Within the first row transition metal (TM) dimer series, Co₂ should occupy an intermediate position between mid-row elements where high bonding orders imply significant “3d–3d” contribution to the bonding, and Ni₂ or Cu₂, which are described as primarily “4s–4s” bonded.

A mass spectrometric study¹ has yielded an estimate for the dissociation energy (1.69 ± 0.26 eV), but this value relied on many assumptions for the electronic and vibrational partition function.

Important information has been provided by the Co₂⁻ photodetachment study of Leopold and Lineberger,² the main points being ground state frequencies for Co₂ and Co₂⁻ of 280 ± 20 cm⁻¹ and 240 ± 15 cm⁻¹, respectively, and features indicative of excited states of Co₂ some 600 and 5000 cm⁻¹ above the ground state.

Optical studies on Co₂ isolated in cryogenic matrices have evidenced strong UV absorptions near 270–280 nm and 340 nm, as well as a much weaker feature centered around 442 nm.³

All these electronic transitions appeared to be broad and structureless.

Next, Dong et al⁴. using mass-selection/reneutralization of Co₂⁺confirmed the assignment to Co₂ isolated in solid argon of a broad, unstructured absorption extending from 425 to 485 nm.

More important, they obtained Raman progressions, using excitations in the 458–488 nm range, yielding ω_e = 296.8 ± 5.4 cm⁻¹, ω_ex_e = 2.2 ± 0.8 cm⁻¹ for the ground state, in agreement with the preceding gas phase measurement.²

In contrast, theoretical studies are numerous and their results conflicting.

An ab initio HF-CI study by Shim and Gingerich⁵ first predicted a ⁵Σ+g ground state, with a very long r_e = 2.56 Å, and a very low vibrational frequency, ω_e = 162 cm⁻¹.

Several DFT studies have been performed, and differ in their conclusions regarding the ground state nature, suggesting either a ⁵Δ_g or a ⁵Σ ground state.^6–12

Barden et al. warn in their systematic study of TM dimers¹⁰ that many states with different electron configurations and spin multiplicities lie very close, thus the predicted ground state depends very much on the method used, which indicates the necessity for multi reference ab initio calculations for a correct approximation of the electronic wavefunction.

In absence of these, Gutsev et al¹¹. and Gutsev and Bauschlicher¹² have used various DFT methods to predict energies, bond lengths, magnetic moments and vibrational frequencies for neutral and charged Co₂⁻.

These DFT studies predict a ⁵Δ_g ground state with a relatively short bond length (1.98 Å), but the vibrational frequency (ω_e = 386–364 cm⁻¹, depending on the functional used), and the dissociation energies (D₀ = 2.51–2.24 eV) are well above the experimental values. ⁵Σ−u and ⁵Σ+g states of the same multiplicities were calculated 0.23 and 0.84 eV above the ground state and five other states of different multiplicities were also predicted to lie within 1 eV of the ground state, which underlines the high density of states presumable for Co₂.¹¹

Ideally, spectroscopic investigations of transition metal diatomics are best carried out in the gas phase, where rotationally resolved fluorescence¹³ or two-photon resonant ionization schemes,¹⁴ as developed by Morse and coworkers, can bring precise knowledge concerning bond length electronic structure or even dissociation energy.

These studies are best performed in solid neon, a less perturbing and polarizing medium than the heavier rare gases, and where electronic transitions are shifted by less than 1% in most cases.¹⁵

The experimental set-up has been described previously in detail.¹⁶

Under these conditions, samples are almost completely absorbing in the UV range and the reported weak optical absorption for Co₂ in argon near 445 nm (22 500 cm⁻¹)^3,4 dominates the spectrum.

Ozin and Hanlan³ observed the progressive “bleaching” of the Co₂ UV bands upon 270 nm selective irradiation and concomitant growth of Co atomic absorptions, implying that the molecule reaches from the upper state a rapidly dissociating channel and produces back Co atoms.

Also, Dong et al⁴. failed to obtain resonance Raman signals when using 364 nm UV excitation.

All bands presented a partly resolved structure, which is likely due to well-known matrix trapping site effects, as the less stable matrix sites could be annealed out by cycling the solid neon temperature up to 10 K and back (Fig. 2).

Recently, Jules and Lombardi²⁰ have extended to transition dimers empirical rules correlating force constants and equilibrium internuclear distances, such as Badger’s or Pauling’s rules.

Other cases of departure of Badger-type empirical rules have been discussed for excited states of TM metal dimers, such as for Re₂.²¹

Most of the accurate data on transition metal diatomics is obtained using two photon ionization schemes of jet-cooled molecules, these techniques yield rotationally resolved absorption data in bound excited states.

It also enables observation of abrupt changes in excited state lifetimes¹⁴ or drop in ionization signal²² as soon as the excitation energy exceeds the molecular binding energy.

The underlying basis is that, for this type of molecule, the density of excited states is very high, and, whenever above the binding energy, an excitation can no longer be considered as occurring to a single potential surface.

This is what has been observed for dimer absorptions in domain slightly above the binding energy for instance with Cu₂^23,24 or Ti₂.²⁵

Leopold and Lineberger²⁶ discuss their results concerning electron photodetachment of Fe₂⁻ and Co₂⁻ in terms of detachment of the least bound electron from a (3d)ⁿ(4s)² configuration of the anion toward two states of the neutral with (3d)ⁿ(4sσ_g)² (4sσ_u) configurations, the 4sσ_u unpaired electron being either low or high spin–coupled to the d electrons.