DFT calculations on gas-phase models were performed with the Amsterdam Density Functional (ADF) package, version 2019.304 . The PBE functional was used for all calculations. Slater-type basis sets of triple-ζ + polarization quality were used on all atoms, with orbitals up to 4d (Pt), 3d (Ge) and 1s (C) included in the frozen core .