Hexaammine complexes

LCo(H20)6] ion, and bidentate /V-donor ligands such as cn, bipy and phen form octahedral cationic complexes [Co(L-L)3] , which are much more stable to oxidation than is the hexaammine [Co(NH3)6l . Acac yields the orange [Co(acac)2(H20)2] which has the tram octahedral structure and can be dehydrated to [Co(acac)2l which attains octahedral coordination by forming the tetrameric species shown in Fig. 26.3. This is comparable with the trimeric [Ni(acac>2]3 (p. 1157), like which it shows evidence of weak ferromagnetic interactions at very low temperatures. fCo(edta)(H20)] is ostensibly analogous to the 7-coordinate Mn and complexes with the same stoichiometry, but in fact the cobalt is only 6-coordinate, 1 of the oxygen atoms of the cdta being too far away from the cobalt (272 compared to 223 pm for the other edta donor atoms) to be considered as coordinated. 

Although somewhat more stable than its hexaammine relative, the air-sensitive [Co(en)3]2+ is still substitutionally labile and racemizes rapidly in solution. Chiral discrimination in its (racemic) solutions has been observed in outer sphere electron transfer reactions with optically active oxidants including [Coin(EDTA)], 209,210 [Cr(ox)3]3-,211,212 Co111 oxalate, malonate, and acetylacetonate (acac) complexes.213... 

However, there are a number of other miscellaneous biological roles played by this complex. The [Co(NH3)6]3+ ion has been shown to inhibit the hammerhead ribozyme by displacing a Mn2+ ion from the active site.576 However, [Co(NH3)6]3+ does not inhibit ribonuclease H (RNase),577 topoisomerase I,578 or hairpin ribozyme,579 which require activation by Mg2+ ions. The conclusions from these studies were that an outer sphere complex formation between the enzyme and Mgaq2+ is occuring rather than specific coordination of the divalent ion to the protein. These results are in contrast to DNase I inhibition by the same hexaammine complex. Inhibition of glucose-induced insulin secretion from pancreatic cells by [Co(NH3)6]3+ has been found.580 Intracellular injection of [Co(NH3)6]3+ into a neurone has been found to cause characteristic changes to the structure of its mitochondria, and this offers a simple technique to label neuronal profiles for examination of their ultrastructures.581... 

A series of 12 complexes of Co(III) with both ionic and covalent azide groups was prepared and most were easily detonable as dry salts, especially at elevated temperatures [1]. Polarography is an accurate and safe method of analysis for azides [2], Hexaammine-cobalt, -chromium and -rhodium hexaazidocobaltates are explosive, particularly in the dry state [3],... 

The H3NAIX3 molecules have been studied in the gas phase by IR/Raman spectroscopy.24 NCA yields Al—N force constants of 1.50N cm-1 (X = C1) and 1.45 N cm-1 (X = Br). The analyses of spectra were supported by ab initio MO calculations which were also extended to H3NAIF3, a molecule which has eluded synthesis. The ammonia adduct of alane (A1H3) is unknown. A hexaammine A1(BH4)3-6NH3 is the product of excess ammonia on KA1(BH4)4.25 It is believed to contain the cation A1(NH3)1+ and it is possible that this species is also present in some of the complexes A1X3(NH3) , noted above, but structural investigation is required. 

The [Cr(en)3]2+ and [Cr(pn)3]2+ salts have reflectance spectra (Table 11) resembling those of the hexaammines, and the six N donor atoms are assumed to complete tetragonally distorted octahedra around the metal. Stability constant measurements (Table 39) have shown that the ions [Cr(en)(aq)]2+ (vmax= 18 300 cm-1, e = 25 dm3 mol-1 cm-1) and [Cr(en)2(aq)]2+ (vma = 17 500 cm-1, e = 17 dm3 mol-1 cm-1) exist in aqueous solution, but that, as in the copper(II) system, the third ethylenediamine molecule is only weakly bound, and care is needed to prevent loss of en from tris(amine) complexes in the preparations. Several bis(amine) complexes, e.g. [CrBr2(en)2], have been isolated, and these are assigned trans structures because of IR spectral resemblances to the corresponding oopper(II) complexes. Since the spectrum of [Cr(S04)(en)2] also shows the presence of bidentate sulfate, this is assigned a trans octahedral structure with bridging anions. 

A few calcium complexes having nitrogen ligands are known. Calcium halides form addition complexes with NH3 such as CaCl2wNH3 (n = 2,4,8) and CaBr2fiNH3 (n = 2,4,6,8). Hexaammine calcium [12133-31 -2], Ca(NH3)6, is formed by reaction of calcium metal and anhydrous NH3 (11) (see... 

In their first publication on this subject,59 Werner and Miolati showed that the molecular conductances (fx) of coordination compounds decreased as successive molecules of ammonia were replaced by acid residues (negative groups or anions). For cobalt(III) salts, they found that fi for luteo salts (hexaammines) > fi for purpureo salts (acidopentaammines) > /t for praseo salts (di-acidotetraammines). The conductance fell almost to zero for the triacidotriammine Co(N02)3-(NH3)3 and then rose again for tetracidodiammines, in which the complex behaved as an anion. By such measurements, Werner and Miolati determined the number of ions in complexes of cobalt(III), platinum(II) and platinum(IV). They not only found support for the coordination theory, but they also elucidated the process of dissociation of salts in aqueous solution and followed the progress of aquations. 

A multicomponent positive-imaging process using ammonia release has been described by Ricoh.211 The components of the system are (1) a cobalt(III) hexaammine complex, (2) a quinone photoreductant, (3) a chelating agent such as dimethylglyoxime, (4) a leuco dye (triarylmethane type), (5) a photooxidant (biimidazole) and (6) an organic acid (toluenesulfonic acid). 

Fe(NH3)6]Br2 When an excess of gaseous ammonia is passed through a freshly prepared solution of elemental iron in hydrobro-mic acid (40%), the bromide salt of the hexaammine iron(II) complex is deposited (86). The cation is also found in certain carbonyl ferrates, such as [Fe(NH3)6][Fe3(CO)u], which are obtained from the reaction of triiron dodecacarbonyl with ammonia in an autoclave (87). 

Prepared as described in reference 3, and recrystallized from dilute HC10 three times as discussed in reference 1. An equivalent amount of [Co(NH ) C1]C12 which has been allowed to aquate for 4 days at room temperature can be used, but lower yields are obtained. The purity of the initial pentaammine complex is important. Hexaammine impurities are carried through the reaction sequence. 

Upon contact of silica with a nickel nitrate ammo-niacal solution, one observes that the purple blue colour, characteristic of nickel hexaammine [Ni(NH3)6]2+, is transferred from the solution to the silica support, while the diffuse reflectance of the resulting wet powder is the same as that of the initial solution. These phenomena can be explained in terms of the following competitive cation exchange, where ammonium ions compete with nickel hexaammine complexes to occupy the =SiO exchange sites of silica ... 

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