The highly exothermic reaction of chromium trioxide when added to an excess of pyridine leads to the formation the CrO3 • 2 Py complex (Sarett Reagent). Compared to the Jones Reagent, Sarett Reagent allows the oxidation of various primary alcohols to aldehydes due to the non-aqueous conditions (see Jones Oxidation for an explanation). Unfortunately, the complex is highly hygroscopic and the preparation of the reagent is not without risk because the solvent occasionally catches fire during preparation. In addition, the use of pyridine as solvent does not permit the oxidation of base-sensitive substrates.
The CrO3 • 2 Py complex diluted in dichloromethane - also known as Collins Reagent - is a more convenient oxidation reagent, that can be more easily prepared and allows the oxidation of a broader substrate scope.
A full review of chromium-based reagents can be found in the book written by Tojo and Fernández (Oxidation of Alcohols to Aldehydes and Ketones, Springer Berlin, 2006, 1-97.).
Attention: Chromium (VI) compounds are toxic and must be handled with care.