Octahedral geometry examples

In a symmetrical octahedral system such as SFg, each polar S—F bond has a counterpart pointing in the opposite direction. The bond polarities cancel in pairs, leaving this molecule without a dipole moment. Example examines molecular variations on octahedral geometry. 

A molecule with a steric number of 6 requires six hybrid orbitals arranged in octahedral geometry. In Chapter 9, sulfur hexafluoride appears as the primary example of a molecule with a steric number of 6 (Figure ). Six equivalent orbitals for sulfur can be constmcted for the inner sulfur atom by combining the 3. S, the three 3 p,... 

To this point the complexes considered have shared the coordination number six and approximate octahedral geometry. It has been argued that they also share the dissociative reaction mode. There are examples of reactions both with and without intermediates of reduced (that is, 5) coordination, but the insensitivity to entering ligands is a consistent feature. It will be interesting, shortly, to see if the dissociative pattern persists in more or less organometallic octahedral systems but first we shall give some attention to the non-labile square planar systems. 

Type II copper enzymes generally have more positive reduction potentials, weaker electronic absorption signals, and larger EPR hyperfine coupling constants. They adopt trigonal, square-planar, five-coordinate, or tetragonally distorted octahedral geometries. Usually, type II copper enzymes are involved in catalytic oxidations of substrate molecules and may be found in combination with both Type I and Type III copper centers. Laccase and ascorbate oxidase are typical examples. Information on these enzymes is found in Tables 5.1, 5.2, and 5.3. Superoxide dismutase, discussed in more detail below, contains a lone Type II copper center in each of two subunits of its quaternary structure. 

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. 

Another example (which has been already discussed in Chapter 2, Section 1.3.2) is offered by the Cu(II) complex with the oxathia crown ligand l-oxa-4,7-dithiacyclonane [Cu(9S20)2]2 +, whose Jahn-Teller distorted (axially elongated) octahedral geometry is illustrated in Figure 130.183... 

A further educational example is offered by the neutral semiquinonate complex [Cr(02C6Cl4)3] (or [Crm(Cl4sq)3]), the octahedral geometry of which is illustrated in Figure 34.56... 

Dithiolenes also form tris complexes of octahedral geometry. Limiting to M(dmit)3 complexes we can cite as a typical example the dianion [W(dmit)3]2-, the molecular structure of which is illustrated in Figure 44.81... 

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