Gas-phase chemistry of ruthenium and rhodium carbonyl complexes

Abstract

Short-lived ruthenium and rhodium isotopes were produced from a ²⁵²Cf spontaneous fission (SF) source. Their volatile carbonyl complexes were formed in gas-phase reactions in situ with the carbon-monoxide containing gas. A gas-jet system was employed to transport the volatile carbonyls from the recoil chamber to the chemical separation apparatus. The gas-phase chemical behaviors of these carbonyl complexes were studied using an online low temperature isothermal chromatography (IC) technique. Long IC columns made up of FEP Teflon were used to obtain the chemical information of the high-volatile Ru and Rh carbonyls. By excluding the influence of precursor effects, short-lived isotopes of ^109–110Ru and ^111–112Rh were used to represent the chemical behaviours of Ru and Rh carbonyls. Relative chemical yields of about 75% and 20% were measured for Ru(CO)₅ and Rh(CO)₄, respectively, relative to the yields of KCl aerosols transported in Ar gas. The adsorption enthalpies of ruthenium and rhodium carbonyl complexes on a Teflon surface were determined to be around ΔH_ads = –33⁺¹₋₂ kJ mol⁻¹ and −36⁺²₋₁ kJ mol⁻¹, respectively, by fitting the breakthrough curves of the corresponding carbonyl complexes with a Monte Carlo simulation program. Different from Mo and Tc carbonyls, a small amount of oxygen gas was found to be not effective for the chemical yields of ruthenium and rhodium carbonyl complexes. The general chemical behaviors of short-lived carbonyl complexes of group VI–IX elements were discussed, which can be used in the future study on the gas-phase chemistry of superheavy elements – Bh, Hs, and Mt carbonyls.

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Introduction

The synthesis of superheavy elements (SHEs; Z ≥ 104) and their chemistry studies are attractive but challenging in the field of nuclear science. The superheavy elements can only be produced in heavy-ion reactions at the “one-atom-at-a-time” level with very short half-lives. These extreme conditions require the development of unique components and methods to achieve the ultimate goal of chemically isolating one single atom that lives for only a few seconds. Due to very strong relativistic effects on the valence electron shells, the SHEs should behave dissimilarly to their lighter homologs in the chemical groups.¹ Significant deviations of the chemical behaviors of SHEs require the studies of their lighter homologs, which can provide corresponding chemical information, and contribute to the further studies of SHEs.

Simple inorganic compounds like oxides, halides, hydroxides, and oxyhalides of SHEs and their homologs have been investigated in the past few decades.^2–5 In recent years, transition metal – organic complexes have been drawing much attention, and the new classes of carbonyl complexes opened the perspectives to the chemical studies of SHEs.¹ As these complexes were known to be volatile under ambient conditions, gas-phase chromatography techniques were used to study their chemical properties.^6–11

The EAN rule (effective atomic number rule), known as the 18-electron rule, primarily gives the qualitative predictions for the coordination and structures of many transition metal complexes.¹² In previous work, Mo(CO)₆ and W(CO)₆ were observed in the gas-phase experiments,⁷ and were confirmed in matrix isolation experiments.¹³ For groups VII–IX, Tc(CO)₅, Ru(CO)₅, and Rh(CO)₄ were proposed according to the 18-electron rule.¹⁴ In other studies, Tc(CO)₅ and Re(CO)₅ were considered to be the only possible structures, which should thermodynamically exist under specific conditions.^10,11 Short-lived Re and Ir carbonyls were studied using a TransActinide Separator and Chemistry Apparatus (TASCA),¹⁰ and the coordination numbers of ¹⁷⁰Re(CO)_x and ¹⁷⁸Ir(CO)_x were proposed to be x = 5 and 4, respectively.¹⁰

The chemical behaviors of Mo and W carbonyl complexes were studied systematically in ref. 8, 10, 11 and 14. Furthermore, the first synthesis of an organometallic compound of a superheavy element, seaborgium hexacarbonyl (Sg(CO)₆), was recently successfully achieved, and its adsorption enthalpy on the quartz surface was deduced.⁹ So far, the gas-phase chemistry of carbonyl complexes of group VI has been studied adequately. The chemical information on group VI–IX carbonyls was reported by Even et al.¹⁰ The adsorption enthalpy of Tc carbonyls and the thermal stabilities of Mo, W, Tc, Ru, and Rh carbonyl complexes were also studied in other groups.^8,11,14 However, in ref. 10, the authors did not give final conclusions on the adsorption enthalpies or chemical yields of Tc, Ru or Rh carbonyls, since these observed isotopes were influenced by strong precursor effects in the experiment with a ²⁴⁹Cf target irradiated at the nuclear reactor; no chemical yields of Ru and Rh carbonyls were reported. Therefore, the systematic studies on Ru and Rh carbonyl complexes are requisite and urgent for the future chemical investigation of carbonyl complexes of element 108, Hs and element 109, Mt, respectively.

