We describe the development of a ligand knowledge base designed to capture the properties of C-donor ligands coordinating to transition metal centres, LKB-C. This knowledge base has been developed to describe both singlet (Arduengo and Fischer) and triplet (Schrock) carbenes, as well as related neutral monodentate C-donor ligands. The descriptors evaluated and used have been derived from a range of coordination environments to maximise their transferability and hence utility for the investigation of such ligands. These descriptors have been analysed with different statistical approaches, both individually to determine their chemical context, and collectively by principal component analysis thereby allowing the derivation of maps of ligand space for different ligand sets. The utility of such maps for investigating ligand similarity and identification of target areas for future ligand designs has been discussed. In addition, linear regression models have been fitted for the prediction of a calculated response variable, highlighting further potential applications of such a knowledge base.

We describe the development of a ligand knowledge base designed to capture the properties of C-donor ligands coordinating to transition metal centres, LKB-C. This knowledge base has been developed to describe both singlet (Arduengo and Fischer) and triplet (Schrock) carbenes, as well as related neutral monodentate C-donor ligands. The descriptors evaluated and used have been derived from a range of coordination environments to maximise their transferability and hence utility for the investigation of such ligands. These descriptors have been analysed with different statistical approaches, both individually to determine their chemical context, and collectively by principal component analysis thereby allowing the derivation of maps of ligand space for different ligand sets. The utility of such maps for investigating ligand similarity and identification of target areas for future ligand designs has been discussed. In addition, linear regression models have been fitted for the prediction of a calculated response variable, highlighting further potential applications of such a knowledge base.

The reductive elimination of 2-hydrocarbyl-imidazolium salts from hydrocarbyl–palladium complexes bearing N-heterocyclic carbene (NHC) ligands represents an important deactivation route for catalysts of this type. We have explored the influence that carbene N-substituents have on both the activation energy and the overall thermodynamics of the reductive elimination reaction using density functional theory (DFT). Given the proximity of the N-substituent to the three-centred transition structure, steric bulk has little influence on the activation barrier and it is electronic factors that dominate the barriers' magnitude. Increased electron donation from the departing NHC ligand acts to stabilise the associated complex against reductive elimination, with stability following the trend: Cl < H < Ph < Me < Cy < iPr < neopentyl < tBu. The intimate involvement of the carbene pπ-orbital in determining the barrier to reductive elimination means N-substituents that are capable of removing π-density (e.g. phenyl) act to promote a more facile reductive elimination.

The effect of substituents at the heteroatom on the electronic structures of different N-heterocyclic carbenes (1, 2 and 3), silylene (4) and germylene (5) are examined using Density Functional Theory. The kinetic and thermodynamic stabilities of these molecules are assessed by examining the HOMO–LUMO gap and hydrogenation energies, respectively. The extent of cyclic electron delocalization present in these five-membered ring systems are quantified with the help of NICS calculations. The ligating properties of 1–5 and the recently synthesized free abnormal carbene 6 (Bertrand et al., Science, 2009, 326, 556–559) are examined by looking at the energies of the σ symmetric electron-donating orbital of the respective molecules. Among the systems considered, 6 is found to have the strongest σ-donating ability. A comparative study of the ligating properties between the two isomeric carbenes 1 and 6 is performed by calculating the carbonyl stretching frequencies of some iridium carbonyl complexes of these two tautomeric carbenes.

The effect of substituents at the heteroatom on the electronic structures of different N-heterocyclic carbenes (1, 2 and 3), silylene (4) and germylene (5) are examined using Density Functional Theory. The kinetic and thermodynamic stabilities of these molecules are assessed by examining the HOMO–LUMO gap and hydrogenation energies, respectively. The extent of cyclic electron delocalization present in these five-membered ring systems are quantified with the help of NICS calculations. The ligating properties of 1–5 and the recently synthesized free abnormal carbene 6 (Bertrand et al., Science, 2009, 326, 556–559) are examined by looking at the energies of the σ symmetric electron-donating orbital of the respective molecules. Among the systems considered, 6 is found to have the strongest σ-donating ability. A comparative study of the ligating properties between the two isomeric carbenes 1 and 6 is performed by calculating the carbonyl stretching frequencies of some iridium carbonyl complexes of these two tautomeric carbenes.