Introduction

Chemical bonding is among the most fundamental concepts in modern chemistry. Depending upon the availability of valence orbitals and electrons, an atom can form single or multiple covalent chemical bonds with neighboring atoms in forming stable compounds. Since the discovery of Re–Re quadruple bond in 1964 by Cotton and coworkers1,2, metal–metal multiple bonding with bond orders of four to six has been extensively explored for transition metals and actinides3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18. Main group elements of the periodic table may form single, double, and triple bonds between two atoms, that is, the maximum bond order can only be three as exemplified in alkyne. Triple bonds between two main group atoms are well established, and a large number of triple-bonded molecules are known19,20,21,22,23,24,25,26. As main group elements have nsnp four valence orbitals accessible for bonding, it has been speculated that a further extension to bond order of four for main group elements should in principle be possible. Diatomic C2 and its isoelectronic molecules CB−, BN, and CN+ each having eight valence electrons were claimed to be quadruple-bonded molecules, comprising not only one σ- and two π-bonds, but also one weak ‘inverted’ bond27. However, this quadruple bond assignment has been debated, which has stimulated hot discussion28,29,30. Quadruple bonding of carbon to uranium with rich valence shell (sdf) has been reported to exist in the triatomic uranium carbide oxide molecule CUO and related species, due to availability of both 2p and unhybridized 2s orbitals of carbon46 analyses shown in Supplementary Table 2. The semi-localized results indicate that the B–Fe bonding interactions in BFe(CO)3− involve two 5c-2e σ bonds and two 5c-2e π bonds. It should be mentioned that here there is some mixing of the B–Fe and Fe-CO bonding MOs, but the major components and shapes of MOs resemble those of the classical electron-pair bonding models. The above bonding analysis also indicates that the B–Fe bonding interactions have quite small effect on the Fe-to-CO 2π* back-donation interactions in the Fe(CO)3− fragment. Charge analyses in Supplementary Table 4 also indicate that the negative charge is largely located on the Fe(CO)3 moiety. The antisymmetric CO stretching frequency (1841 cm−1) of BFe(CO)3− is only slightly higher than that of free Fe(CO)3− (1780 cm−1), but is lower than those of neutral Fe(CO)4 and Fe(CO)5.

Fig. 3
figure 3

The canonical Kohn-Sham valence molecular orbitals of BFe(CO)3−. The B–Fe nonbonding (11e), bonding (13a1, 10e, 14a1) and the corresponding antibonding molecular orbitals (17a1, 12e, 15a1) are plotted with isosurfaces = 0.05 au

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Figure 4 qualitatively illustrates the bonding picture of BFe(CO)3− and its correlation to the interacting fragments B and Fe(CO)3−. The major interactions between B and Fe(CO)3− involve one σp-type (14a1) Fe–B electron-sharing bonding, two degenerate π-type (10e) Fe→B dative bonding, and one σs-type (13a1) B→Fe dative bonding, leading to the aforementioned quadruple bonding. The ground state BFe(CO)3− anion can thus be regarded as being formed via the interactions between the ground state 2P-B atom with the (2s)2(2pz)1 configuration and the Fe(CO)3− fragment in its 2A1 electronic ground state with (a1)1(e)4(e)4 configuration. The B≣Fe quadruple bonding interactions are further characterized using the principal interacting orbital (PIO) approach47. The calculated PIOs and PIMOs shown in Supplementary Figs. 8–11 clearly reveal the fourfold bonding interactions between B and Fe in the BFe(CO)3− anion, which further support the above assignment.

Fig. 4
figure 4

Bonding scheme of the C3v structure of 1A1–BFe(CO)3−. The scheme qualitatively illustrates the bonding interactions between boron 2s-2p orbitals and Fe 3d/4p orbitals in the Fe(CO)3− fragment

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The strengths of the above-mentioned pairwise orbital interactions can be quantitatively estimated by the EDA-NOCV method48,49. The numerical results calculated at the M06-2X /TZ2P level are listed in Supplementary Table 5. The EDA-NOCV calculations suggest that 43% of the attractive forces in BFe(CO)3− are due to orbital (covalent) interactions. The breakdown of the orbital interaction term ∆Eorb into individual orbital contributions reveals that the four bonding components contribute 42.8% (σp), 46.6% (two π), and 8.2% (σs) of the orbital bonding interactions. Although the fourth component (σs B→Fe dative bonding interaction) is much weaker than the electron-sharing σp and two Fe→B dative π bonding interactions, it has a calculated interaction energy of 18.4 kcal/mol, indicating that it is a non-negligible, albeit weak, bonding interaction. The deformation densities Δρ(σ) and Δρ(π) that are connected to the σ and π interactions in BFe(CO)3− are displayed in Supplementary Table 5. The deformation densities Δρ show the direction of the charge flow and the orbitals that are involved, where the color code of the charge flow is red → blue. The results clearly indicate the electron-sharing bonding nature of the stronger σp interaction and the dative bonding character of the two π and the weaker σs interactions. The weaker σs interaction is strongly polarized with obvious charge flow from B atom to Fe(CO)3−. In contrast, the two degenerate π interactions show reversed charge flow from Fe(CO)3− to B.

