Electronic and geometric structure analysis of neutral and anionic alkali metal complexes of the CX series (X = O, S, Se, Te, Po): The case of M(CX) n =1–4 (M = Li, Na) and their dimers Isuru R. Ariyarathna , and Evangelos Miliordos* Bonding mechanisms, potential energy curves, accurate struc- tures, energetics, and electron affinities are obtained for all M(CX) 1–3 species with M = Li, Na, and X = O, S, Se, Te, and Po, at the coupled-cluster level with triple-ζ quality basis sets. We discuss and rationalize the trends within different molecular groups. For example, we found larger binding ener- gies for M = Li, for CX = CPo, and for the tri-coordinated (n = 3) complexes. All three facts are explained by the fact that the global minimum of the titled complexes originate from the first excited 2 P (2p 1 for Li or 3p 1 for Na) state of the metal, with each ligand forming a dative bond with the metal. All of the complexes, except Na(CO) 3 , have stable anions, and their elec- tron affinity increases as MCX < M(CX) 3 < M(CX) 2 . This sequence is attributed to the binding modes of these complexes. The Li(CO) 3 and Li(CS) 3 complexes are able to accommodate a fourth ligand, which is attached to the system electrostatically. Finally, two Li(CO) 3 molecules can bind together covalently to make the ethane analog. The staggered conformer was found lower in energy and unlike ethane the CO ligands bend toward the neighboring Li(CO) 3 moiety. © 2019 Wiley Periodicals, Inc. DOI: 10.1002/jcc.25791 Introduction Transition metal carbonyl chemistry is a well explored field because of the catalysis applications, especially in organometal- lic chemistry. Among them, Mo(CO) 6 , W(CO) 6 , Cr(CO) 6 , [FeCp (CO) 2 ] 2 , Co 2 (CO) 8 , and Ni(CO) 4 have been identified as efficient catalysts for liquefaction of coal, [1] while M(CO) 3 (NCCH 3 ) 3 and MI 2 (CO) 3 (NCCH 3 ) 2 (M = Mo, W) are capable of facilitating acetylene polymerization. [2] Industrially, Co 2 (CO) 8 and Ni(CO) 4 catalysts are utilized for the production of aldehydes from alkenes [3] and in the synthesis of propanoic acid, [4] respectively. Further, anions of metal carbonyls such as Mn(CO) 5 - and C 6 H 6 Fe(CO) 2 - have been used to synthesize various organome- tallic compounds. [5] To understand and predict their chemical activity, the elec- tronic structure of metal carbonyl bonds has been extensively studied. [6–11] Carbonyls can bind in two ways with a metal M: Carbon monoxide is a σ-donor (electron flow from CO to M along the M C axis) and a π-acceptor (flow from M to CO through the π-frame). The π back donation strengthens the M C bond but weakens C O producing elongated C O bonds and lower C O stretching frequencies. [12] In anionic metal car- bonyl complexes this C O bond weakening is enhanced over neutral complexes. [12] On the other hand, the C O bond of sev- eral late transition metal complexes such as M(CO) 1,2 + (M = Cu, Ag, Au) is found to be stronger than free CO. These complexes are referred as nonclassical metal carbonyls. [6,13] Considerable interest has been also shown for the s-block metal carbonyl systems in the literature. Several alkali metal car- bonyl compounds, such as Li(CO) n =1–4 , NaCO, KCO, Li(CO) 2 , and Li(CO) 3 , have been detected by means of matrix isolation, [14–16] and the vibrational spectra of Li(CO) n =1–4 in krypton have been experimentally recorded and analyzed by Ayed et al. [14] Electronic paramagnetic resonance spectrum of NaCO is reported by Joly and Howard who clearly identified an unpaired electron localized on Na. [17] From the theoretical stand point, in 1986 Silvi et al. has performed Hartree-Fock and configuration interaction (CI) calculations for Li CO Li and Li CO species and suggested that the latter is ionic (Li + CO - ). [18] Ten years later Kalemos et al. studied the chemical bonding of linear LiCO, LiCS, and LiSC including potential energy curves (PECs) with respect to the M-ligand distance. [19] According to their findings all three molecules are generated by the interaction of the first excited state of Li( 2 P; 2p 1 ) with the ground state, 1 Σ + , of CO/CS. The ground state of CO and CS has one electron pair at the carbon end which facilitates a dative bond with Li, Li(2p σ )  CO/CS. Additionally, Li(2p π ) ! CO/CS(π*) electron transfer is proposed by the same authors. Their population analysis demonstrated that almost half an electron is transferred from CO to Li in the σ-frame, and about 0.8 electrons flow toward the opposite direction in the π-frame. LiCO, LiOC, and LiSC were all found metastable with respect to Li( 2 S) + CO( 1 Σ + ) or Li( 2 S) + CS( 1 Σ + ), and only LiCS is bound by 25.7 kcal/mol at CI singles and doubles (CISD). Our calculations confirm fully the previous bonding patterns not only for LiCO [a] I. R. Ariyarathna, E. Miliordos Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama, 36849-5312 E-mail: emiliord@auburn.edu © 2019 Wiley Periodicals, Inc. WWW.C-CHEM.ORG FULL PAPER Wiley Online Library Journal of Computational Chemistry 2019 1