In the present work, short-lived ruthenium and rhodium isotopes were generated from a ²⁵²Cf spontaneous fission (SF) source, and their volatile carbonyl complexes were formed in gas-phase reactions. ^109–110Ru and ^111–112Rh carbonyls, which could exclude the precursor effects, were studied using a low-temperature IC technique. The adsorption enthalpies were obtained from the breakthrough curves of the corresponding carbonyl complexes. The relative chemical yields and the influence of a small amount of oxygen gas were also studied.

Experimental

The same experimental setup was used in the previous work,¹¹ see Fig. 1. The system consists of (a) the gas-jet transport system, (b) the ²⁵²Cf source chamber, (c) the low-temperature isothermal chromatography apparatus, and (d) the collecting, detection, and data acquisition system. The components were connected via fluorinated ethylene propylene (FEP) Teflon tubes. For more details, please see ref. 11.

Fig. 1 Schematic of the experimental setup ((1) mass flow controllers; (2) two-way valves; (3) three-way valves; (4) ²⁵²Cf source chamber; (5) isothermal chromatography column at low temperature; (6) bypass; (7) charcoal filter; (8) HP Ge-γ detector; (9) KCl aerosol generator; (10) ²⁵²Cf fission source; (11) 0.5 μm-thick polyester film covering; (12) Teflon cylinder collector; (13) pressure gauge).

According to the Monte Carlo simulation, a 5-m-long FEP IC column (i.d. = 4 mm) was used to obtain the complete breakthrough curves of high-volatile ruthenium carbonyls in the limited temperature range. And a 3-m-long (i.d. = 2 mm) FEP IC column was used in the rhodium carbonyl experiment.

All experiments were performed at 19 °C and at 1.05 bar with a total gas flow rate of 1000 mL min⁻¹, except for these with oxygen containing gas with a gas flow rate of 1500 mL min⁻¹. The online collecting time for each experiment was 30 min.

Results and discussion

Formation of Ru and Rh carbonyl complexes

Fig. 2 shows the typical γ-ray spectra of ²⁵²Cf fission fragments transported with KCl aerosols in Ar gas. All fission fragments with suitable half-lives and γ-ray energies can be found without chemical selectivity. Fig. 3 shows only the volatile species among the fission products, e.g.^137–139Xe, transported in pure Ar gas without any aerosol.

Fig. 2 γ-ray spectra of fission products transported in Ar gas with KCl aerosol particles. The total gas flow rate was 1 L min⁻¹. The sample was collected and detected on-line for 30 min.

Fig. 3 γ-ray spectra of fission products transported in pure Ar gas. The total gas flow rate was 1 L min⁻¹. The sample was collected and detected on-line for 30 min.

When CO gas was added to Ar gas, several Mo, Tc, Ru, and Rh isotopes appeared in the spectra, see Fig. 4. This proved that volatile carbonyl complexes of these 4d elements were synthesized and transported from the ²⁵²Cf source chamber to the charcoal trap.

Fig. 4 γ-ray spectra of fission products transported in a mixed gas of 50% Ar and 50% CO. The gas flow rate was 1 L min⁻¹. The total sample was collected and detected on-line for 30 min.

Since there is no direct experimental evidence to provide any information on the configuration of Ru or Rh complexes under such conditions, the structures of these complexes can only be inferred theoretically.

