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Why are calcium, strontium, and barium compounds not always completely ionic?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkaline-earth metal Modern version of the periodic table of the elements (printable).

While the chemistry of alkaline-earth metals is largely interpreted through an ionic model, compounds of the heavier members—calcium, strontium, and barium—are not always completely ionic due to structural features that deviate from purely ionic expectations.1

Evidence for this includes the observation that certain gaseous dihalides and organometallic compounds exhibit nonlinear or "bent" geometries rather than the expected linear structures. One primary explanation for this behavior is that the filled "semi-core" (n − 1)p and valence (n − 1)d orbitals of these heavier metals participate in chemical bonding.1

During the bonding process, the wave functions of these orbitals can mix, which leads to the observed nonlinear structures. Calculations confirm that explicitly including the participation of these metal d orbitals is necessary to accurately reproduce these bent molecular shapes.1