MOLAR VOLUMES OF METAL-COMPLEX IONS IN WATER .1. HEXAAMMINE AND TRISDIAMINE COMPLEXES

Partial molar volumes at infinite dilution V(O)BAR have been examined [ML6]z+ in water at 25-degrees-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). V(O)BAR 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)(= V(O)BAR + 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(int) vs. 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.