As the production rates of metal atoms from the ²⁵²Cf fission source are very low (not higher than few thousands per second), the probability of atoms meeting each other in the gas-phase, and forming polynuclear complexes, e.g. M_x(CO)_y (x ≥ 2), can be excluded. It is convinced that these short-lived isotopes would form mononuclear complexes rather than polynuclear complexes.^10,11

Fig. 5 shows the valence electron (VE) configurations of central atoms in Ru, Rh, Mo, and Tc carbonyls according to the valence bond (VB) theory. A free ruthenium atom has 8 VEs. The bonding arrangement results in four paired electrons in two of the 4d orbitals and four unpaired electrons in the other three 4d orbitals and 5s orbitals. When ruthenium forms a carbonyl complex, one of the 4d orbitals, one 5s and three 5p orbitals are made available to form a set of empty dsp³ hybrid orbitals by means of electron pairing.¹² Five empty orbitals can accept at most five pairs of electrons which are donated by CO ligands, and reach the number of electrons in the next noble gas atom. As a result, Ru(CO)₅ is the final saturated structure, which is known to be stable in macrochemistry, see Fig. 5a.

Fig. 5 The valence electron configurations of central atoms in Ru(CO)₅, Rh(CO)₄, Mo(CO)₆, and Tc(CO)₅.

In the formation of rhodium carbonyls, 9 VEs from the Rh atom require all the five 4d orbitals, and one electron remains unpaired. Only one 5s and three 5p orbitals are used in the hybrids. The resulting sp³ hybrid orbitals can accommodate at most four pairs of electrons, and for CO ligands the maximum coordination number is 4. In this case, Rh(CO)₄ gives a total of 17 electrons in the valence shell of Rh, see Fig. 5b. Since the formation of polynuclear carbonyls is excluded, this may be the only possibility which is closest to the stable structure according to the EAN rule.

In the same way, the maximum coordination number of Mo and Tc mononuclear carbonyls, which were mentioned in previous studies,^7–11 can also be inferred to be 6 and 5, respectively, see Fig. 5c.

In a laser-ablation matrix-isolation infrared spectroscopy experiment,¹³ the free Mo/W atoms form carbonyl complexes directly in CO and inert gas mixtures. The low-coordinate complexes were found to be spontaneously transformed into high-coordinate complexes under such conditions, and saturated Mo(CO)₆/W(CO)₆ was the final stable product. It can be deduced that the low-coordinate carbonyl complexes of Ru/Rh have the same trend to transform into saturated complexes under the same conditions. Therefore, Ru(CO)₅ and Rh(CO)₄ should be the final products in reactions with free atoms and CO containing inert gas.

Additionally, Bogdan et al. reported that the ruthenium carbonyls like Ru(CO)₅ could be observed using transient infrared spectroscopy.^15,16 Ozin et al. reported that Rh(CO)_1–4 was observed in solid argon in earlier thermal atom experiments.¹⁷ Even et al. also proposed that ruthenium and rhodium were most likely to form pentacarbonyl ruthenium and tetracarbonyl rhodium under similar conditions.¹⁰ Though Ru(CO)₅ and Rh(CO)₄ could be reactive and unstable in macrochemistry, they are the only possible binary, mononuclear, neutral and saturated chemical forms in the current study.

Relative chemical yields

In this work, the first step is to decide which isotopes can be used to represent the corresponding elements.

In Even's work,¹⁰ short-lived isotopes of ^106–107Tc, ^107–108Ru and ^{107–110,110m}Rh produced using the ²⁴⁹Cf source were identified in the spectra. The transport yields of ^106–107Tc, ¹⁰⁸Ru, and ¹⁰⁹Rh as a function of CO concentrations were illustrated.

But these isotopes were all influenced by precursor effects, and cannot represent the behaviors of the corresponding elements. As a result, no quantified chemical information of ruthenium and rhodium could be deduced in that study.

In our recent study,¹¹ for a certain isotope the precursor effects can be excluded only if (1) the fraction (F_r) of the independent yields (Y_i) to the cumulative yields (Y_c) is large enough, and (2) the precursor's half-life is short enough to decay in the recoil chamber.

The Y_i and Y_c values of short-lived Tc, Ru, and Rh isotopes observed in the γ-ray spectra are shown in Table 1. For ruthenium, most of the ¹⁰⁷Ru (t_1/2 = 3.8 min) atoms are formed from the β⁻-decay of ¹⁰⁷Tc (t_1/2 = 21.2 s), and only 13% are formed directly. This means that the ¹⁰⁷Tc atoms form carbonyl complexes and pass through the IC column, but give the signals of ¹⁰⁷Tc and ¹⁰⁷Ru in the charcoal trap during the measurement, giving strong precursor effects on the daughter ¹⁰⁷Ru. ¹⁰⁸Tc and ¹⁰⁸Ru are in the same situation. These two isotopes should show almost the same behaviors as ¹⁰⁸Tc.

