Paired interacting orbitals (PIO) analysis of electronic structures of copper(I) complexes with hydrotris(pyrazolyl)borate ligands

The influence of the hydrotris(pyrazolyl)borate (tpzb) ligand substituents on the electron delocalization of the copper(I) center with carbon monoxide and triphenylphosphine, i.e. the copper(I) carbonyl complexes, [Cu{HB(3,5-iPr2pz)3}(CO)] (1), [Cu{HB(3,5-Me2pz)3}(CO)] (2), [Cu{HB(pz)3}(CO)] (3), [Cu{HB(3-CF3-5-Mepz)3}(CO)] (4), and the copper(I) triphenylphosphine complexes [Cu{HB(3,5-iPr2pz)3}(PPh3)] (5), [Cu{HB(3-CF3-5-Mepz)3}(PPh3)] (6), are studied by using paired interacting orbitals (PIO) analysis. Carbon monoxide is a weak σ-donor and strong π-acceptor ligand. The Cu(I)-CO interaction is entirely dominated by π back bonding between the two degenerate π∗ orbitals of carbon monoxide and two t2 type d orbitals of copper(I) ion. This interaction is clearly expressed by two PIOs, PIO-2 and PIO-3, and the strength of this π-acceptability is estimated by the summation of their overlap populations (ΣOP). On the other hand, triphenylphosphine is a strong σ-donor ligand. The Cu(I)-P interaction is dominated by donation from the P atom to the copper(I) ion and expressed by PIO-1, and the strength of this donative interaction (σ-donationability) is estimated by the overlap population of PIO-1. These results are consistent with experimental data of the CO stretching frequencies by IR spectroscopy and of 13C chemical shifts of 13CO and the broadness of the 31P signals of PPh3 by 13C and 31P NMR spectroscopy.