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

Reduction of Fp<sub>2</sub> (Fp = CpFe­(CO)<sub>2</sub>) or [Co­(CO)<sub>3</sub>(PCy<sub>3</sub>)]<sub>2</sub> (<b>15</b>) with Mg-mercury amalgam gave [Mg­{TM­(L)}<sub>2</sub>(THF)]<sub>2</sub> (TM­(L) = Fp or Co­(CO)<sub>3</sub>(PCy<sub>3</sub>) (<b>19</b>)) in which the TM is bonded to two Mg atoms. Reduction of <b>15</b> with Ca-, Sr-, Ba-, Yb-, Eu- and Sm-mercury amalgam gave a series of compounds “M­{Co­(CO)<sub>3</sub>(PCy<sub>3</sub>)}<sub>2</sub>(THF)<sub><i>n</i></sub>” (M = Ae or Ln) in which the M–Co bonding varies with the charge-to-size ratio of M. For M = Ca or Yb (<b>24</b>), each metal forms one M–Co bond and one M­(μ-OC)­Co η<sup>1</sup>-isocarbonyl linkage. With M = Sr (<b>21</b>) or Eu (<b>25</b>), a switch from M–Co bonding to side-on (η<sup>2</sup>) CO ligand coordination is found. Sm<sup>II</sup>{Co­(CO)<sub>3</sub>(PCy<sub>3</sub>)}<sub>2</sub>(THF)<sub>3</sub> disproportionates in pentane to form Sm<sup>III</sup>{Co­(CO)<sub>3</sub>(PCy<sub>3</sub>)}<sub>3</sub>(THF)<sub>3</sub> containing two Sm<sup>III</sup>–Co bonds, in contrast with <b>25</b>, showing the importance of the Ln charge on Ln–TM bonding. Diffusion NMR spectroscopy found that in solution, <b>21</b> and <b>24</b> are dimeric compounds [M­{Co­(CO)<sub>3</sub>(PCy<sub>3</sub>)}<sub>2</sub>(THF)<sub>3</sub>]<sub>2</sub> 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 (η<sup>2</sup>) CO ligand coordination competes with Ae–Co bonding. The PCy<sub>3</sub> ligand plays a pivotal role by increasing solubility in nondonor solvents and the Ae–Co interaction energy.