A B3LYP-DBLOC empirical correction scheme for ligand removal enthalpies of transition metal complexes: parameterization against experimental and CCSD(T)-F12 heats of formation

Average ligand removal enthalpies of 30 differently coordinated mono-nuclear fourth-row transition metal complexes taken from a database recently considered by Johnson and Becke [Can. J. Chem., 2009, 8, 1369] have been computed in the gas phase using unrestricted pseudo-spectral (LACV3P) and fully analytic (qzvp(-g)) B3LYP including a recently developed empirical dispersion correction. Heats of formation of neutral singlet reactants and neutral, potentially high spin, products have been taken from NIST's Organometallic Thermochemistry Database. Comparison of B3LYP-MM//qzvp(-g) and experimental average ligand removal enthalpies reveals a systematic error in the reported experimental enthalpies for manganese-containing complexes which is verified with high-level, CCSD(T)-F12//family of cc-pVTZ, explicitly correlated coupled-cluster methods. Other B3LYP-MM//qzvp(-g) error patterns give rise to a d-block localized orbital correction (DBLOC) scheme containing six transferable parameters that correct the functional's description of metal-ligand bonding, cation-pi, and dispersion interactions as well as metal and/or ligand multi-reference effects. Metal-ligand cation-pi and dispersion interactions have been fit to the monopole/induced-dipole, C-4/R-4, and induced-dipole/induced-dipole, C-6/R-6, interaction functions, respectively. This DBLOC model has been built upon a previously determined set of metal atom parameters which are necessary to properly describe the free metal atom reaction products. The final DBLOC model brings the mean unsigned error of B3LYP-MM//qzvp(-g) from 3.74 +/- 3.51 kcal/mol to 0.94 +/- 0.68 kcal/mol and corrects the functional's under binding in nearly every case. Several important connections among DBLOC parameters have been made.