Zinc (0) chemistry: does the missing 18-electron zinc tricarbonyl really exist?

Abstract

Recently, a combined laser ablation and density functional theory study (Jiang and Xu, J. Am. Chem. Soc. 2005, 127, 8906) claimed the existence of the long-sought 18-electron member of the first-row transition metal carbonyl complex, Zn(CO)₃. In this paper, we systematically investigate the thermodynamic and kinetic stability of Zn(CO)₃ towards CO-extrusion at the BP86, B3PW91, BPW91, PBEPBE, BH&HLYP, B3LYP, MP2, MP4SDQ, QCISD, CCSD and CASPT2 levels as well as the Born–Oppenheimer molecular dynamic (BOMD) simulation. All these calculations consistently reveal that the 18e Zn(0) complex Zn(CO)₃ is neither a genuine minimum point nor kinetically stable with negligibly low barriers. In particular, Zn(CO)₃ is quite thermodynamically unstable with respect to the fragments ¹Zn + 3CO by around 40 kcal mol⁻¹ at all the three sophisticated correlation levels, i.e., MP4SDQ, QCISD and CCSD. We thus conclude that the tricarbonyl Zn(0) complex, Zn(CO)₃, should not exist even for spectroscopic characterization. Interestingly, our extensive structural search predicts that two triplet di-zinc carbonyls, i.e., ³(CO)ZnZn and ³(CO)₂ZnZn, have noticeable kinetic stability (10.41 and 8.11 kcal mol⁻¹ at the CCSD level) against the respective CO- and Zn-extrusion, which can be compared with the value 8.70 kcal mol⁻¹ for the already detected ³Zn(CO)₂ (Jiang and Xu, J. Phys. Chem. A2006,110, 7092). Our designed ³(CO)ZnZn and ³(CO)₂ZnZn together with the experimentally known ³ZnCO and ³Zn(CO)₂ are formally associated with the zinc (0) “spin-based zinc carbonyls” and should be considered as remarkable, since most of the known zinc complexes usually contain +2 or +1 oxidation state Zn.