Hexamines metal complexes

Nonbonded interactions are possible between atoms that are neither bound to one another (1,2 interactions) nor bound to a common atom (1,3 interactions). They include both van der Waals and electrostatic interactions. Nonbonded interactions with the metal are often omitted in calculations that have been performed with either the VFF or the POS approach. The reason for the omission is that the majority of metal complexes that have been modeled are coordinatively saturated. In these structures, the metal center is shielded from nonbonded interactions by a shell of donor atoms followed by a second shell of ligand atoms. It is not possible for the outer shells of ligand atoms, i.e., those with 1,4 or greater connection to the metal, to achieve close contacts with the metal center. It has been demonstrated that the inclusion of M-X van der Waals terms does not significantly alter calculated geometries or relative energies for different conformers of hexamine Co(III) and Cr(III) complexes and pentacoordinate nitrogen macrocyclic complexes of Co(III) and Cu(II). ... 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

The unsubstituted linear tri-, tetra-, penta-, and hexamines are known to coordinate nearly all metals. Sometimes they are used as additional neutral donors to saturate the coordination sphere of the central atom.28 Considerable interest in the design of such ligands is concentrated on the synthesis of new chelating donors able to form kinetically inert or thermodynamically stable complexes of transition and nontransition metals. The coordination ability of the amine can be... 

The value of e (under the conditions specified in Procedure) is 5.2 10" (a=1.9) at 545 nm. In the absence of acetate buffer, e values can be higher [16,17]. Hexamine buffer is recommended. The absorbance of the free reagent is insignificant at 545 nm. Solutions of the CAS-Al complex do not obey Beer s law. The intensity of the colour of the complex depends on the concentration of CAS. The colour weakens with increasing concentration of acetate. Alkali-metal salts affect the colour and its stability. To compensate for this salt effect, corresponding amounts of the salts should be added to the reference calibration solutions [19]. 

Figure 5.6 Infrared spectra of metal hexamine complexes. From Nakamoto, K., Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B, Applications in Coordination, Organometallic and Bioinorganic Chemistry, 5th Edn. Copyright (1997 John Wiley Sons, Inc.). This material is used by permission of John WUey Sons, Inc.

The spectra presented in Figure 5.6 show a trend in the wavenumber shifts for the three hexamine complexes the N H bands shift to lower wavenumbers from Co to Cr to Ni. This indicates that the N-H bond order (bond strength) decreases as the metal-N bond order increases in the stability order mentioned. 

Cobalt(II), like other transition metal ions, forms a stable complex with EDTA. The suitable pH for the titration, adjusted by hexamine, is pH6.0 and the indicator is xylenol orange. 

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