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 solvent 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 establish Acceptor Numbers (ANM,NM) for cations, Mn+, dissolved in solvents of low donicity. Nitromethane (NM) has been used as a reference solvent because it is both a very weak donor and a relatively weak acceptor solvent. (The solvent must exhibit some donor and acceptor properties in order to ensure solubility of the cationic species and the Fe(phen)2.(CN)2 indicator.) Apparent Acceptor Numbers (ANM,solv) for cations dissolved in various solvents other than nitromethane are also given and used to show that the acceptor properties of cations vary 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),(CN)2 used as the Lewis-acid indicator). Results are compared with the spectroscopic shift of the analogous complex Ru(phen)2(CN)2. The relationship between the proposed AN scale for cations and other practical and theoretical measurements of the Lewis acidity of cations is discussed.