Table 1 The half-lives (t_1/2), independent yields (Y_i), and cumulative yields (Y_c) of Tc, Ru, and Rh isotopes formed using the ²⁵²Cf SF source.¹⁹F_r shows the fraction of Y_i in the corresponding Y_c as a percentage

Isobar	Nuclide	Half-life	Independent yield (Yi)	Cumulative yield (Yc)	Fraction (%) (Fr)
107	^107Tc	21.2 s	3.630	5.730	63
	^107Ru	3.8 min	0.872	6.600	13
108	^108Tc	5.17 s	3.330	4.010	83
	^108Ru	4.5 min	1.980	5.990	33
109	^109Tc	0.86 s	1.890	2.040	93
	^109Ru	34.5 s	2.990	5.030	59
	^109mRh	50 s	0.254	2.770	9
	^109Rh	1.34 min	0.652	5.930	11
110	^110Tc	0.83 s	0.855	0.878	97
	^110Ru	11.6 s	3.620	4.500	80
	^110mRh	29 s	0.675	0.675	100
	^110Rh	3.1 s	0.672	5.170	13
111	^111Ru	2.12 s	2.260	2.440	93
	^111Rh	11 s	2.460	4.890	50
112	^112Ru	1.75 s	0.939	0.962	98
	^112Rh	6.73 s	2.390	3.350	71

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For ¹⁰⁹Ru (t_1/2 = 34.5 s; E_γ = 206 keV, I_γ = 22%; E_γ = 226 keV, I_γ = 20%), the Y_i value is large enough (59%), and the half-life of its mother ¹⁰⁹Tc (t_1/2 = 0.86 s) is too short to be transported from the recoil chamber to the charcoal trap. This means that the precursor effect of ¹⁰⁹Tc on ¹⁰⁹Ru can be completely excluded. Similarly, ¹¹⁰Ru (t_1/2 = 11.6 s; E_γ = 112.2 keV, I_γ = 23%) will not be influenced by precursor effects, since its mother ¹¹⁰Tc (t_1/2 = 0.83 s) has a very short half-life.

For rhodium, the isotopes of ^109–112Rh can be observed. In view of above-mentioned reasons, ^109–110Rh is strongly affected by the precursor effects and showed the same behavior as ^109–110Ru. Only ¹¹¹Rh (t_1/2 = 11 s; E_γ = 275 keV; I_γ = 100%) and ¹¹²Rh (t_1/2 = 6.8 s; E_γ = 348.7 keV; I_γ = 89%) can represent the behavior of element Rh.

The relative chemical yields of the investigated carbonyl complexes as a function of CO concentration are shown in Fig. 6 and 7. The data were fixed by the constant yields of the volatile fission product ¹³⁹Xe, to eliminate the influence of the gas flow rate. The yields are given relative to those of KCl aerosol transport. The relative chemical yields of ^109–110Ru carbonyl complexes reach the maximum of about 75% (Fig. 6). Meanwhile, the maximum yields of ^111–112Rh approach about 20% (Fig. 7). As mentioned above, the maximum yields of ^109–110Rh isotopes are far higher than those of ^111–112Rh, and similar to their precursors ^109–110Ru. For all these carbonyl complexes, the relative chemical yields reach the maximum basically when CO concentration is higher than 40%.

Fig. 6 Relative chemical yields of ^109–110Ru as a function of CO concentration in Ar carrier gas, normalized to the yields obtained with the KCl aerosol transport in Ar gas, and corrected by the yields of the volatile fission product of ¹³⁹Xe. The total gas flow rate was 1 L min⁻¹. The sample was collected and detected on-line for 30 min.

Fig. 7 Relative chemical yields of ^109–112Rh as a function of CO concentration in Ar carrier gas, normalized to the yields obtained with the KCl aerosol transport in Ar gas, and corrected by the yields of the volatile fission product of ¹³⁹Xe. The total gas flow rate was 1 L min⁻¹. The sample was collected and detected on-line for 30 min.

