Square pyramidal electronic spectra

Cr(solv)Me4CyclamJ(BPh4)2 solv = MeCN, DMF Distorted square pyramidal, electronic spectra, 1 1 electrolytes 5... 

In their pursuit of modeling Type I copper proteins, Kitajima et al. reported112 a rare, tetrahedrally coordinated complex (105), which displayed an EPR spectrum consistent with the presence of the unpaired electron in the dz2 orbital.1 They also isolated a square-pyramidal DMF adduct (complex (106)). They were successful in providing structural proof of a copper(II) complex (trigonal pyramidal) with C6F5S -coordinated complex (107), with CuN3S chromo-phore.113 The X-ray analysis (poor data set) of a closely similar complex with Ph3CS as the... 

A Cu(II) complex with square-pyramidal geometry, and its 1H NMR spectrum are shown in Fig. 5.41. The relatively broad signals, in the range from 20.4 to —13 ppm, as typical for mononuclear copper(II) systems, are related to a correlation time of 4 x 10 11 s, which is the rotational time, as the electron relaxation time is about two orders of magnitude longer [119]. The reader is referred to Section 6.3.2 for a comparison with a dimeric copper species with similar ligands. 

The value of J is not known in the case of the copper-nickel derivative. However, the same line of discussion should hold for this derivative, whose spectrum is shown in Fig. 6.8B [29]. It is noteworthy that the R values of the protons of the copper domain are smaller in this case than in the cobalt case. This is probably due to a shorter electronic relaxation time of tetrahedral nickel(II) than tetrahedral cobalt(II) (see Section 3.3). In this respect it may be interesting to note that in the case of the cobalt-cobalt derivative we have a tetrahedral and a square pyramidal cobalt(II) ion [30,31]. The former has longer electronic relaxation times. Upon establishment of magnetic coupling, the electronic relaxation times of the tetrahedral cobalt(II) ion decrease and tend to reach the values of the square pyramidal cobalt(II) ion. This is evident by comparing the spectra of the protein containing only the tetrahedral cobalt(H) ion with those containing both ions (Fig. 6.9). 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

Another interesting square pyramidal complex is Co(dacoda)(H2O)10Z which is isomorphous to the nickel analogue. The latter has been reported to have an electronic spectrum similar to that of nickel carboxypeptidase. Although the same similarity does not hold for the cobalt complex, it can be used as an interesting model of cobalt enzymes which have one coordinated water molecule. 

A yellow form of the pentacyanocobaltate(II) ion has been observed in DMF solution. Various monomeric (NR4)3[Co(CN)5] complexes have been crystallized26 and a structural study on one shows the anion to possess a truly five-coordinate square-pyramidal geometry.27 Electron irradiation of solid K3[Co(CN)6] gives a presumed pentacyanocobaltate(II) ion,28 the visible spectrum of which is virtually identical with that of [Co(CN)5]3a ). The structural parallels between the green and yellow forms of [Co(CN)5]3 and isoelectronic [Co(CNR)5]2+ ions indicate that all the green forms observed both in the solid state and in solution are weakly coordinated in the sixth axial position. 

A safer procedure for the synthesis of Ta(NMe2)s has been described.81 Electron diffraction indicated a square pyramidal stereochemistry with Ta—N bond lengths of 1.937 (ax) and 2.040 (eq)A.82 Its solid-state electronic spectrum exhibited absorption bands assigned to N Tav ligand-to-metal charge transfer transitions.83... 

The structure of the cation IFj is similar—removal of one electron from IFj converts a lone electron pair into an unpaired electron. There will be less repulsion between a lone electron and bonding electron pairs in I-F bonds but the arrangement of atoms would remain square pyramidal. Thus, the F-NMR spectrum of IFs" contains two resonances—one is doublet for four basal F atoms coupled to the axial F, and another one is quintet for the axial F atom coupled to the four basal F atoms. 

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