Metal Complexes a s Color lndlcators for Solvenl Parameters To the Editor: The use of metal complexes as color indicators for solvent parameters was a subject of discussion in this Journal recently [1985,62,459]. In the paper i t was claimed that using Cu(tmen)(acac)C104 as a reference compound (where tmen = N,N,N',N'-tetramethylethylenediamine and acac = actylacetonate) a linear relationship between the wavenumber of the maximum absorption band in the visible region and the solvent donor number (DN) ( 1 ) exists. 1 wish to report that surh a linear relationship does not exist. In order to prove my point the optical absorption spectraof thenbovementioned campound wereobtnined ina number 01 purified sdventi. The ahiorption maximum in wavrnumher ;and the donor numhersare listed in the table.
Absorption Maxlmum of Cu(tmen)(acac)CIOl in VarfousSolvents Solvent
DN
i(ztl00cm-I)
1,2dichloroethane nilrobenzene ethylacetate wafer methanol tetrahydrofuran N,N-dimefhylformamide dimethylsulfoxide
0
18600 18800 17400 16900 16900
4.4 17.1 18 19 20
26.6 29.8
17300 16600 16300
An examination of the table shows clearlythe invalidity of the claimedrelationship if we compare the u valuesin 1,2,-dichloroethane (DN = 0) and nitrobenzene (DN = 4.4), in water (DN = 18)lmethanol (DN = 19) and tetrahydrofuran (DN = 20). I t is not surprisine not to find a linear relationship hetween the two baramkers. D N is based on SbCIb as a reference acid and the expected different solvation effects and steric factors between ShC16 and Cu(tmen)(acac)+ would make D N a poor parameter to measure the Lewis-basicity of the solvent molecules (2,3). On the other hand v is dependent on the strength of axial interaction, which in turn deuends on the Lewis basicitv of the solvent molecules and the strength of electrostatic Lteraction between the complex cation and the oerchlorate anion (4).For example, the electrostatic intcr*crion acting along the axial d&rion in 1 2 dichloroethane is responsible t i ~ the r smaller I , value as compared to that in nitrihenzene. The inclusion of pyridine in the correlation in the paper is not appropriate because the results of the ESR work carried out in my laboratory (5)show unequivocally the presence of a t least two species in the pyridine solution one of which is the normal 1:l Lewis acid-base adduct and the other with identity still unknown a t present.
904
Journal of Chemical Education
Lnerature Clted 1. Gutmsnn, V. The Donor~AcceptorApprooeh Lo Maleerrlor Intaroefiona: Plenum: New Yurk. 1978. 2. Lim,Y.Y.;Drsg0.R. S.Inorg. Chem. 1912,11,202. 2 Urn, Y Y.lnoigwic Chimico Acto 1984,86, L1. 4. Sane. K.: Fukuda,Y. Studies in Phwieol and Theor~licnlChemistry, 1982.27.251. 5. Lim,Y. Y.. unpublirhed cesu1ts.
Y. Y. Lim University of Malaya Kuala Lumpur 59100. Malaysia
To the Editoc One of the reasons why so great a number of empirical solvent parameters have been introduced, and still are being introduced, is that solvent-solute interactions are highly complex in nature even to the extent of making each situation a special case. Thus any chemical compound to be advocated as a standard has its advantages and disadvantages. For instance. in the emolovmeut of ionic metal com~lexes . " two limitations-in addition to the principal one concerning the hiehlv structured solvents-are to be noted. (1)i n i o w donor solvents the formation of ion pairs between the cationic metal complex and its counterion may mimic a solvent effect. (2) Solvents of high donor strength may compete with the ligands for coordination and give rise to ligand exchange processes. Actually, Lim's examples come from all three areas of limited validity. Moreover, the demonstration of the limits of a solvent parameter should not be mistaken for the physical sensibility of the latter. On the cnntrary, a knowledge of the limitations aids one to translate the correlations into physical models of solvent-solute interactions. Finally we would like to draw attention to a note recently published in Bull. Chem. Sac. J p n . (1987, 60, 2286) where just these features are under discussion. In this. the tetra~heuvlborate - . of the copper complex is introduced as solvent basicity indicator instead of the perchlorate so as to circumvent point 1 above. If, however, Lim's comment is virtuallv aimed a t an attack of the usefulness uf the donor numher; for waling iolvenr basicity, nu wurd needs to he added ru the t'(,llowiny reierences. References Hnhn.S.:Miller, W. M.; Liehtenfhaler, R. N.:Prausnitz, J. M. J. Solution Chem. 1985.14.
Schmid. R.: Sspunov. V. N. Non-lorn
Maria. P.:Gal,J.J.Phvs.Chem. 1985.89.1296. Pernmn. I. Purr rind Appl. ?hem 1986.58.1153.
R. W. Soukup R. Schmld nsfltLte of lnarganlc Cnem stry Techn cat Unlvers~ty01 V enna. A-1060 V enna Getremaemark1 9. Auslr s