The results show the difference in chemical yields of the carbonyl complexes of group VIII and group IX. According to the EAN rule, the numbers of effective electrons in Ru(CO)₅ are 18, and it suggests that Ru(CO)₅ has a stable structure, and may have a relatively high chemical yield. However, Rh(CO)₄ has an unstable structure, because of its 17 effective electrons. This could make its chemical yields much lower than Ru(CO)₅.

Adsorption enthalpies

Fig. 8 shows the breakthrough curves of ^109–110Ru with a 5-m-long FEP IC column (i.d. = 4 mm) using 1 L min⁻¹ mixed gas (Ar : CO = 1 : 1; 1.05 bar; 19 °C). Since the Ru carbonyls are very volatile,⁸ a Monte Carlo simulation program¹⁸ was used to select the length and the radius of the IC column according to the estimated adsorption enthalpy values and simulated breakthrough curves. At each experimental temperature, the products were collected and measured on-line for 30 minutes.

Fig. 8 Breakthrough curves of ^109–110Ru and ^109–110Rh carbonyls with a 5-m-long FEP Teflon IC column (i.d. = 4 mm). The symbols (black boxes and circles) are the experimental data. The solid curves are the results of the Monte Carlo simulations¹⁸ with the corresponding adsorption enthalpies. For Rh isotopes, the curves are consistent with the simulation results of their precursors.

In Fig. 8, the simulation results show a near-perfect match with the breakthrough curves. By fitting the experimental data with the Monte Carlo simulation, the adsorption enthalpy of Ru carbonyls was determined to be ΔH_ads = –33⁺¹₋₂ kJ mol⁻¹ on the FEP surface. The T_50% values, where 50% of the molecules pass the IC column, are −74 °C and −66 °C for ¹⁰⁹Ru and ¹¹⁰Ru carbonyls, respectively, due to the difference of their half-lives.

Since most of the ¹⁰⁹Rh atoms are formed from the β⁻-decay of ¹⁰⁹Ru in the charcoal trap, the precursor effects strongly affect ¹⁰⁹Rh. The breakthrough curve of ¹⁰⁹Rh is similar to ¹⁰⁹Ru and shows an equal T_50% value (Fig. 8a and b). ¹¹⁰Rh is in the same situation and shows the behaviors of ¹¹⁰Ru (Fig. 8c and d).

Fig. 9 shows the breakthrough curves of ^111–112Rh with a 3-m-long FEP IC column (i.d. = 2 mm), fitting with the Monte Carlo simulation. The gas flow rate was 1 L min⁻¹ (Ar : CO = 1 : 1; 1.05 bar; 19 °C). An adsorption enthalpy of ΔH_ads = −36⁺²₋₁ kJ mol⁻¹ was deduced for Rh carbonyls on the FEP Teflon surface. The T_50% appears at −58 °C for both ¹¹¹Rh and ¹¹²Rh. In contrast to Ru carbonyls, Rh carbonyl complexes are less volatile and have lower relative chemical yields.

Fig. 9 Breakthrough curves of Rh carbonyls on the FEP Teflon surface, fitting with Monte Carlo simulation. The symbols are the experimental data. The solid curves are the results of Monte Carlo simulations¹⁸ with the corresponding adsorption enthalpies.

Influence of oxygen on the chemical yields of ruthenium and rhodium carbonyl complexes

In previous studies, a trace amount of O₂ was found to have a strong negative influence on chemical yields of Mo, Tc, and W carbonyl complexes.^10,11 Wang et al. reported that 2% of oxygen gas already makes the relative yields drop from 100% to 70% for Mo, and from 25% to 15% for Tc. With 6% of O₂ gas, about 70% of Mo and Tc carbonyls were lost.¹¹

To verify such an influence on carbonyl complexes of group VIII and group IX elements, experiments under the same conditions were performed. A small amount of O₂ was added into the gas mixture (Ar : CO = 1 : 1) with a total flow rate of nearly 1.5 L min⁻¹. The relative chemical yields of Ru and Rh were obtained in different O₂ concentrations. The relative chemical yields of corresponding carbonyls were obtained, relative to the transport yields with KCl aerosol in Ar gas.

In Fig. 10, we can see that with the O₂ concentrations varying from 0% to 17%, the relative chemical yields of Ru and Rh carbonyls remain basically constant, which remain around 70% and 20%, respectively. Thus, no significant influence of O₂ gas was observed on the chemical yields of Ru and Rh carbonyls under the present experimental conditions.

