Molar volumes of metal complex ions in water. Part 1.—Hexaammine and trisdiamine complexes
Abstract
Partial molar volumes at infinite dilution V0 have been examined [ML6]z+ in water at 25 °C on the basis of the scaled particle theory (SPT), where M = Cr, Mn, Fe, Co, Ni, Rh or Ir; z= 2 or 3; L = NH3, ethane-1,2-diamine/2 (en/2), propane-1,2-diamine/2 (pn/2), cyclohexane-1,2-diamine (chxn/2), 2,2′-bipyridine/2 (bipy/2) or 1,10-phenanthroline/2 (phen/2). V0 decreases by 25 cm3 mol–1 as z increases from 2 to 3, keeping L and r(M–N) constant, where r(M–N) is the mean distance between metal and coordinated N atoms. Applying this difference to the Drude–Nernst equation Vel(the effect of z on the intrinsic volume Vint), is given as –5.0z2 cm3 mol–1. Linear relationships having the same slope were found between Vint(=V0+ 5.0z2) and r(M–N) for all groups of [ML6]z+ whose L are fixed. This shows that Vint of [ML6]z+ is the sum of the core volume Vint(core) which determines Vel and the volume of additional parts grown from the core Vint(add). The slope of Vintvs. r(M–N) gives Vint(core), and suggests that the nearest water molecules to the core contact at the face centres of the octahedral core formed by six N atoms.