text: Mechanisms for C-H activations by metal centers can be classified into three general categories: 
  *(i) Oxidative addition, in which a low-valent metal center inserts into a carbon-hydrogen bond, which cleaves the bond and oxidizes the metal:
  :LM  + RH  →  LMR(H)
  *(ii) Electrophilic activation in which an electrophilic metal attacks the hydrocarbon, displacing a proton:
  :LM  + RH  →  LMR  +  H
  *(iii) Sigma-bond metathesis, which proceeds through a "four-centered" transition state in which bonds break and form in a single step:
  :LMX  + RH  →  LMR  +  XH