Since a transition metal methyl complex corresponds to a compound in which one hydrogen atom of methane is replaced by a transition metal (M), the M–C bond is considered to be a covalent bond or σ-bond. To understand the properties and reactivity of organometallic complexes, it is important to understand the character of M–C σ-bonds in transition metal alkyl complexes.

How is the σ-bond between a transition metal and carbon polarized? When considering a C–Cl bond, for example, since Cl is more electronegative than C, it is expected to be polarized C^{δ +}–Cl^{δ −}. M–C bonds may be considered in a similar way. A table of Allred–Rochow electronegativities is given in the Appendix. The electronegativity of carbon is given as 2.5. For transition metals, the electronegativity values are between 1.1 and 1.8, showing that all transition metals have lower electronegativity than carbon. Thus in any complex L_nM–CR₃, it is almost certain that the polarization of the M–C bond is M^{δ +}–C^{δ −}, regardless of the identity of M, the kind of ligand L and the substituent R.