Is CO a special ligand in organometallic chemistry?: Theoretical investigation of AB, Fe(CO)4AB, and Fe(AB)5 (AB = N2, CO, BF, SiO)

Carbon monoxide, CO, is ubiquitous ligand in organometallic and coordination chemistry. In the present paper we investigate the neutral isoelectronic molecules AB = N-2, CO, BF, and SiO and their coordination in the model complexes Fe(CO)(4)AB and Fe(AB)(5), using nonlocal density functional theory and a large, polarized STO basis set of triple-zeta quality (NL-SCF/TZ(2P)). Our aim is to get more insight into the ligating properties of SiO and BF in comparison to CO and N-2. The computed 298 K Fe(CO)(4)-AB bond dissociation enthalpies of C-3v- symmetric symmetric Fe(CO)(4)AB are 18.1, 42.3, 67.9, and 35.6 kcal/mol for N-2, CO, BF, and SiO, respectively; the corresponding values for C-2v-symmetric Fe(CO)(4)AB are comparable: 19.0, 42.3, 66.7, and 39.7 kcal/mol. Good, balanced sigma donation (through 5 sigma) and pi acceptance (through 2 pi) are what makes CO a good donor, of course, The gap between these frontier orbitals (5 sigma and 2 pi) becomes even smaller in SiO and BF. The analysis of the bonding mechanism of the Fe-AB bond shows that SiO is a better sigma donor but a worse pi acceptor ligand than CO and that BF should be superior to CO in terms of both sigma donor and pi accepter properties. However, these polar ligands are therefore also more reactive: and more sensitive, e.g. to nucleophilic attack, because of a low-energy 2 pi LUMO. Our results suggest that BF and SiO should, in principle, be excellent ligands. We also interesting side-on and O-bound local minima, not very unstable, for SiO bound to an Fe(CO)(4) fragment.