Fig. 10 Relative chemical yields of Ru and Rh isotopes as a function of the O₂ concentration in the mixed Ar and CO gas, normalized to the yields obtained with the KCl aerosol transport in Ar gas, and corrected by the yields of the volatile fission product of ¹³⁹Xe in each γ-spectrum. The dashed lines show the average values of relative chemical yields of Ru and Rh carbonyls.

Gas-phase behaviors of group VI–IX element carbonyl complexes

The chemical information of group VI–IX element carbonyl complexes is shown in Table 2. The VE numbers of the corresponding carbonyls are listed. For group VI and group VIII elements, their carbonyls have 18 VEs, and should be stable according to the EAN rule. Not surprisingly, they were found to have high chemical yields in the experiments. For group VII and group IX elements, their carbonyls have 17 VEs. They would be unstable and low-yield according to the EAN rule. The low chemical yields were confirmed in the present experiments.

Table 2 Chemical information on the Mo/W, Tc/Re, Ru/Os, and Rh/Ir carbonyls

Group	VE numbers	Complexes	Relative chemical yield (%)	−ΔHads on the Teflon surface (kJ mol^−1)	−ΔHads on the SiO2 surface (kJ mol^−1)
a Data from ref. 11. b Data from ref. 10. c Data from ref. 8.
VI	18	Mo(CO)6	100^a	38 ± 2^a	39 ± 2^a
		W(CO)6			46.5 ± 2.5^b
VII	17	Tc(CO)5	25^a	43 ± 2^a	43 ± 2^a
		Re(CO)5			43 ± 3^b
VIII	18	Ru(CO)5	75	33^+2−1	35 ± 1^c
		Os(CO)5			43.5^+3.5−2.5^b
IX	17	Rh(CO)4	20	36^+1−2	37 ± 3^b
		Ir(CO)4

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For group VI and group VIII elements, the −ΔH_ads values of W/Os carbonyls on the quartz surface are about 8 kJ mol⁻¹ higher than those of Mo/Ru carbonyls. However, for group VI and group VIII elements, Re/Ir and Tc/Rh carbonyls have basically the same adsorption enthalpies, respectively.

This work gives important information for the future study on the carbonyls of SHEs – ₁₀₈Hs and ₁₀₉Mt. If their chemical properties are not strongly affected by the relativistic effects, it may suggest that Hs carbonyl complexes will have higher stability than Mt carbonyl complexes.

Additionally, the Hs carbonyls may be less volatile than Os carbonyls, while the ΔH_ads value of Mt carbonyl complexes may still be close to that of Ir carbonyls, which is around −37 kJ mol^–1.

Conclusions

Short-lived ruthenium and rhodium carbonyl complexes were synthesized and their gas-phase chemistry was studied using an online isothermal chromatography apparatus coupled to a ²⁵²Cf SF source. In this work, the VB theory and the EAN rule were used to deduce the coordination and VE configuration of the 4d element carbonyls formed in gas-phase reactions of ²⁵²Cf SF fission fragments and CO gas. As a result, Mo(CO)₆, Tc(CO)₅, Ru(CO)₅, and Rh(CO)₄ should be the final products in the current study. Chemical yields of 75% and about 20% were measured for Ru(CO)₅ and Rh(CO)₄, respectively. The CO concentration of 40% was found to be enough to give the highest yield for Ru and Rh carbonyls in the Ar/CO gas mixture.

By fitting the breakthrough curves with the Monte Carlo simulation, the adsorption enthalpies of Ru(CO)₅ and Rh(CO)₄ on the FEP Teflon surface were deduced to be around ΔH_ads = −33 kJ mol⁻¹ and −36 kJ mol⁻¹, respectively. Furthermore, a small amount of oxygen gas was found to have no remarkable influence on the chemical yields of Ru and Rh carbonyls, which was totally different from Mo and Tc carbonyls.

In combination with the previous studies, the general chemical behaviors of group VI–IX element carbonyl complexes were discussed. And several important information was provided for the future study on the gas-phase chemistry of SHEs – Bh, Hs and Mt.

Acknowledgements

The present work was supported by the National natural Science Foundation of China (Grant No. 11079006, 11505250, and 11575260) and the West Light Foundation of Chinese Academy of Sciences (CAS).

Notes and references

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