Investigating the Effects of Basis Set on Metal-Metal and Metal-Ligand Bond Distances in Stable Transition Metal Carbonyls: Performance of Correlation Consistent Basis Sets with 35 Density Functionals.

Density functional theory (DFT) is a widely used method for predicting equilibrium geometries of organometallic compounds involving transition metals, with a wide choice of functional and basis set combinations. A study of the role of basis set size in predicting the structural parameters can be insightful with respect to the effectiveness of using small basis sets to optimize larger molecular systems. For many organometallic systems, the metal-metal and metal-carbon distances are the most important structural features. In this study, we compare the equilibrium metal-ligand and metal-metal distances of six transition metal carbonyl compounds predicted by the Hood-Pitzer double-ζ polarization (DZP) basis set, against those predicted employing the standard correlation consistent cc-pVXZ (X = D,T,Q) basis sets, for 35 different DFT methods. The effects of systematically increasing the basis set size on the structural parameters are carefully investigated. The Mn-Mn bond distance in Mn2(CO)10 shows a greater dependence on basis set size compared to the other M-M bonds. However, the DZP predictions for re(Mn-Mn) are closer to experiment than those obtained with the much larger cc-pVQZ basis set. Our results show that, in general, DZP basis sets predict structural parameters with an accuracy comparable to the triple and quadruple-ζ basis sets. This finding is very significant, because the quadruple-ζ basis set for Mn2(CO)10 includes 1308 basis functions, while the equally effective double-ζ set (DZP) includes only 366 basis functions. Overall, the DZP M06-L method predicts structures that are very consistent with experiment.