Synthesis, stability and structure of the complex of bismuth(III) with the nitrogen-donor macrocycle 1,4,7,10-tetraazacyclododecane. The role of the lone pair on bismuth(III) and lead(II) in determining co-ordination geometry

The complex [Bi(cyclen)(H2O)(ClO4)(3)] (cyclen = 1,4,7,10-tetraazacyclododecane) has been synthesized and its crystal structure determined, The Bi has square-antiprismatic co;ordination to the four nitrogens of cyclen (mean Bi-N 2.39 Angstrom) with the upper square being provided by one oxygen from each of the three perchlorates and a water molecule (mean Bi-O 2.77 Angstrom). The formation constant (log K-1) of Bi-III with cyclen has been determined as 23.45 at I = 0.5 mol dm(3) and 25 degrees C by differential pulse polarography in an out-of-cell technique, where equilibration was slow and required 6 months. This is a high log K-1 value for a complex with cyclen, in accord with predictions made of the high affinity that Bi-III would have for polyamines on the basis of a model of acid-base interactions in aqueous solution. The structures of some complexes of Bi-III and the isoelectronic Pb-II were analysed in terms of the effect that the lone pair of electrons has on the M-L bond length (M = Bi or Pb, L = ligand donor atom). A relationship was found between the M-L bond length and the distance of L from the lone pair as quantified by the 1.p.-M-L (1.p. = lone pair) angle, with M-L decreasing as 1.p.-M-L increases. This observation is rationalized in terms of an analogy with the linear co-ordination geometry found in many complexes of Hg-II, where the lone pair on Bi-III or Pb-II occupies one of the two co-ordination sites in the analogous linear two-co-ordinate mercury(II) complex.