Strong thermochromic and solvatochromic effects of Ni(II) and Cu(II) complexes in DMF/nitromethane mixtures: A probe for investigation of binary solvents.

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Thermochromic and solvatochromic equilibria involving the ions [Ni(tmen)(acac)]<SUP>+</SUP> and [Cu(tmen)(acac)]<SUP>+</SUP> respectively are highly sensitive to the polarity of the solvent. These probes are thus suited for the study of binary mixtures. In nitromethane (NM)/dimethylformamide (DMF) binary solvent the Ni(II) complex displays strong thermochromism in a zone of low DMF concentration. This result is surprising because neither of the two solvents are intrinsically thermochromic since NM is a weak donor while DMF is a strong donor. NM/DMF binary mixtures behave like solvents with intermediate donor properties. In the same solvent mixtures (low DMF concentration), the solvatochromic behavior of the Cu(II) complex is reflected by a large shift in the maximum absorption of more than 70 nm. This was accounted for by the considerable variation in donor power of the solvent mixture in this concentration range. Quantitative analysis of the thermochromic and solvatochromic equilibria as a function of DMF concentration in NM indicated the existence of self-associations of DMF. The relatively small effects of dilution of DMF on the donor properties of the solvent mixture was explained in terms of the discharge of free DMF from self-associations with increasing dilution. In DCE, DMF self-associates to a greater extent than in NM.
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