Hexacoordinate geometry

Diaryltin dicupferronates have been isolated by the reactions of acetonic solutions of diaryltin dichloride with a solution of cupferron in the same solvent.123 On the basis of IR spectra, a hexacoordinate geometry is suggested for tin in these complexes. 

The molecular structure of Sip4 2py (59) shows a centrosymmetric octahedral geometry at silicon with the pyridine molecules trans to each other and an Si-N distance of 1.93 Hexacoordinate geometry may also occur when... 

A hexacoordinate geometry is observed for bis(phenylethynyl)magnesium. An X-ray determination of (Ph—C = C)2Mg(THF) demonstrates the presence of two isomeric compounds in which the organic groups are either trans or ds to each other [41]. The crystal structure of an analogous compound, (Ph—C = C)2Mg(TMEDA)2 [Compound 41],... 

Chirality is an important part of today s chemistry and, in this respect, the pseudo-octahedral geometry of hexacoordinated phosphorus derivatives is interesting as it suffices to coordinate to the central atom three identical sym-... 

With P-donors a variety of different structures are formed. Two complexes (359a,b) with a five-coordinate square pyramidal geometry and one S-donor in the apical position are reported.934,933 In a related complex (360) the nickel center reveals a distorted square planar geometry, because one xanthate ligand switched to a monodentate coordination mode. 6 By reaction of [NiL2] complexes with Ph2P(CH2)2P(Ph)(CH2)2PPh2, compound (362) is formed, which consists of a five-coordinate cation and a hexacoordinate anion. 7... 

Hexacoordinate, hypervalent sulfur fluorides have an octahedral geometry that is symmetrical for SF6, which appears as a sharp singlet at +56 ppm, but which has magnetically nonequivalent (axial and equatorial, ab4 system) fluorines for compounds of the structure R-SF5. Compounds with the general structure R-SF4-X can exist as cis- and trans-isomers, the former having three types of fluorine, and the latter only one (Scheme 7.17). 

In the case of neutral systems the geometry corresponds to a bicapped tetrahedron or, in other words, to a tetrahedron which undergoes two nucleophilic coordinations. They have been observed [5] to be stable in solution. A careful H variable temperature NMR study shows that these compounds undergo an intramolecular isomerization which takes place without cleavage of Si-N bonds. The AGt of this so-called "Baylar Twist" isomerization has been estimated at between 12 and 18 kcal/mole depending on the substituents at the silicon. We can conclude that this work supports strongly hexacoordinated silicon structures as possible intermediates in the usual course of nucleophilic reactions at silicon. 

More illustrative is the structure of the fluorosilane which exhibits the same tricapped tetrahedron geometry. The three N-Si interactions take place in the frontal position towards the Si-F bond instead of the rear coordination observed in penta- and hexacoordinated structures. Furthermore, the benzylamino groups have the possibility not to be coordinated at silicon since these groups have the capability of free rotation around the carbon-nitrogen bond. 

During the last 5 years, Inabe and co-workers [73-76] have used electrochemical oxidation to synthesize a series of partially oxidized ionic Pc compounds [PXX]n[MmPc(CN)2] (M = Fe or Co) (87-90) and [TPP][ComPc(X)2]2 (X = 0(91) or Br(92)) with two trans CN groups, Cl- or Br ions. The central metal ions all have distorted hexacoordinated octahedral geometries. The Pc skeletons in these complexes are nearly strictly planar and the central metal ions lie in the (Niso)4 planes. The M-C=N angles are almost linear and the axial CN groups, Cl- or Br ions are relatively small. Therefore, these Pc complexes always form ID columnlike or 2D sheet-like Pc stacks via various Pc n-n interactions. In addition, the planar... 

Ribozymes are a class of metallo-enzymes based on RNA rather than proteins. They have potential in clinical medicine, for example, as potential anti-HIV agents (568, 569) and as possible new tools for the treatment of cancer (570). The active structures of ribozymes contain domains of stacked helices which pack together through tertiary contacts. Divalent metal ions such as Mg(II), Zn(II), and Mn(II) can tune the reactivity and shape the structures of ribozymes (571). Manganese(II) and Mg(II) have similar hexacoordinate ionic radii (0.86 and 0.97 A, respectively) (572) and octahedral geometry ( )Ka of hydrates Ca(II), 12.7 Mg(II), 11.4 Mn(II), 10.7 Zn2+, 9.6) (571). There are several potential oxygen donors on the ribose sugar moiety. 

A related hexacoordinate macrocycle complex is the Fe(III) derivative with the scorpiand 6,13-diamino-6,13-dimethyl-1,4,8,11-tetraazacyclo-tetradecane (the traws-diammac ligand already discussed in Section 2), [Fe(tra s-diammac)]3 +. Its octahedral geometry is shown in Figure 67.103... 

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