A new type of organometallic catalyst can control the rates of hydroborations and potentially other reactions by using different wavelengths of light. The work may lead to distinct products with the flip of a light switch and may simplify multistep syntheses by using single catalysts in sequential reactions. Bethany M. Neilson and Christopher W. Bielawski of the University of Texas, Austin, achieved the organometallic photocontrol with an N-heterocyclic carbene (NHC) ligand bearing a photoresponsive group (Organometallics 2013, DOI: 10.1021/om400348h). When exposed to UV light, the group cyclizes, weakening NHC’s (shown, black) electron-donating ability and catalytic activity, whereas visible light opens the ring, enhancing NHC’s (red) electron donation and catalysis. Using NHC-metal complexes as photoswitchable catalysts was first suggested in 2009 by Vivian Wing-Wah Yam and coworkers at the University of Hong Kong (J. Am. Chem. Soc., DOI:10.1021/ja808791e). Similar reactions have been reported previously. But the new report is the first to control metal-catalyzed reactions by using NHCs and the first to do so by modifying a ligand’s electron-donating ability.