Natural Bonding Orbital (NBO) calculations of complex 3a provided information on the bonding of the heterometallic motif (Figure ). Based on the analysis the hydrides are best considered as bridging ligands which participate in 3-centre 2-electron bonding with Zn and Fe. There is limited support for a significant Zn---Fe interaction. Wiberg Bond Indices (WBI) for the Fe-H bonds are higher than those of the corresponding Zn-H bonds. In comparison the Zn---Fe WBI is lower than those between the metals and hydrides. NPA charges are negative on the Fe and H atoms but electropositive on Zn (Figure ). The calculations suggest that the hydrides form the most significant covalent interaction with Fe rather than Zn.