Octahedral species

There are two types of stereoisomerism associated with octahedral species. In EX2Y4, the X groups may be mutually cis or trans as shown for [SnF4Me2) (2.34 and 2.35). In the solid state structure of [NH4]2[SnF4Me2], the anion is present as the /rans -isomer. 

If two species have the same molecular formula and the same atom connectivity, but differ in the spatial arrangement of different atoms or groups about a central atom or a double bond, then the compounds are stereoisomers. 

Stereoisomers fall into two categories, diastereoisomers and enantiomers. Diastereoisomers are stereoisomers that are not mirror-images of one another. Enantiomers are stereoisomers that are mirror-images of one another. In this section, we shall only be concerned with diastereoisomers. We return to enantiomers in Sections 4.8 and 20.8. 

In a square planar species such as [IC ] or [PtC ] (2.30), the four Cl atoms are equivalent. Similarly, in [PtCl3(PMe3)] (2.31), there is only one possible arrangement of the groups around the square planar Pt(II) centre. (The use of arrows or lines to depict bonds in coordination compounds is discussed in Section 7.11.) 

In this section we discuss geometrical isomerism. Examples are taken from both p- and J-block chemistry. Other types of isomerism are described in Section 19.8. 

If two species have the same molecular formulae and the same structural framework, but differ in the spatial 

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Octahedral Having the symmetry of a regular octahedron. In an octahedral species, a central atom is surrounded by six other atoms, one above, one below, and four at the comers of a square, 176 complex in transitional metals, 418-420 geometric isomerism, 415 Octane number, 584... 

The successful rationalization of these transition-metal inverse spinel structures in terms of the relative LFSE s of tetrahedral and octahedral sites is another attractive vindication of ligand-field theory as applied to structure and thermodynamic properties. Once again, however, we must be very careful not to extrapolate this success. Thus, we have a clear prediction that LSFE contributions favour tetrahedral over octahedral coordination, except for d" with n = 0, 5 or 10. We do not expect to rationalize the relative paucity of tetrahedral nickel(ii) species relative to octahedral ones on this basis, however. Many factors contribute to this, the most obvious and important one being the greater stabilization engendered by the formation of six bonds in octahedral species relative to only four bonds in tetrahedral ones. Compared with that, the differences in LSFE s is small beer. Why , one asks, was our rationalization of spinel structures so successful when we neglected to include consideration of the bond count The answer is that cancellations within the extended lattice of the spinels tend to diminish the importance of this term. 

Substituted thioureas have been extensively studied over the decades. Reaction of CoX2 (X = C1, Br) with substituted phenylthioureas yield a range of complexes involving halide and thiourea as ligands, characterized by spectroscopy and thermogravimetric analysis.503 Both [Co(Rtu)4(OH2)2]2+ (Rtu = thiourea, phenylthiourea, allylthiourea) and [Co(Rtu)2(OH2)4]2+ (Rtu = diphenylthiourea) have been prepared and characterized as low-spin octahedral species.504 The octahedral bis(phenylthiourea)bis(dithiolate)cobalt(II) complex, one of a number of complexes of phenylthiourea, chlorophenylthiourea and bis(diphenylphospinothioyl)methane prepared and characterized,505 proved the most biologically active of those tested. 

Square planar Ni11 complexes with saturated macrocyclic ligands usually have Ni—N bond distances ranging from 1.90 A to 1.95 A, depending on the type of ligand. The Ni—N bond distances increase when square planar Ni11 complexes bind axial ligands to form octahedral species.91... 

The distorted octahedral species [10] and [11] are the essential part of the cytochromes acting as redox catalysts. In these, a very specific porphyrin redox potential may have been adjusted by an appropriate choice of the axial ligands exerting a cis effect on the porphyrin system transmitted through the iron atom. In cytochrome c, these axial ligands are the imidazole of a histidine and the thioether function of methionine, as in [10], and in the cytochromes a or b5 they are presumably two imidazoles of histidine, as in [77] (7). 

A variety of geometries have been established with Co(II). The interconversion of tetrahedral and octahedral species has been studied in nonaqueous solution (Sec. 7.2.4). The low spin, high spin equilibrium observed in a small number of cobalt(Il) complexes is rapidly attained (relaxation times < ns) (Sec. 7.3). The six-coordinated solvated cobalt(ll) species has been established in a number of solvents and kinetic parameters for solvent(S) exchange with Co(S)6 indicate an mechanism (Tables 4.1-4.4). The volumes of activation for Co " complexing with a variety of neutral ligands in aqueous solution are in the range h-4 to + 1 cm mol, reemphasizing an mechanism. 

