The close agreement of NBO data between 3 and 3' is consistent with the proposed ionic bonding situation (Figure ). QTAIM calculations further support this bonding formulation. For 3-Ir, bond paths are identified between each calcium and oxygen atom of the bridging motif. Bond critical points show (r) and  2 (r) values of 0.04 e bohr -3 and 0.19-0.22 e bohr -5 respectively, consistent with binding of the formyl ligand to calcium being primarily through electrostatic interactions.