Final answer:
The decline of the vital force theory led to the formation of complex organic compounds.
Step-by-step explanation:
The theory that led to the formation of complex organic compounds is the decline of the vital force theory. For many years, scientists believed that organic compounds could only be made by living organisms because they possessed a vital force found only in living systems. However, in 1828, Friedrich Wöhler synthesized urea, an organic compound, from inorganic starting materials, disproving the vital force theory. This led to the recognition that organic molecules obey the same natural laws as inorganic substances, and that both natural and synthetic compounds containing carbon can be classified as organic.