Hence, across the series, NPA charges on calcium are high and positive (+1.80 to +1.81), while those on carbon are close to neutral (-0.03 to -0.07). Most of the charge resides on the oxygen atoms (-0.93 to -0.94). The Wiberg Bond Indices (WBI) for the M-C bonds increase down the group taking values of 0.95 < 0.97 < 1.08 for Co < Rh < Ir. The WBIs for the C=O bonds are consistent between 1.27 and 1.31 with no discernible trends down the triad. The M-C values above 1 and C=O values significantly below 2 are consistent with the formyl ligand adopting metallocarbene character. For comparison, the free dianionic complexes [M( 5 -C5Me5)(CHO)2] 2-(3'-M) were calculated. For 3'-M, NPA charges on carbon (0.17 to 022) and oxygen (-0.74 to -0.76) were more positive than in 3 but followed the same general trends.