ESTIMATION OF THE ACCEPTOR NUMBERS OF CATIONS BY MEANS OF AN ACID-BASE INDICATOR

The shift of the visible charge-transfer absorption bands of Fe(phen)( 2)(CN)(2) in various solvents resulting from the interaction of solven t molecules or of cationic species with the free electron pair on the -CN group(s) has been investigated and the results have been used to e stablish Acceptor Numbers (AN(M,NM)) for cations, Mn+, dissolved in so lvents of low donicity. Nitromethane (NM) has been used as a reference solvent because it is both a very weak donor and a relatively weak ac ceptor solvent. (The solvent must exhibit some donor and acceptor prop erties in order to ensure solubility of the cationic species and the F e(phen)(2)(CN)(2) indicator.) Apparent Acceptor Numbers (AN(M,solv)) f or cations dissolved in various solvents other than nitromethane are a lso given and used to show that the acceptor properties of cations var y in a way directly related to the Donor Number of the solvent (i.e., a decrease in the effective Acceptor Number of the cations is observed with increasing DNSolv. This arises from competition between solvent and acceptor ion for the Fe(phen)(2)(CN)(2) used as the Lewis-acid ind icator). Results are compared with the spectroscopic shift of the anal ogous complex Ru(phen)(2)(CN)(2). The relationship between the propose d AN scale for cations and other practical and theoretical measurement s of the Lewis acidity of cations is discussed.