Ab initio and DFT calculations have been performed on a series of organometallic compounds, according to the formula MCH_n, where M = K, Ca, Sc, Ti, V, Cr, Mn and n = 1-3. Various theoretical methods are compared, the B3LYP level yielding the same agreement with the experimental geometries available as the correlated MP2 and CISD methods, with the 6-311++G(3df,2p) basis set for C,H and Wachter's (15s11p6d3f1g)/[10s7p4d3f1g], basis set for transition metals. The main geometric features of the molecules studied are described, and the corresponding electronic structure and M-C bonding characteristics are analyzed in terms of the Atoms in Molecules theory (AIM) and the Electron Localization Function (ELF). Although multiple bonding is expected from the Lewis bonding scheme, the results indicate an almost pure ionic bond for all the systems studied. The net charge transfer from the metal to the carbon atom ranges from 0.5 to 1 e-, and the electronic structure of the CH_n- moiety is unaltered after the interaction with the metal cation, showing little or no effect on the shape of the electron pairing. The bond paths corresponding to a possible α-agostic bond for these systems are not present.