Reduction of Fp(2) (Fp = CpFe(CO)(2)) or [Co(CO)(3)(PCy3)](2) (15) with Mg-mercury amalgam gave [Mg{TM(L)}(2)(THF)}(2) (TM(L) = Fp or Co(CO)(3)(PCy3) (19)) in which the TM is bonded to two Mg atoms. Reduction of 15 with Ca-, Sr-, Ba-, Yb-, Eu- and Sm-mercury amalgam gave a series of compounds "M{Co(CO)(3)(PCy3)}(2)(THF)(n)" (M = Ae or Ln) in which the M-Co bonding varies with the charge-to-size ratio of M. For M = Ca or Yb (24), each metal forms one M-Co bond and one M(mu-OC)Co eta(1)-isocarbonyl linkage. With M = Sr (21) or Eu (25), a switch from M-Co bonding to side-on (eta(2)) CO ligand coordination is found. SmR{Co(CO)(3)(PCy3)}(2)(THF)(3) disproportionates in pentane to form Sm-III{Co(CO)(3)(PCy3)}(3)(THF)(3) containing two Sm-III-Co bonds, in contrast with 25, showing the importance of the Ln charge on Ln-TM bonding. Diffusion NMR spectroscopy found that in solution, 21 and 24 are dimeric compounds [M{Co(CO)(3)(PCy3)}(2)(THF)(3)](2) that, according to DFT calculations, contain either one (Ae = Ca) or two (Ae = Sr) Ae-Co bonds per Co atom. DFT calculations in combination with Ziegler Rauk energy decomposition and atoms in molecules analysis were used to assess the nature and energy of Ae-Co bonding in a series of model compounds. The Ae-Co interaction energies decrease from Be to Sr, and toward the bottom of the group, side-on (eta(2)) CO ligand coordination competes with Ae-Co bonding. The PCy3 ligand plays a pivotal role by increasing solubility in nondonor solvents and the Ae-Co interaction energy.