Bonding in Barium Boryloxides, Siloxides, Phenoxides and Silazides: A Comparison with the Lighter Alkaline Earths.

Barium complexes ligated by bulky boryloxides [OBR ] (where R=CH(SiMe ) , 2,4,6- Pr -C H or 2,4,6-(CF ) -C H ), siloxide [OSi(SiMe ) ] , and/or phenoxide [O-2,6-Ph -C H ] , have been prepared. A diversity of coordination patterns is observed in the solid state for both homoleptic and heteroleptic complexes, with coordination numbers ranging between 2 and 4. The identity of the bridging ligand in heteroleptic dimers [Ba(μ -X )(X )] depends largely on the given pair of ligands X and X . Experimentally, the propensity to fill the bridging position increases according to [OB{CH(SiMe ) } )] <[N(SiMe ) ] <[OSi(SiMe ) ] <[O(2,6-Ph -C H )] <[OB(2,4,6- Pr -C H ) ] . This trend is the overall expression of 3 properties: steric constraints, electronic density and σ- and π-donating capability of the negatively charged atom, and ability to generate Ba ⋅ ⋅ ⋅ F, Ba ⋅ ⋅ ⋅ C(π) or Ba ⋅ ⋅ ⋅ H-C secondary interactions. The comparison of the structural motifs in the complexes [Ae{μ -N(SiMe ) }(OB{CH(SiMe ) } )] (Ae = Mg, Ca, Sr and Ba) suggest that these observations may be extended to all alkaline earths. DFT calculations highlight the largely prevailing ionic character of ligand-Ae bonding in all compounds. The ionic character of the Ae-ligand bond encourages bridging coordination, whereas the number of bridging ligands is controlled by steric factors. DFT computations also indicate that in [Ba(μ -X )(X )] heteroleptic dimers, ligand predilection for bridging vs. terminal positions is dictated by the ability to establish secondary interactions between the metals and the ligands.