Cobalt complexes, hexacoordinate

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

At the same time, according to x-ray data for zinc chelate 909, the nitrogen atom is turned to the side of the metal. The distance Npy-Zn is 2.80 A, that allows us to consider the possible participation of the examined donor center in binding with the metal, leading to formation of a hexacoordinated structure (two-capped tetrahedron) [243]. In relation with this result, let s pay attention to the data reported in Refs. 244 and 248. The tetrahedral configuration without coordination of the nitrogen atom of pyridine is attributed to the cobalt complex 907 (X = NTs, M — Co), although this N atom is rotated to the side of the metal [244]. The pentacoordinated complex 910 is described in Ref. 248, in which only one pyridine substituent is coordinated (the distance Npy-Co is 2.45 A) ... 

Upon adsorption of excess ammonia in a Co(II)Y zeolite a white, high-spin cobalt(Il)-ammonia complex with a spin configuration of (fe )5-(eg)2 is formed. According to studies of cobalt (II) complexes in solutions, salts, and in zeolites, a hexacoordinate Co(II)-ammonia complex is the most likely form when an excess of ammonia is present (3, 4> ) Indeed,... 

We may conclude that the divalent cobalt ions move out into the large cavities upon adsorption of NH3 to form a hexacoordinate cobalt(II)-ammonia complex. Following adsorption of 02 in the ammoniated Co(II)Y zeolites, oxygen enters the coordination sphere of the Co2+ ions. This is accompanied by a charge-transfer process to form a [Co(III) (NH3)502 ]2+ complex. The general intermolecular redox process can be approximated by the reactions... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

The electronic ground state that a particular metal center adopts is a function of the chromophore. In many cases the ground state may be derived from chemical knowledge (e. g., octahedral cobalt(HI) ( Aig) or tetrahedral Ni(II) (3T() complexes). However, based on the molecular mechanics formalism alone, this problem cannot be solved in a general way. Let us consider coordination compounds which are close to the spin-crossover limit (for example hexacoordinate iron(II) (1Aig/5T2g)). In these cases it is not possible to assign the atom type of the metal center without further information (experimental or theoretical). Therefore, molecular mechanics alone is not always able to predict the structural properties. 

The stabilizing effect of an axial ligand has been previously observed in the synthesis of cobalt corrolates. Such an effect has been used to synthesize the complex where no peripheral p substituents are present on the macrocycle, which decomposes if attempts are made to isolate it in the absence of triphenyl-phosphine [10]. The behavior of rhodium closely resembled that of cobalt and it seems to be even more sensitive to the presence of axial ligands. [Rh(CO)2Cl]2 has also used as a metal carrier with such a starting material a hexacoordinated derivative has been isolated. The reaction follows a pathway similar to that observed for rhodium porphyrinates the first product is a Rh+ complex which is then oxidized to a Rh3+ derivative [29]. 

Jcfrgensen (20) has analyzed a number of spectra of hexacoordinated transition metal ions in relation to pressure. In K20sBrg the peaks at 22.200 cm and at 16.750 cm are shifted distinctly to lower energy with increasing pressure at the same time the spUtting increases drastically. Spectroscopic changes occur as a result of the pressure induced transformation of tetrahedral cobalt(U)-complexes in solution into octahedral arrangements (21, 22), and indeed are used as evidence for this transformation. 

Figure 12.5 Plots of the experimentally determined structures of the hexacoordinate, s = 0, (a) and pentacoordinate, s= 1, cobalt(lll) (b) complexes, an energy level diagram (c) based on DFTcalculations, the steric strain (d) and the HOMO-LUMO gap (e) as a function of the angle shown in (d) [474].

Cobalt(II) porphyrins can be tetra-, penta- or hexacoordinated, while the Co(III) complexes are generally 5- or 6-coordinate, depending on the solution conditions. Basolo and coworkers showed that the potential for oxidation for a Co(II) porphyrin or related macrocycle could be related linearly to the base strength of the bound axial ligand [305, 306], and a similar relationship has been observed between... 

Similar equilibria between tetra and hexacoordinated complexes exist also for the oxygen-free form of the Co-monoamine complexes whose spectra are dependent upon the amine concentration versus cobalt ions and upon the temperature (17). The hexacoordinated complexes formed in these systems are less stable than in the Co-BSB--amine system, and the quantitative approach to the problem is difficult because the system was investigated in a strong coordinating solvent as dimethylformamide. Also in this case dependence on steric hindrance was observed. 

In the former case, polymers were assembled by complexation of a pyridyl-substituted porphyrin with the hexacoordinate metal ion, cobalt(II). The resulting supramolecular polymers, (60) (Figure 37), were characterized by UV/vis spectroscopy, NMR diffusion studies, and SEC. Polymers with concentration-dependent molecular weights up to 136 kDa (DP 100) formed, as indicated by SEC data. The dynamic nature of polymer formation was implied in the concentration dependence of the molecular weights as well as in capping studies with a porphyrin chain stopper [122],... 

For the acid-independent and acid-dependent aquation pathways of [Co(NH3)sS04], the respective volumes of activation A Vo (at zero pressure) and AVh (pressure averaged over 100 MPa) are -18.3 and -3.5cm mor at 35°C and -19.7 and -3.9cm mor at 55° and / = 1.0 The temperature dependence of A Vo can be accounted for in terms of solvational change indicated by its pressure dependence. The work also questions the common supposition that the molar volumes V of the penta- and hexacoordinate ammine complexes of the same metal ion are equal and suggests that there is a difference of 17-20 cm moV for the cobalt(III) case. 

It is important to underline the fact that a tetrahedral complex, which would have the same global formula as the preceding one, cannot exhibit this isomerism. Hence, for example, the isolation of two isomers of the diamminedichloroplatinum(II) complex [Pt (Cl)2(NH3)2] has been a very strong argument in favor of its planar stmcture. The octahedral hexacoordinated complex dichlorobis(ethylenediamine)cobalt(III) [Co (Cl2)(en)2]+ exists in the cis and trans forms (Fig. 23.4). 

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