  Recent observations of selective emergence (suppression) of superconductivity
in the uncollapsed (collapsed) tetragonal phase of LaFe$_2$As$_2$ has rekindled
interest in understanding what features of the band structure control the
superconducting T$_c$. We show that the proximity of the narrow Fe-d$_{xy}$
state to the Fermi energy emerges as the primary factor. In the uncollapsed
phase this state is at the Fermi energy, and is most strongly correlated and
source of enhanced scattering in both single and two particle channels. The
resulting intense and broad low energy spin fluctuations suppress magnetic
ordering and simultaneously provide glue for Cooper pair formation. In the
collapsed tetragonal phase, the d$_{xy}$ state is driven far below the Fermi
energy, which suppresses the low-energy scattering and blocks
superconductivity. A similar source of broad spin excitation appears in
uncollapsed and collapsed phases of CaFe$_{2}$As$_{2}$. This suggests
controlling coherence provides a way to engineer T$_c$ in unconventional
superconductors primarily mediated through spin fluctuations.
