The electron densities of 3-Yb and 4-Yb were calculated using PBE to determine the nature of the Yb-P interactions (see SI Figures ). Frontier molecular orbitals show that there is no substantial orbital overlap between Yb and P (Figures ), where the lone pair remains localized on P, although a larger P 3p contribution is present in the highest occupied molecular orbital of 3-Yb. Delocalization indices, (Yb-P) and Wiberg bond orders (WBO) report the number of electrons shared between two nuclei; for 4-Yb, the average (Yb-P) = 0.24 and WBO is 0.28, however these values are higher for 3-Yb: (Yb-P) = 0.33 and WBO is 0.36. The electron density at the bond critical point BCP(Yb-P) shows the Yb-P interaction in 3-Yb (0.034 a.u.) is stronger than in 4-Yb (0.015 a.u.).