink in aqueous solution and is beUeved to be the ion PuO Pu(VI) is tan or orange in acid solution, and exists as the ion PuO. In neutral or basic solution Pu(VI) is yellow cationic and anionic hydrolysis complexes form. Pu(VII) has been described as blue-black. Its stmcture is unknown but may be the same as the six-coordinate NpO (OH) (91). Aqueous solutions of each oxidation state can be prepared by chemical oxidants or reductants... 

The possibihty of six-coordinate sihcon species in aqueous solution has been suggested. Raman studies have indicated, however, that monosilicic acid in solution contains a tetracoordinate sihcon species (37). 

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. 

Titanium Trifluoride. The trifluoride (121) is a blue crystalline soHd, density 2980 kg/m, ia which the titanium atoms are six-coordinate at the center of a slightly distorted octahedron, where the mean Ti—F distance is 197 pm. Titanium trifluoride [13470-08-1] is stable ia air at room temperature but decomposes to titanium dioxide when heated to 100°C. It is insoluble ia water, dilute acid, and alkaUes but decomposes ia hot concentrated acids. The compound sublimes under vacuum at ca 900°C but disproportionates to titanium and titanium tetrafluoride [7783-63-3] at higher temperatures. 

Titanium Tetrafluoride. Titanium tetrafluoride [7783-63-3] is a white hygroscopic soHd, density 2798 kg/m, that sublimes at 284°C. The properties suggest that it is a fluorine-bridged polymer in which the titanium is six-coordinate. The preferred method of preparation is by direct fluorination of titanium sponge at 200°C in a flow system. At this temperature, the product is sufficiently volatile that it does not protect the unreacted sponge and the reaction proceeds to completion. The reaction of titanium tetrachloride with cooled, anhydrous, Hquid hydrogen fluoride may be used if pure hydrogen fluoride is available. 

The six coordinated titanium(IV) compounds, Ti(acac)2(X)2, where X is methoxy, ethoxy, isopropoxy, -butoxy, or chloro, all adopt the cis-configuration. This is beheved to result from the ligand-to-metal TT-electron donation (88,89). 

Coordination compounds of vanadium are mainly based on six coordination, in which vanadium has a pseudooctahedral stmcture. Coordination number four is typical of many vanadates. Coordination numbers five and eight also are known for vanadium compounds, but numbers less than four have not been reported. The coordination chemistry of vanadium has been extensively reviewed (8—12) (see Coordination compounds). 

The neutral complexes of chromium, molybdenum, tungsten, and vanadium are six-coordinate and the CO molecules are arranged about the metal in an octahedral configuration as shown in stmcture (3). Vanadium carbonyl possesses an unpaired electron and would be expected to form a metal—metal bond. Steric hindrance may prevent dimerization. The other hexacarbonyls are diamagnetic. 

Figure 16.24 A six-coordinated pentamer of SV40 "extracted" from the model shown in Figure 16.23. The extended carboxy-termlnal arms are shown in the conformations they adopt in the assembled particle in the free pentamer they are disordered and flexible. (Courtesy of S. Harrison.)...

Figure 16.23 Overview of the structure of the SV40 virus particle, showing the packing of pentamers. The subunits of pentamers on fivefold positions are shown in white those of pentamers in six-coordinated positions are shown in colors. The six colors indicate six quite different environments for the subunit. (Courtesy of S. Harrison.)...

Figure 5 Solid state NMR spectra of Vanadium oxide on y-alumina as a function of vanadium loading (wt.%) and surface coverage 0. Note the gradual emergence of the six-coordinated vanadium site with increased loading.

Five- and six-coordinated nitrogen in azaborane clusters (c/o.so-NB9Hlo, nido-NBlqHl3, c/o.s o-NBllHl2, and their derivatives) 98EJI143. 

Coordination number The number of bonds from the central metal to the ligands in a complex ion, 409,412t four-coordinate metal complex, 413 six-coordinate metal complex, 413-414 Copper, 412 blister, 539... 

Single bond A pair of electrons shared between two bonded atoms, 167 Six-coordinate metal complex, 413-414 Skeleton structure A structure of a species in which only sigma bonds are shown, 168... 

In its complex compounds, of which there are many thousands, Co almost invariably has a +3 oxidation number. Apparently, Co+s ion accompanied by six coordinating groups is particularly stable. Cobalt complexes are important in biochemistry. Some enzyme reactions go through a cobalt-complexing mechanism. Although only small traces are needed, cobalt is essential to the diet. 

When, however, the ligand molecule or ion has two atoms, each of which has a lone pair of electrons, then the molecule has two donor atoms and it may be possible to form two coordinate bonds with the same metal ion such a ligand is said to be bidentate and may be exemplified by consideration of the tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+. In this six-coordinate octahedral complex of cobalt(III), each of the bidentate ethylenediamine molecules is bound to the metal ion through the lone pair electrons of the two nitrogen atoms. This results in the formation of three five-membered rings, each including the metal ion the process of ring formation is called chelation. 

The 14e compound MTO readily forms coordination complexes of the type MTO-L and MTO-L2 with anionic and uncharged Lewis bases [96], These yellow adducts are typically five- or six-coordinate complexes, and the Re-L system is highly labile. Apart from their fast hydrolysis in wet solvents, MTO-L adducts are much less thermally stable then MTO itself. The pyridine adduct of MTO, for instance, decomposes even at room temperature. In solution, methyltrioxorhenium displays high stability in acidic aqueous media, although its decomposition is strongly accelerated at increased hydroxide concentrations [97, 98], Thus, under basic aqueous conditions MTO decomposes as shown in Equation (4). 

The P450 reaction cycle (Scheme 10.4) starts with four stable intermediates that have been characterized by spectroscopic methods. The resting state of the enzyme is a six-coordinate, low-spin ferric state (complex I) with water (or hydroxide) coordinated trans to the cysteinate ligand. The spin state of the iron changes to high-spin upon substrate binding and results in a five-coordinate ferric ion (com-... 

Six-coordination is obtained in [PtMe3(acac)]2 by bidentate (0,0 ) behaviour and by a bond to the 7-carbon, a situation maintained in solution at room temperature (on warming, the bond to the 7-carbon breaks). In the bipyridyl adduct, it is the bond to the 7-carbon that completes the octahedral coordination [192],... 

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