Cobalt five-coordinate complexes

Azomethines bear a formal resemblance to azo compounds and many parallels exist in the coordination chemistry of the two series of compounds. Thus the bidentate azomethines (191) behave in a strictly comparable manner to the bidentate azo compounds (20) (Section 58.2.2.1). The isomeric, bidentate azomethines (192), however, form metal complexes which undergo very facile hydrolysis as a result of polarization of the azomethine linkage. The difference between the two types of complex is dramatically illustrated by the results of a study135 of metal complex formation by the bis(azomethine) (193). This cannot function as a tetradentate ligand for steric reasons and reacts with copper, nickel and cobalt halides in cold ethanolic solution to form the five-coordinate complexes (194). Crystallization of these products from ethanol gives the five-... 

Fitting of the data for pseudotetrahedral complexes shows that they have Ts of 10 s, whereas five-coordinate complexes have a shorter Tj, on the order of 10 s. The latter derivatives also have exchangeable protons that could correspond to a water molecule in the coordination sphere, whereas the former do not. The Tj values are thus proposed as indicators of the coordination number in low-symmetry, four- and five-coordinate cobalt complexes. The shorter electronic relaxation times are related to low-lying excited states, which, independently of the particular mechanism, favor electron relaxation. " ... 

The proposed square-planar geometry of the cobalt centers is supported by X-ray absorption spectroscopy. The spectra for the immobilized cobalt sites in P-22 are consistent with cobalt centers that are centrosymmetric, which is indicative of a square-planar coordination geometry. The lose of centrosymmetry in P-22(NO) is caused by the formation of a five-coordinate complex in the presence of NO. 

Low-valent cobalt [Co, Co° and Co" ] are known to complex with unconjugated dienes (e.g., 1,5-COD, NBD, etc.) and acetylenes. The Co or Co " precatalyst is presumably reduced by EtjAlCl to either Co° or Co. Co, which has the electronic configuration [Ar]3d and commonly forms five-coordinate complexes in a trigonal bipyramidal form. Thus, although very little is known about the structure of the catalytic intermediates, an intermediate with structures Ij and/or seems reasonable (Fig. 2). 

Acetonitrile solvent exchange at the five-coordinate complex ions [ML(AN)] (with M = Co or Ni and L = l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclote-tradecane) appears to follow an la mechanism, with large, negative entropies of activation (-69.5 and —47.8 J K" mol" respectively) and low enthalpies of activation (19.5 and 20.3 kJ mol" ). Interestingly, the nickel complexes [NiL(AN)] and [NiL(OH2)] are significantly more labile than the cobalt(II) analogs. 

One possibility is a limiting SN1 mechanism with a five-coordinate cobalt complex as an intermediate,... 

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

These complexes may be either six-coordinate with an octahedral configuration or five-coordinate with a square-pyramidal configuration, in which the organo ligand occupies the apical position a few form dimers through the interaction of each cobalt with a coordinated atom of the equatorial ligand of the other half (see Section In virtually all groups... 

Ethanol-dimethoxypropane solutions of either 1-formylisoquinoline or 2-formylquinoline thiosemicarbazone and cobalt(II) salts yield [Co(L)A2] complexes where A = Cl, Br, I, NO3, NCS, or NCSe [147]. All are non-electrolytes, have magnetic moments of 4.30-4.70 B.M. and are five coordinate with approximate trigonal bipyramidal stereochemistry involving NNS coordination based on electronic and infrared spectra. [Co(21-H)2] 2H2O was isolated from a cold methanolic solution of cobalt(II) chloride and 1-formylisoquinoline thiosemicarbazone [187]. Infrared spectral studies show NNS coordination the electronic spectral bands fit a distorted octahedral symmetry, and the magnetic moment is 4.48 B.M. 

The electronic spectra and magnetic susceptibility of [Ni(21)Cl2] were found to be consistent with a five-coordinate high spin complex. This complex has greater antitumor activity against P388 lymphocytic leukemia test system in mice than cobalt(II), copper(II), zinc(II) and platinum(II) complexes of 21 [187],... 

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