Why are some gaseous calcium, strontium, and barium dihalides bent rather than linear?

The gaseous dihalides of the heavier alkaline-earth metals—specifically calcium, strontium, and barium—are observed to be bent rather than linear, a structural feature that deviates from the expectations of a purely ionic bonding model.1

The prevailing explanation for this geometry is that the bonding in these molecules involves more than just the valence electrons of the metal. It is suggested that the filled "semi-core" $(n-1)p$ and the valence $(n-1)d$ orbitals of these heavier metals participate in the bonding process. For example, in calcium, this involves the 3p and 3d orbitals; in strontium, the 4p and 4d; and in barium, the 5p and 5d.1

The mixing of these orbitals during the formation of the chemical bonds leads to the nonlinear, or bent, structures observed in molecular beam measurements. This interpretation is supported by computational models, which must explicitly include the participation of the metal d orbitals to accurately reproduce the bent geometries.1