Relativistic Effects on Donor-Acceptor Interactions in Coinage Metal Carbonyl Complexes [TM(CO)(n)](+) (TM=Cu, Ag, Au; n=1, 2)

DFT calculations at the BP86+D3(BJ)/TZ2P level, with and without relativistic contributions, using the ZORA approximation have been carried out for the coinage metal carbonyl complexes [TM(CO)](+) and [TM(CO)(2)](+) with TM = Cu, Ag, Au. The nature of the metal-CO interactions and the relativistic effects on the different energy terms were analyzed with the EDA-NOCV method. The three terms Pauli repulsion, Coulomb attraction, and orbital interactions become stronger when relativistic effects are accounted for; the strengthening exhibits the order Delta E-Pauli > Delta E-elstat > Delta E-orb. The largest change in the calculated energy terms is, as expected, found for gold, followed by silver and copper. The relativistic contributions on the Cu+-CO interactions are significant and thus, relativistic effects should not be neglected in quantum chemical calculations in copper compounds. Breakdown of the orbital term into individual contributions shows that the relativistic effect in [TM(CO)](+) is for the TM+<- CO sigma-donation stronger than for TM+-> CO pi-backdonation, except for TM=Cu. The trend in the dicarbonyls [TM(CO) 2]+ has the order (+,+) sigma-donation > pi-backdonation > (+, -) sigma-donation. The bonding analysis reveals that there is a sizeable contribution from TM+-> CO sigma-backdonation in all carbonyl complexes that further stabilizes the metal-carbonyl bonds. In [Au(CO)(2)](+) it becomes even larger than the (+, -) OC -> TM+<- CO sigma-donation. The trends of the various orbital interactions and the effect of relativity on their strength can be understood when the valence orbitals of the metals and CO are considered.