Ligand-field theory applied to diatomic transition metals. Results for the d A9d B9σ2 states of Ni2, the dNi9dCu10σ2 states of NiCu, and the dNi8(3F)dCu10σ2σ*1 excited states of NiCu

A ligand-field theory has been developed for transition-metal diatomics having electronic configurations of dA9dB10σ2, dA9dB9σ2, and dA8(3F)dB10σ2σ*1. The theory treats each atom as a point charge and includes spin–orbit interactions. No contributions due to d-orbital chemical bonding are included. Since the d orbitals are quite small compared to the bond lengths in these molecules, the only inputs to the theory are the ligand charges (ZA and ZB), the radial expectation values <rA2≳nd, <rB2≳nd, <rA4≳nd, and <rB4≳nd, the atomic spin–orbit parameters ζA and ζB, and the bond length, R. Calculations employing no adjustable parameters (setting ZA, B =+1.0, and using radial expectation values and spin–orbit parameters from atomic tables) provide essentially quantitative agreement with ab initio results on the dNi9dCu10σ2 manifold of states in NiCu, and on the dA9dB9σ2 manifold of states in Ni2. This demonstrates that the ligand-field model has some validity for metal molecules containing nickel, primarily because of the compact nature of the 3d orbitals in this element. Similar calculations of the dA9dB9σ2 manifold of states in Pt2 and the dNi9dPt9σ2 manifold of states in NiPt are presented for comparison to future ab initio or experimental measurements, although the possibility of d-orbital contributions to the bonding in these species makes the ligand-field model less favorable in these examples. The dNi8(3F)dCu10σ2σ*1 excited electronic states of NiCu, which are well known from resonant two-photon ionization spectroscopy, are also investigated in the ligand-field model. As a final example, the dNi8(3F)σ2σ*1 excited electronic states of NiH are also examined using the same treatment as that employed for the dNi8(3F)dCu10σ2σ*1 excited manifold of NiCu.