To make certain that these single-configuration quantum chemical methods are reliable, the wavefunction of BFe(CO)3− is further examined in a multi-configurational framework. Ab initio CASSCF calculation involving 12 valence electrons in a space of 12 MOs was performed at the M06-2X optimized geometry, and the Löwdin natural orbitals (NOs) were analyzed using the CASSCF density matrix. The six strongly occupied and six weakly occupied natural orbitals and their occupation numbers (NOONs) are displayed in Supplementary Fig. 12. There is no significant occupation number of B–Fe antibonding NOs, 1σ*0.05π*0.182σ*0.11, indicating that the multi-reference feature of this molecule is not very large. The B≣Fe effective bond order (EBO)9 based on the ground-state CASSCF wavefunctions is calculated as 3.7, which is rather close to four and further supports the B≣Fe quadruple bonding assignment.

In summary, the BFe(CO)3− anion complex has been identified by a combined quantum chemical and experimental study. The anion is generated in the gas phase and is studied by mass-selected infrared photodissociation spectroscopy. The complex has been characterized to have a cylindrical BFe(CO)3− structure with C3v symmetry and a very short B–Fe bond distance. Electronic structure and chemical bonding analyses indicate that the complex exhibits an unprecedented B≣Fe quadruple bonding interactions, featuring one electron-sharing σ bond, two Fe→B dative π bonds as well as one additional weak B→Fe dative σ bonding interaction. The results extend the maximum bond order of boron element to four. This study reveals that a variety of such quadruple-bonded systems of main-group and transition-metal elements may exist in inorganic and organometallic chemistry.

Methods

Experimental details

The anion complexes were generated in the gas phase using a pulsed laser vaporization/supersonic expansion ion source. A bulk target compressed from an isotopically enriched (11B- or 10B-depleted) or natural abundance boron powder was used. The ions were produced from the laser vaporization process in expansions of helium seeded with 5−10% CO using a pulsed valve (General Valve, Series 9) at 0.5−1.0 MPa backing pressure. After free expansion and cooling, the anions were skimmed into a second chamber where they were pulse-extracted into a Wiley−McLaren type time-of-flight mass spectrometer. The anions of interest were mass-selected and decelerated into the extraction region of a second collinear time-of-flight mass spectrometer, where they were dissociated by a tunable IR laser. The tunable IR laser used is generated by a KTP/KTA//AgGaSe2 optical parametric oscillator/amplifier system (OPO/OPA, Laser Vision) pumped by a Nd: YAG laser, producing about 0.5−1.0 mJ per pulse in the range of 1600−2200 cm−1. Resonant absorption leads to fragmentation of the anion complex. The infrared photodissociation spectrum is obtained by monitoring the yield of the fragment anion as a function of the dissociation IR laser wavelength and normalizing to parent anion signal.

Computational methods

Geometry optimization and vibrational frequency calculations of the most stable structure of BFe(CO)3− were performed using the density functional theory (DFT) methods with the PBE50,51, B3LYP52,53, and M06-2X 54 functionals using the ADF 2016 program packages [ADF 2016.101, http://www.scm.com]. The Slater-type-orbital (STO) basis sets with the quality of triple-ζ plus two polarization functions (TZ2P)55 were applied with the consideration of scalar-relativistic (SR) effects at the zero-order regular approximation (ZORA)56. In order to obtain more accurate B–Fe bond distance, a series of single-point calculations at different B–Fe distances were carried out using a domain-based local pair natural orbital coupled cluster method, DLPNO-CCSD(T)57,58,59, with the def2-TZVP basis sets (11s6p2d1f)/[5s3p2d1f] for the B, C and O, and (17s11p7d1f)/[6s4p4d1f] for Fe60. The DLPNO-CCSD(T) calculations were performed with the ORCA program package61.

The chemical bonding properties were analysed by employing several different methods including adaptive natural density partitioning (AdNDP)46 and the principal interacting orbital (PIO)47. The def2-TZVP basis sets, (11s6p2d1f)/[5s3p2d1f] for the B, C, and O, and (17s11p7d1f)/[6s4p4d1f] for Fe in Gaussian 09 program62, were applied in the AdNDP calculations. For determining the most stable structure of BFe(CO)3−, BFe(CO)4−, and BFe(CO)5−, B3LYP with Dunning’s correlation consistent basis sets aug-cc-pVTZ63, (10s5p2d1f)/[4s3p2d1f] for B, C, and O, and (20s16p8d2f1g)/[7s6p4d2f1g] for Fe64 were applied with Gaussian 09 program. The PIO analyses were also carried out using the Gaussian 09 program with aug-cc-pVTZ basis sets.

The multi-configurational complete-active-space SCF (CASSCF)65 calculations were performed using the MOLPRO 2012.1 program package to examine the description of electronic configurations based on the DFT-optimized geometries with the Dunning’s correlation consistent basis sets aug-cc-pVTZ63, (10s5p2d1f)/[4s3p2d1f] for B, C, and O, and (20s16p8d2f1g)/[7s6p4d2f1g] for Fe64. The active space used 12 valence electrons in 12 orbitals (12e,12o), which includes six occupied orbitals and six unoccupied orbitals at the frontier region of the molecular orbitals.