The destabilization of a 1 Δ compared to X 3 Σ -is rationalized by Hund's rule. The highly excited c 1 Σ + state carries the same multi-reference electronic configuration as a 1 Δ but is likely higher in energy due to potential mixing with other excited states. The heavily multi-reference A 3 Φ, B 3 Π, and d 1 Π states are generated by promoting one electron from a 1π orbital of X 3 Σ -to 1δ orbitals (1dxy and 1d " ! #$ ! ) with various compositions ( HfCO is the b 5 Δ state that has the unique 1σ 2 2σ 1 1π 2 1δ 1 configuration. Similar to the ground state, we can expect a bond order of two for b 5 Δ, which in fact is almost identical to its effective bond order, 1.9, as could be expected based on its 98% single-reference nature.