  We report that the 122 type Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ solid solution
undergoes an anomalous structural transition from the uncollapsed to the
collapsed ThCr$_2$Si$_2$ structure at a distinct onset composition near
$x=0.5$. Correlated with the structural changes, the electronic system evolves
from a nearly ferromagnetic Fermi liquid to an antiferromagnetic metal, through
a complex crossover regime. The structural collapse, driven by P-P bonding
across the (Sr,Ca) layers, is much more pronounced in this system than it is in
the analogous Fe-based system, indicating a strong sensitivity of structure to
total electron count in the transition metal pnictide 122 family.
