π Ligands in Alkaline Earth Complexes

π ligands such as olefins and alkynes bind intramolecularly to the metal atom in d 0 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.