Why CpAl–Cr(CO) is linear while CpIn–Cr(CO) is not? Understanding the structure and bonding of the CpE–Cr(CO) (E = Group 13 element) complexes

Density functional theory computations at the BP86-D3/def2-TZVP level are reported for the CpE–Cr(CO) complexes (E = Group 13 element). In principle, we have answered two important facts: first the nature and trend of the E–Cr bonding along B to Tl complexes; second, the deviation of Cp (centroid)-E–Cr angle in In and Tl from linearity. The bonding situation in the complexes is examined via the natural bond orbital, adaptive natural density partitioning, and energy decomposition analysis schemes. Our results reveal that the E–Cr bonding in the lighter compounds is mainly ionic, while this bonding in the In and Tl complexes is dominated by an orbitalic contribution. We also clarify the origin of deviation of Cp (centroid)-E–Cr angle for the In and Tl complexes using simple molecular orbital arguments and find that the repulsive intermolecular contacts in the crystals are not the real source of this deviation as was claimed.