D. De Vito et al., Can octahedral t(2g)(6) complexes substitute associatively? The case of the isoelectronic ruthenium(II) and rhodium(III) hexaaquaions, INORG CHEM, 39(24), 2000, pp. 5547-5552
For the low-spin t(2g)(6) Ru(OH2)(6)(2+) (DeltaV(double dagger) = -0.4 cm(3
) mol(-1)) and Rh(OH2)(6)(3+) (Vi(double dagger) = -4.2 cm(3) mol(-1)) hexa
aquaions, the respective I-d and I-a water exchange mechanisms had been ass
igned, mainly on the basis of activation volumes DeltaV(double dagger) and
entering ligands effects for water substitution. For Ru(II) the near-zero D
eltaV(double dagger) was supposed to be due to the compensation between a p
ositive contribution (the loss of a water molecule) and a negative one (the
contraction of the bonds of the five spectator ligands at the transition s
tate). Recently, it has been suggested that Rh(III), because of its higher
positive charge, could promote further spectator ligands bond contraction s
ufficient to change the sign of DeltaV(double dagger) to a negative value.
If true, this would be an example of Limitation in the use of DeltaV(double
dagger) for a direct diagnosis of the mechanism. Quantum chemical calculat
ions including hydration effects show that the activation energies for the
water exchange on Rh(OH2)(6)(3+) via the I-a (114.8 kJ mol(-1)) and the D p
athways is 21.8 kJ mol(-1) in favor of the former. In the case of Ru(OH2)(6
)(2+) all attemps to compute a transition state for an interchange mechanis
m failed, but the calculated DeltaE(double dagger) for the D mechanism (71.
9 kJ mol(-1)) is close to both experimental DeltaG(298)(double dagger) and
DeltaH(298)(double dagger) values. The calculated Delta Sigmad(M-O) values
of -0.53 Angstrom for rhodium(III) and +1.25 Angstrom for ruthenium(II) agr
ee with the experimented DeltaV(double dagger) values and suggest I-a and D
(or I-d) mechanisms, respectively. In the case of Ru(OH2)(6)(2+) the short
ening of the bonds of the five spectator ligands to reach the transition st
ates corresponds to a volume change of -1.7 cm(3) mol(-1). For Rh(OH2)(6)(3
+) these spectator ligands' volume decrease is much smaller (maximum of -0.
8 cm3 mol(-1)) and the bond lengths of the two exchanging ligands at the tr
ansition state are characteristic of an interchange pathway with a small "a
" character. Because of the strong Rh-III-O bonds, water exchange on Rh(OH2
)(6)(3+) proceeds via the I-a pathway with retention of the configuration,
whereas the same reaction of Ru(OH2)(6)(2+), which has considerably weaker
Ru-II-O bonds, follows the Icl or the D mechanism.