| In the transition-metal coordination chemistry, the carbonyl-coordination ability of scandium has been rather poorly understood. Up to date, only smaller Sc(CO)n (n=1-3) have been clearly characterized with the aid of experimental and computational studies. In this work, in order to explore the carbonyl-coordination ability of Sc and whether the Sc-carbonyl compounds satisfy the well-known"18-electron rule"or not, we performed the first computational study on the equilibrium geometries, stability and vibrational frequencies of a series of Sc(CO)n (n=4-7) and Sc(CO)7- by using various DFT functionals and the coupled cluster (single-point) method with the large 6-311+G(3df) basis set, in comparison with Sc(CO)n (n=1-3).The main results are summarized as follows:(1) We found that the studied Sc(CO)n (n=1-7) generally favor the low-spin ground state (doublet) structures except ScCO and Sc(CO)3 that are in the high-spin quartet states. For instance, the ScCO molecule has a 4∑- ground state derived from 1∑+ CO and atomic Sc with the first excited 3d24s1 occupation. The ground state of Sc(CO)2 is a doublet 2B1 with C2v symmetry, while the linear 4∑g- lies 11.0 kcal/mol higher in energy. The Sc(CO)3 molecule has a C3v symmetry with an 4A1 ground electronic state, which is consistent with the experiment that showed the Sc(CO)3 to have a trigonal pyramidal structure. The Sc(CO)4 molecule has a 2A1 ground state with C4v symmetry, and a 4B2 state with C2v symmetry is 4.1 kcal/mol higher than the 2A1 state. Sc(CO)5 has the 2A' ground state structure in Cs symmetry, whereas the square pyramidal C4v(4B1) structure lies 18.3 kcal/mol higher. The Sc(CO)6 molecule has a 2B1 ground state with C2v symmetry. Yet, the more symmetric structure, i.e., octahedral quartet isomer Oh (4A1g), lies 24.3 kcal/mol higher in energy. The Sc(CO)7 molecule has a ground state structure with the face-capped trigonal prism (C2v) in the doublet ground state 2A1. It was found that the 18-electron Sc(CO)7-, which is formed by adding an electron to the 17-electron Sc(CO)7, becomes more stable and symmetric, possessing a C3v face-capped octahedron ground-state geometrical configuration.(2) The obtained sequential bond dissociation energies (BDE) show that the sequential bond dissociation energies (BDE) of Sc(CO)n (n=4-7) and Sc(CO)7- are comparable to those of smaller Sc-carbonyls (n<4) as well as Ti(CO)7+ that have all been experimentally characterized. Thus, the studied high scandium carbonyls should all be experimentally accessible. Furthermore, it is our optimistic expectation that the last 18-electron first-row transition metal carbonyls, i.e., Sc(CO)7-, are to be seen in the coming future.(3) In our studies, the previous uncertain spectrum bands are assigned for the first time. The good agreement between our computed and previous experimental C-O stretching vibrational frequencies and isotope ratios indicate that Sc(CO)4 and Sc(CO)5 should have been already generated and present in the previous matrix experiments. |
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