Probing the Limits of Alkaline Earth–Transition Metal Bonding: An Experimental and Computational Study

Reduction of Fp2 (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(μ-OC)­Co η1-isocarbonyl linkage. With M = Sr (21) or Eu (25), a switch from M–Co bonding to side-on (η2) CO ligand coordination is found. SmII{Co­(CO)3(PCy3)}2(THF)3 disproportionates in pentane to form SmIII{Co­(CO)3(PCy3)}3(THF)3 containing two SmIII–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 (η2) 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.