Toward Accurate Theoretical Thermochemistry of First Row Transition Metal Complexes

Abstract

The recently developed correlation consistent Composite Approach for transition metals (ccCA-TM) was utilized to compute the thermochemical properties for a collection of 225 inorganic molecules containing first row (3d) transition metals, ranging from the monohydrides to larger organometallics such as Sc(C₅H₅)₃ and clusters such as (CrO₃)₃. Ostentatiously large deviations of ccCA-TM predictions stem mainly from aging and unreliable experimental data. For a subset of 70 molecules with reported experimental uncertainties less than or equal to 2.0 kcal mol^–1, regardless of the presence of moderate multireference character in some molecules, ccCA-TM achieves transition metal chemical accuracy of ±3.0 kcal mol^–1 as defined in our earlier work [<i>J. Phys. Chem. A</i> <b>2007</b>, <i>111</i>, 11269–11277] by giving a mean absolute deviation of 2.90 kcal mol^–1 and a root-mean-square deviation of 3.91 kcal mol^–1. As subsets are constructed with decreasing upper limits of reported experimental uncertainties (5.0, 4.0, 3.0, 2.0, and 1.0 kcal mol^–1), the ccCA-TM mean absolute deviations were observed to monotonically drop off from 4.35 to 2.37 kcal mol^–1. In contrast, such a trend is missing for DFT methods as exemplified by B3LYP and M06 with mean absolute deviations in the range 12.9–14.1 and 10.5–11.0 kcal mol^–1, respectively. Salient multireference character, as demonstrated by the <i>T</i>₁/<i>D</i>₁ diagnostics and the weights (<i>C</i>₀²) of leading electron configuration in the complete active self-consistent field wave function, was found in a significant amount of molecules, which can still be accurately described by the single reference ccCA-TM. The ccCA-TM algorithm has been demonstrated as an accurate, robust, and widely applicable model chemistry for 3d transition metal-containing species with versatile bonding features.