The rapid interconversion between tetrahedral and planar geometries, between cis- and traws -planar forms and between planar and octahedral species has been studied using Ni(II) complexes (Secs. 7.2.1-7.2.3). Chelates can be resolved into optical forms and their racemiza-tion studied. (Sec. 7.6.1). 

The hw-amine adducts characterized are green, paramagnetic, irans-octahedral species whereas the nature of the mono-amine adducts has been more controversial 143,147,148,152,158) stabihty constant measurements I44,i53,i56,is8) foj. above reactions have shown that for pyridine and primary amines... 

Table 2. Configurational effects in ligand loss from low-spin octahedral species...

The extension to other cases is straightforward but tedious, and the principal results for low-spin octahedral species are summarised in Table 2, which shows some interesting features. At this level of discussion, R loss is never assisted. The question of demotion only arises where the t2g subshell is less than full. Two-electron demotion is is only possible for R loss from cf , cf, and systems, and in all of these it is actually term-term assisted. R" loss is assisted by the demotion of one electron fotd, d, or d curves, but among these it is only term-term assisted for d. R loss is clearly assisted by the demotion of a single electron for all d" (n < 6), but is only term-term assisted for n = 1 and n = 2. (These predictions are quite different from those of Ref. which refers exclusively to second order terms in R loss). The only configurations with n < 6 for which no process shows first order term-term assistance are d and d. This is a gratifying result and tends to promote confidence in the usefulness of the theory. The relative ease of preparation of Cr(III) alkyl complexes has often been noted and t/ is exemplified by the Co(IV) alkyls now known to be accessible by electrochemical oxidation of Co(III) Presumably a parallel chemistry of Fe(III) awaits discovery. 

Macrocyclic complexes (continued) nickel(II), 44 93-94 eatalysis, 44 119-125 configurational isomerization, 44 126 electrochemical properties, 44 112-113 electronic absorption spectra, 44 108-112 reactions, 44 118-119 square-planar and octahedral species, 44 116-118... 

SlOO proteins, calcium binding, 46 451-456 Spruhtrocken process, 4 26 Square-planar complexes, 4 157-164 octahedral, compared, 4 162-174 in solution, 34 270-271 Square-planar iridium complexes, 44 295, 297 Square-planar nickel macrocyclic complexes equilibrium with octahedral species, 44 116-118... 

Of the Ru(IV) complexes recorded here most are mono-oxo species which, despite the strong axial distortion brought about by the terminal oxo ligand, are probably all paramagnetic. Semi-empirical molecular orbital calculations (INDO/1) for epoxidations effected by oxo-Ru(IV) complexes have been reported (a non-concerted [1 h- 2] pathway was preferred) [642], [643] and for alcohol oxidations by octahedral species containing an Ru" (0) unit [644]. The reactivity of high oxidation-state polypyridyl complexes of osmium and Ru, with particular emphasis on Ru(IV) and Os(IV) oxo species, has been reviewed [43]. 

These data provide some qualitative guides to the kinetic behavior of octahedral species. If the change in the d-electron stabilization energy (i.e., the CFAE) is negative for a particular mechanism, the reaction is favored and the complex should be relatively labile—i.e., the substitution process should occur easily. Conversely, if the CFAE is positive, the reaction is disfavored and the complex should be relatively kinetically inert. 

E. Equilibrium Between Square-Planar and Octahedral Species in Coordinating Solvent... 

S) such as water, acetonitrile, or Me2SO (7, 10, 12-14, 16, 65, 66). Interconversion between these two forms depends on the ligand structure and reflects a balance between endothermic solvent coordination and exothermic Ni—N bond lengthening in octahedral species. 

The thermodynamic data of Eq. (11) for the various Ni(II) macrocyclic complexes are summarized in Table IV. Since the octahedral species should have longer Ni—N bond distances than the square-planar species, the complexes with flexible macrocyclic ligands have larger values. For example, the formation constants of octahedral species for... 

It has been reported that interconversion of square-planar to octahedral species for the Ni(II) complexes of the lipophilic macrocycle L-a... 

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