text: An important aspect of improving chemical reactions is the understanding of the underlying reaction mechanism. To answer this question for C-H activation, time-resolved spectroscopic techniques can be used to follow the dynamics of the chemical reaction. This technique requires a trigger for initiating the process, which is in most cases illumination of the compound. Photoinitiated reactions of transition metal complexes with alkanes serve as a powerful model systems for understanding the cleavage of the strong C-H bond. 
  In such systems, the sample is illuminated with UV-light which excites an electron from the metal center to an unoccupied, antibonding ligand orbitals (MLCT), leading to ligand dissociation. This creates a highly reactive, electron deficient 16-electron intermediate, with a vacant coordination site. This species then binds to an alkane molecule, forming a σ-complex coordinating to a C-H bond. In a third step, the metal atom inserts into the C-H bond, cleaving it and yielding the C-H bond activated product.
  The intermediates and their kinetics can be observed using different time-resolved spectroscopic techniques (e.g. TR-IR, TR-XAS, TR-RIXS). Time-resolved infrared spectroscopy (TR-IR) is a rather convenient method to observe these intermediates. However, it is only limited to complexes which have IR-active ligands and is prone to correct assignments on the femtosecond timescale due to underlying vibrational cooling. To answer the question of difference in reactivity for distinct complexes, the electronic structure of those needs to be investigated. This can be achieved by X-ray absorption spectroscopy (XAS) or resonant inelastic X-ray scattering (RIXS). These methods have been successfully used to follow the steps of C-H activation with orbital resolution and provide detailed insights into the responsible interactions for the C-H bond breaking.
  Full characterization of the structure of methane bound to a metal center was reported by Girolami in 2023: isotopic perturbation of equilibrium (IPE) studies involving deuterated isotopologs showed that methane binds to the metal center through a single M···H-C bridge; changes in the J coupling constants indicate clearly that the structure of the methane ligand is significantly perturbed relative to the free molecule.