What was the capital of the union of provinces in the Río de la Plata comprised by most of the former dependencies from the last to be organized and also the shortest-lived of the Viceroyalties of the Spanish Empire in the Americas?
Buenos Aires
Title: Viceroyalty of the Río de la Plata
Passage: The Viceroyalty of the Río de la Plata (Spanish: "Virreinato del Río de la Plata" ) was the last to be organized and also the shortest-lived of the Viceroyalties of the Spanish Empire in the Americas.
Title: United Provinces of the Rio de la Plata
Passage: The United Provinces of the Río de la Plata (Spanish: "Provincias Unidas del Río de la Plata" ), earlier known as the United Provinces of South America (Spanish: "Provincias Unidas de Sudamérica" ), a union of provinces in the Río de la Plata region of South America, emerged from the May Revolution in 1810 and the Argentine War of Independence of 1810–1818.  It comprised most of the former Viceroyalty of the Río de la Plata dependencies and had Buenos Aires as its capital.
Title: Santiago de Liniers, 1st Count of Buenos Aires
Passage: Jacques de Liniers (July 25, 1753 – August 26, 1810) was a French officer in the Spanish military service, and a viceroy of the Spanish colonies of the Viceroyalty of the Río de la Plata.  He is more widely known by the Spanish form of his name, Santiago de Liniers.  He was popularly regarded as the hero of the reconquest of Buenos Aires after the first British invasion of the Río de la Plata, which led to his designation as viceroy, replacing Rafael de Sobremonte.  Such a thing, the replacement of a viceroy without the King's direct intervention, was completely unprecedented.  He was confirmed in office by Charles IV of Spain, and endured a second ill-fated British Invasion attempt and a mutiny that sought to replace him.  He was replaced in 1809 by Baltasar Hidalgo de Cisneros, appointed as viceroy by the Junta of Seville, and retired from public activity.  However, when the May Revolution took place, Liniers decided to come out of his retirement and organized a monarchist uprising in Córdoba.  However, Liniers was defeated, captured, and executed without trial.
United Provinces of the Rio de la Plata Viceroyalty of the Río de la Plata