Thermal Decomposition of First-Generation Systems

The methylidene complex RuCl2(=CH2)(PCy3)2 (19) is much shorter lived than 14, with a half-life of 40 min in CgDg solutions (0.023 M) at 55 C [31]. The decomposition rate of 19 was found to be independent of the concentration of free PCy3 and follow first-order kinetics [31, 35]. The thermal decomposition 

The decomposition of the deuterium-labeled analog of 19, RuCl2(=CD2) (PCy3)2, led to the observation of a broad signal at 2.5 ppm in the H-NMR spectrum, suggesting that the decomposition of these methylidene complexes may involve the activation of phosphine C-H bonds [31]. This presumed phosphine activation could be a step in the primary decomposition pathway - a unimolecular process that includes the attack of PCyg on the methylidene - or in a secondary decomposition route. For instance, it is conceivable that some secondary decomposition of methylidene complexes may occur via a bimolecular process. 

For example, the formation of a ruthenium-ethylene complex was observed during the attempted preparation of a monophosphine bimetallic species by reaction of RuCl2(=CH2)(PCy3)2 (19) with [Ru(p-cymene)Cl2]2 [31, 37]. 

A similar decomposition process has been observed for a second-generation ruthenacyclobutane intermediate, which will be discussed in Section 11.3.2 [40]. 

The degree to which this decomposition pathway is operating in complex 19 under ethylene is unclear. In addition, the unimolecular decomposition route involving phosphine attack on the methylidene carbon seems to also be abundant when a reaction is conducted in the presence of ethylene [2]. Interestingly, studies 

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