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Vacancies: Protonation of the ruthenium carbide compounds [Cl2(L)(PR3)RuC:] gives the 14-electron four-coordinate ruthenium phosphonium alkylidenes [Cl2(L)RuCH(PR3)]+[B(X)4]− (see scheme). These compounds which already have a vacant coordination site provide direct access to the active species in olefin metathesis catalysis and thus very fast initiation.
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Romero et al. (2004) studied this question.
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