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 V_el(the effect of z on the intrinsic volume V_int), is given as –5.0z² cm³ mol^–1. Linear relationships having the same slope were found between V_int(=V⁰+ 5.0z²) and r(M–N) for all groups of [ML₆]^z+ whose L are fixed. This shows that V_int of [ML₆]^z+ is the sum of the core volume V_int(core) which determines V_el and the volume of additional parts grown from the core V_int(add). The slope of V_intvs. r(M–N) gives V_int(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.