Analyzing the results of the MRCI potential energy scan, we observe that the Hf( 3 F)+CO(X 1 Σ + ) fragment produces three attractive states, X 3 Ʃ -, A 3 Φ, B 3 Π, and one strongly repulsive 3 Δ state. X 3 Ʃ -, A 3 Φ, and B 3 Π are the ground state and the third and fourth lowest-lying electronic states of the molecule, respectively. The first excited state (i.e., a 1 Δ) dissociates to the Hf( 1 D)+CO(X 1 Σ + ) asymptote. We analyzed the electron configurations of the electronic states and the associated molecular orbitals (Table and Figure ). These electron configurations highlight the σ-dative bonding between Hf and CO. Specifically, the dative interaction between these fragments is described by the doubly occupied 1σ orbital, which arises from 5d ! ! (Hf)+σ(CO) hybridization (Figure and ESI Figure ). The occupied 6s atomic orbital of Hf polarizes away from CO to facilitate an efficient σ-dative attack (2σ orbital of Figure ). Other than the σ-dative bond, a strong metal-to-ligand π back-donation (dxz/dyz of Hf to πx * /πy * of CO) was also observed in all electronic states. This π back-donation is evident from the 1πx and 1πy molecular orbitals of HfCO, which are occupied in all cases (Figure ). Specifically, according to natural bonding orbital analysis, 24% of the two electrons has back-donated from Hf to the two π* orbitals of CO in the ground state. On the other hand, only A 3 Φ, B 3 Π, b 5 Δ, and d 1 Π excited states occupy non-bonding 1dxy (1δxy) and 1d " ! #$ ! (1δ " ! #$ ! ) orbitals (Table and Figure ). We next analyzed the relationship between electron configuration and bond order expected in these complexes. The single-reference ground state of HfCO (X 3 Σ -) has a 1σ 2 2σ 2 1πx 1 1πy 1 (= 1σ 2 2σ 2 1π 2 ) dominant electron configuration. Based on its 1σ 2 1π 2 electron population, a double bond (i.e., Hf=C) can be expected for the ground state (Table and Figure ). Indeed, its calculated effective bond order based on the CASSCF weights is ~1.8, which agrees well with our qualitative expectation. The first excited state of HfCO (a 1 Δ) has the same electronic configuration as the ground state but with a different spin configuration. Indeed, the coupling of two π electrons in symmetrically equivalent 1πx and 1πy orbitals produces this multi-reference singlet electronic state.