Atomic size increases:
As we move down the group of alkaline earth metals, the atomic size increases. This is because each successive element has an additional energy level, which increases the distance between the nucleus and the outermost electron. The increase in atomic size leads to a decrease in the effective nuclear charge experienced by the outermost electrons.

Availability of empty d and f-orbitals:
The alkaline earth metals do not have any electrons in their d or f orbitals. As we move down the group, the number of energy levels increases, and the d and f orbitals become available. However, the d and f orbitals do not participate significantly in chemical bonding for alkaline earth metals. Therefore, the availability of empty d and f orbitals does not have a significant effect on the complex formation tendency of these elements.

Nuclear charge to volume ratio increases:
The nuclear charge to volume ratio is the ratio of the charge of the nucleus to the volume occupied by the electrons. As we move down the group, the nuclear charge increases due to the increase in the number of protons, while the atomic size increases due to the increase in the number of energy levels. Therefore, the nuclear charge to volume ratio decreases as we move down the group.

Explanation:
The complex formation tendency of alkaline earth metals decreases down the group primarily because of the increase in atomic size. When an alkaline earth metal forms a complex, it donates its valence electrons to the central metal ion. The larger the atomic size, the more diffuse the valence electrons are, and the less effective they are at forming bonds with the central metal ion. As a result, the complex formation tendency decreases down the group.

The availability of empty d and f orbitals does not have a significant effect on the complex formation tendency of alkaline earth metals. This is because these orbitals do not participate significantly in chemical bonding for these elements.

The increase in the nuclear charge to volume ratio down the group does not significantly affect the complex formation tendency. Although the nuclear charge increases, the atomic size also increases, resulting in a decrease in the effective nuclear charge experienced by the outermost electrons. Therefore, the decrease in complex formation tendency down the group is primarily attributed to the increase in atomic size.