Carbon–hydrogen bond activation or C-H activation is a reaction that breaks a carbon–hydrogen bond.  Most times, the reaction involves organometallic complexes. The reaction path has coordination of a hydrocarbon to the inner-sphere of the metal atom (“M”). The reaction path includes either an intermediate “alkane or arene complex” or as a transition state leading to a "M-C" intermediate. The reaction path makes C-H activation different from other reactions. During the C-H breaking event the hydrocarbyl group remains linked in the inner-sphere and under the influence of the metal atom.

Chemists study C-H activation because for many years chemists believed that C-H bonds were unreactive.  Both theoretical studies as well as experiments now show that C-H bonds can be broken. This is done by a nearby metal atom changing the electron distribution of the C-H bond (coordination).  Much chemical research looks at the design and synthesis of new reagents and catalysts that can affect C-H activation.  The goal of this research is the conversion of cheap and abundant alkanes into valuable functionalized organic compounds.

Historians say that Otto Dimroth discovered the first C-H activation reaction. In 1902, he reported that benzene reacted with mercury(II) acetate (See: organomercury), but some scholars do not view this reaction as being a C-H activation.  Goldman & Goldberg later wrote, C-H activation looks like H-H activation: both can be achieved by electrophilic or oxidative addition. The first true C-H activation reaction was reported by Joseph Chatt in 1965 with insertion of a ruthenium atom ligated to dmpe in the C-H bond of naphthalene.

References

Further reading
 “Activation of C-H Bonds by Metal Complexes”, A. E. Shilov, G. B. Shul’pin, Chem. Rev. 1997, 97, 2879–2932. 
 “Activation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes”, A. E. Shilov, G. B. Shul’pin, Kluwer Academic Publishers, Dordrecht/Boston/London, 2000 (552 p) (Springer, ). https://www.springer.com/chemistry/physical+chemistry/book/978-0-7923-6101-5
 “Alkane C-H activation and functionalization with homogeneous transition metal catalysts: a century of progress – a new millennium in prospect”, R. H. Crabtree, J. Chem. Soc., Dalton Trans. 2001, 17, 2437–2450. 
 “Organometallic alkane CH activation”, R. H. Crabtree, J. Organometal. Chem. 2004, 689, 4083–4091. 
 “Mechanistic Aspects of C−H Activation by Pt Complexes”, M. Lersch, M.Tilset, Chem. Rev. 2005, 105, 2471−2526. 
 “Recent Advances in the Platinum-mediated CH Bond Functionalization”, A. N. Vedernikov, Curr. Org. Chem. 2007, 11, 1401−1416.
 “Catalytic C-H functionalization by metalcarbenoid and nitrenoid insertion”, H. M. L. Davies, J. R. Manning, Nature, 2008, 451, 417−424, 
 "Mechanisms of C-H bond activation: rich synergy between computation and experiment”, Y. Boutadla, D. L. Davies, S. A. Macgregor, A. I. Poblador-Bahamonde, Dalton Trans. 2009, 5820−5831. 
 “C-H Bond Activation in Transition Metal Species from a Computational Perspective”, D. Balcells, E. Clot, O. Eisenstein, Chem. Rev. 2010, 110, 749–823. 
 “Palladium-Catalyzed Ligand-Directed C-H Functionalization Reactions”, T. W. Lyons, M. S. Sanford, Chem. Rev. 2010, 110, 1147–1169. 
 “Selectivity enhancement in functionalization of C-H bonds: A review”, G. B. Shul’pin, Org. Biomol. Chem. 2010, 8, 4217–4228. 

Organometallic chemistry