Three series of N-heterocyclic dicationic ligands, [R(CH2)"R]2+, [R(o-xyl)R]2+, and [R(p-xyl)R]2+, where R = pyrazine (pyz, = 3-8), 4,4'-bipyridine (bpy, = 1-8), and trans-1,2-bis(4-pyridyl)ethylene (bpe, -1-8), have been synthesized by the reaction of the N-heterocycle with the appropriate , -dibromoalkane, , '-dibromo-oxylene, or , '-dibromo-p-xylene. In addition, ligands with R = 3-methyl-and 3-aminopyrazine, in which the free N donor atom is sterically hindered, have been prepared. These ligands react with the aquapentacyanoferrate(II) ion, [Fe(CN)50H2]1 23", to form monomeric, [Fe(CN)5L]-, and bridged dimeric, [(NQsFeOt-LJFefCN^]4-, complexes in aqueous solution. The rate and activation parameters for the aqua substitution and ligand dissociation reactions of these complexes have been measured and are found to be consistent with dissociative mechanisms of ligand substitution. The dependence of the rate constants for the formation of [Fe(CN)5(R(CH2)"R)]-on the polymethylene chain length n decreases in the order R = pyz > bpy > bpe, as the donor nitrogen atom is further removed from the site of the positive charges. Saturation kinetics are observed in the formation reactions with the dicationic ligands, consistent with an ion-pair dissociative mechanism, with [Fe(CN)5OH2||L]~ion-pair constants in the range 40-100 M-1. The rate constants for the rate-determining loss of the coordinated aqua ligand decrease from approximately 300 to 90-130 s-1 upon ion-pairing with the dications. The presence of the amino or methyl substituents ortho to the nitrogen coordinated to the [Fe(CN)s]3-center results in ligand dissociation rate constants which are 103 to 104 56789greater than for ligands in which these substituents are in meta positions or are absent. Steric strain energies of 17 and 22 kJ mol-1 have been determined for the aminoand methyl-substituted pyrazinium ligands, respectively.

This paper is dedicated to Professor Ronald J. Gillespie on the occasion of his 65th birthday DONAL HUGH MACARTNEY and LAUREN JEAN WARRACK. Can. J. Chem. 67, 1774 (1989). Kinetic and spectroscopic studies have been carried out in aqueous solution on the formation (from F~( c N )~O H~' -) and dissociation of pentacyanoferrate(I1) complexes containing 1-(4-pyridy1)pyridinium and the neutral, protonated, and N-methylated forms of 4,4'-bipyridine (BPY), 1,2-bis(4-pyridy1)ethane (BPA), and trans-l,2-bis(4-pyridy1)ethylene (BPE). The pH dependences of the formation kinetics have been analyzed in terms of the specific rate and acid dissociation constants for these ligands. The rate constants (25.0°C, I = 0.10 M) for the formation of the dinuclear complexes ( N C )~F~L F~( C N )~~-have been determined for BPY (50 M-' s-I), BPA (66 M-' s-), BPE (95 M-' s-I), and pyrazine (10 M-I S-I), and are compared with the respective rate constants for the formation of (NC)5FeLCo(NH3)5. The relationships between the formation rate constants and the size of the ligand, the number of donor sites, and the magnitude and position of charges on the ligand are discussed in terms of an ion-pair dissociative mechanism.