In order for neurotransmitters to be released into the synaptic region..what ions are involved in the docking?

Calcium 100%  From Wikipedia:

The release of a neurotransmitter is triggered by the arrival of a nerve impulse (or action potential) and occurs through an unusually rapid process of cellular secretion (exocytosis). Within the presynaptic nerve terminal, vesicles containing neurotransmitter are localized near the synaptic membrane. The arriving action potential produces an influx of calcium ions through voltage-dependent, calcium-selective ion channels at the down stroke of the action potential (tail current).[11] Calcium ions then bind with the proteins found within the membranes of the synaptic vesicles, allowing the vesicles to fuse with the presynaptic membrane, resulting in the creation of a fusion pore. The vesicles then release their contents to the synaptic cleft through this fusion pore[12] within 180 µsec of calcium entry.[11] Vesicle fusion is driven by the action of a set of proteins in the presynaptic terminal known as SNAREs. As a whole, the protein complex or structure that mediates the docking and fusion of presynaptic vesicles is called the active zone.[13] The membrane added by this fusion is later retrieved by endocytosis and recycled for the formation of fresh neurotransmitter-filled vesicles.