The Enthalpy of Formation of Nitrosylpentaammineruthenium(II) from NO⁺_(aq) and Aquopentaammineruthenium(II)

James F. Wishart, Henry Taube, Kenneth J. Breslauer and Stephan S. Isied

Inorg. Chem. 25, 1479-1481 (1986)

Abstract:

An estimate of the enthalpy change associated with the substitution of H₂O on (NH₃)₅RuOH₂²⁺ with NO⁺(aq) has been made by thermochemical measurements on a cycle of reactions, which includes the reaction of (NH₃)₅RuOH₂²⁺ with NO₂^-(aq) and which involves the assumption that the heat of dissolution of NOBF₄(s) to produce NO⁺(aq) + BF₄^-(aq) is close to the heat of dissolution of CsBF₄(s). The chemistry is complicated because the reaction of (NH₃)₅RuOH₂²⁺ with NO₂^-(aq) ultimately produces trans-[(NH₃)₄Ru(OH)NO]²⁺(aq) rather than [(NH₃)₅RuNO]³⁺(aq). Reasonably good estimates of the enthalpy changes of the reactions needed for the cycle to close on [(NH₃)₅RuNO]³⁺ can be made, and when these are combined with the experimental results, they yield as (delta)H° for the reaction of interest -52 kcal mol^-1. We believe that this estimate is accurate to within 5 kcal mol^-1, and the conclusion that the reaction of NO⁺(aq) with Ru(II) is more exothermic than that of the isoelectronic CO(aq) (-38.2 ± 1.2 kcal mol^-1) seems safe. The assumption as to the heat of solution of NOBF₄(s) and CsBF₄(s) is validated by the finding that the enthalpy change for the formation of 2H⁺ + NO₂^-(aq) by the reaction of NO⁺(aq) with water, -10.6 ± 0.5 kcal mol^-1, which can be calculated from our results, lies between upper and lower limits set by the work of others.