Partial Molal Volumes and Volume Changes for Complex Formation of [M(phen)3]Cl2 in Aqueous Solutions

Abstract

The partial molal volumes, \barV°, of [M(phen)3]Cl2 (M=Fe(II), Co(II), Ni(II), Cu(II), and Zn(II)) and 1,10-phenanthroline were determined from density measurements at 25 °C. From the values for \barV° of [M(phen)3]2+, M2+ and phen the volume changes for complex formation of [M(phen)3]2+, Δ\barV°, were caluculated. Values for \barV° of [M(phen)3]Cl2 varied from 445.1 cm3 mol−1 for [Fe(phen)3]Cl2 to 458.1 cm3 mol−1 for [Zn(phen)3]Cl2 and were in the order, [Fe(phen)3]2+<[Co(phen)3]2+>[Ni(phen)3]2+<[Cu(phen)3]2+<[Zn(phen)3]2+. The Δ\barV° of [Fe(phen)3]2+ was significantly small, 4.7 cm3 mol−1. While those of other metal chelate ions were similar values, ca. 20 cm3 mol−1. The smaller Δ\barV° of [Fe(phen)3]2+ is considered to be due to the spin change from the high-spin aqua ion, [Fe(OH2)6]2+, to the low-spin complex ion, [Fe(phen)3]2+.

Separation of Intrinsic and Electrostrictive Volume Effects in Redox Reaction Volumes of Metal Complexes Measured Using High-Pressure Cyclic Staircase Voltammetry

“…Partial molal volumes reported 15 for the tris(phenanthroline) compounds of Fe(II), Co(II), and Ni(II) show a clear empirical correlation between metal-ligand bond length and V°. Molal volumes for the chloride salts 15 and average metal-ligand bond lengths, d M-N , for Fe(phen) 3 2+ , 36 Ni(phen) 3 2+ , 37 and Co(phen) 3 2+ 38 cations are summarized in Table 3 and presented in Figure 3. The volume contributions from the chloride counterion will be constant.…”

“…For larger coordination complexes, such estimates of “effective radius” are inadequate. For example, it is well established that calculations of the intrinsic volume based on a sphere, using the furthest atomic position from the center of the ion as the radius, are clearly overestimates. , More sophisticated approaches based on a van der Waals volume or a solvent-excluded volume determined from the known structure of the compound have been developed. , …”

“…For example, it is well established that calculations of the intrinsic volume based on a sphere, using the furthest atomic position from the center of the ion as the radius, are clearly overestimates. 15,16 More sophisticated approaches based on a van der Waals volume or a solvent-excluded volume determined from the known structure of the compound have been developed. 17,18 We have also set out to investigate the effects of ligand type and complex-ion size on electrostrictive volume changes in a more detailed manner than in the charge dependence study reported previously.…”

Partial molar volumes for inorganic complexes

“…Table 2 [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] lists values for the partial molar volumes of a range of binary (homoleptic) complexes ML nAE 6 and MðLLÞ nAE 3 , where L and LL are mono-and bidentate ligands, respectively. The partial molar volume of CrðNH 3 Þ 3þ 6 is larger than that of CrðOH 2 Þ 3þ 6 (V i ¼ 57.0 cm 3 mol À1 ) [27], in contrast to À53.0 cm 3 mol À1 for Cr 3þ ( similarly has a larger value (127 cm 3 mol À1 [27,29] than CoðenÞ 3þ 3 , whose recommended value (V i ¼ 121 cm 3 mol À1 ) is based on data in references [25,27,29,38] [38,40,41], although the value reported for Znð phenÞ 2þ 3 is, unexpectedly, not smaller than that for Cuð phenÞ 2þ 3 [40]. The partial molar volumes for a series of tris-diimine-iron(II) cations cover the range Values are from references [4,5], converted to V i (H þ ) ¼ À4.5 cm 3 mol À1 , unless otherwise indicated.…”

“…Such is believed to be the case for the large and predominantly hydrophobic organic ions BPh À 4 , Ph 4 P þ , Ph 4 As þ , and Bu 3 N(CH 2 ) 8 NBu 2þ 3 , which are 263.5, 287.7, 296.2, and 520 cm 3 mol À1 [70], respectively. The large and hydrophobic iron(II)-diimine complex FeðMe 2 bsbÞ 2þ 3 (V i ¼ 605 cm 3 mol À1 [42]), probably aquacobalamin and methylcobalamin (V i ¼ 918 and 940 cm 3 mol À1 [71]) and Mð phenÞ 2þ 3 complexes (M ¼ Fe, Co, Ni, Cu, Zn; V i ¼ 400-415 cm 3 mol À1 [40,41]), and perhaps the tetranuclear cobalt(III) complex [Co{(OH) 2 Co(en) 2 } 3 ] 6þ , V i ¼ 304 cm 3 mol À1 [25], also fall into this category. The pyrazolylborate complex FefHBð pzÞ 3 g 2þ 2 has V i ¼ 341 cm 3 mol À1 , whereas in the solid state V 0 ¼ 337 cm 3 mol À1 [72]; this complex, together with Al 13 O 40 H 7þ 48 {V i ¼ $460 cm 3 mol À1 [73]} provide examples of inorganic species whose partial molar and intrinsic volumes are essentially equal.…”