Osmylation of alkenes has become a critically important method of functionalizing an alkene due to two discoveries: 1) the reaction rate is accelerated by addition of amines that bind to Os; 2) use of chiral amines (particularly those derived from cinchona alkaloids such as quinine and quinolidine) allow not just high stereocontrol (syn addition), but also complete enantiocontrol — any absolute stereochemistry at the diol can be prepared based on the configuration of the double bond and the alkaloid ligand chosen.

Generation of 4 stereoisomers (two pairs of enantiomers) from 1-phenylpropene using catalytic OsO4 and different chiral alkaloid ligands.
There is a mechanistic question — key to understanding what the ligand is doing — over whether this is a concerted reaction (both C-O bonds form at once), or whether the alkene binds to the metal and makes the C-O bonds in a 2-step sequence:
Two proposed mechanisms: one concerted to directly form a 5-member osmacycle, or a 2-step mechanism going through a 4-member ring that expands.
Computational modeling can identify transition states for each step in both mechanisms:
Concerted:

 Stepwise, formation of the metallacycle:
 Stepwise, expansion of the ring:
To tell the difference between these we use a measurement of Kinetic Isotope Effects. In this kind of system, molecules with a heavy isotope (13C or 2H) will react a little bit faster or slower than the "normal" isotope, if they are on a position that is undergoing chemical change in the transition state. We measure the "KIE" by running the reaction to a known level of completion, and measuring the isotopic distribution compared to that in unreacted alkene — the buildup of the heavy isotope is related to the ratio of rates for heavy and light isotopes. A large KIE (much different than 1.000) means the center is undergoing extensive change in bonding at the transition state; a small KIE means any change happens after the transition state. Results:
Hα Hβ c Hβ t Cα Cβ
Calculated
Concerted [3+2]
0.909 0.912 0.921 1.025 1.024
Stepwise, 1st Step is the TS
0.892 0.957 0.972 1.050 1.026
Stepwise, 2nd step is the TS
0.880 0.964 1.094 0.989 1.039
Experimental Results (± 0.005)
0.907 0.918 0.925 1.027 1.027