π Ligands in Alkaline Earth Complexes

π ligands such as olefins and alkynes bind intramolecularly to the metal atom in d0 complexes of the large alkaline earths (Ae) calcium and strontium supported by fluoroalkoxo ligands with dangling unsaturated CC or CC bonds, and having the amide N­(SiMe2H)2– as the coligand. These O-bridged dinuclear complexes are further stabilized by secondary C–F→Ae and β-Si–H···Ae interactions. In a set of structurally related Ca-olefin complexes, the strength of these interactions gradually increases as the coordination of the olefin onto Ca2+ becomes weaker (from η2-coordinated to η1 to fully dissociated) upon increasing steric congestion, thus ensuring that overwhelming electronic depletion does not occur at calcium. NMR data imply that the olefins are metal-bound in [D8]­toluene solutions. The Ae···Cπ, C–F→Ae, and β-Si–H···Ae noncovalent interactions are also strong in the parent Ae-alkyne complexes, the first examples of non-acetylide Ae-alkynes compounds. Calcium-arene complexes could not be made, as the aromatic tether did not bind to the metal atom. Instead, a trinuclear complex with noninteracting C6H5 groups was obtained. It exhibits exceptionally strong C–F→Ca and β-Si–H···Ca interactions. NMR data indicate that the congeneric calcium-allene complex can be made, but it spontaneously isomerizes toward the more stable Ca-alkyne via an unusual 1,3-hydride shift intramolecular process.