Chemical Bonding Analysis: COHP analysis was carried out using LOBSTER version 4.1.0 108-112 and plane wave projections onto PbeVaspFit2015 atomic basis functions. Topological analyses of electron densities were made with Critic2. Electron density and volume integrations relied on the Yu-Trinkle algorithm. Superconductivity: Calculations were performed at the PBE-D3(BJ) level with a kinetic energy cutoff of 120 Ry (ca 1632 eV), on a geometry relaxed with Quantum Expresso version 7.3 115 with PAW potentials comparable to those used in VASP. Electronic energies were converged to 10 -12 Ry and forces to 10 -5 a.u. For structural relaxation, a 12x12x24 Monkhorst-Pack kmesh 116 was used with a first order Methfessel-Paxton smearing of 0.005 Ry. Phonons were computed using density-functional perturbation theory on a 4x4x8 q-mesh. The EPW code version 5.5 117, 118 was employed to calculate electron phononcoupling and Tc, with the latter estimated using the McMillan-Allen-Dynes approximation to the Eliashberg equations and a Coulomb pseudopotential parameter μ*= 0.1. Electronic energies, phonon frequencies and electron-phonon matrix elements were evaluated on interpolated 8x8x16 k-and q-grids. Tc, λ, and the superconducting gap were carefully converged with respect to electronic smearing (Table ).