Hole interactions are known by different names depending on the key atom of the bond (halogen bond, chalcogen bond, hydrogen bond, etc.), and the geometry of the interaction (σ if in line, π if perpendicular to the Lewis acid plane). However, its origin starts with the creation of a Lewis acid by an underlying covalent bond, which forms an electrostatic depletion and a virtual antibonding orbital, which can create non-covalent interactions with Lewis bases. In this (maybe subjective) perspective, we will claim that hole interactions must be defined via the molecular orbital origin of the molecule. Under this premise we can better explore the richness of such bonding patterns. For that, we will study old, recent and new systems, trying to pinpoint some misinterpretations that are often associated with them. We will use as exemplars the triel bonds, a couple of metal complexes, a discussion on convergent σ-holes, and many cases of anti-electrostatic hole interactions.

Yet another perspective on hole interactions

The replacement of a CH group of benzene by a triel (Tr) atom places a positive region of electrostatic potential near the Tr atom in the plane of the aromatic ring. This σ-hole can interact with an X lone pair of XCCH (X=F, Cl, Br, and I) to form a triel bond (TrB). The interaction energy between C5 H5 Tr and FCCH lies in the range between 2.2 and 4.4 kcal/mol, in the order Tr=B<Ga<Al. This bond is strengthened by halogen substituents on the ring, particularly on the site adjacent to Tr. There is a much stronger strengthening trend as the F of the FCCH nucleophile is replaced by the heavier halogen atoms, rising up to 22 kcal/mol for ICCH. Adding a Li+ cation above the ring pulls density toward itself and thus magnifies the Tr σ-hole. The TrB to the XCCH nucleophile is thereby magnified as is the strength of the TrB. This positive cooperativity is particularly large for Tr=B.

Weak σ-Hole Triel Bond between C5H5Tr (Tr=B, Al, Ga) and Haloethyne: Substituent and Cooperativity Effects

It is well known that the presence of an electron-withdrawing substituent (EWS) placed near the halogen (X) atom on a Lewis acid molecule amplifies the ability of this unit to engage in a halogen bond with a base Quantum calculations are applied to examine how quickly these effects fade as the EWS is moved further and further from the X atom Conjugated alkene and alkyne chains of varying lengths with a terminal C-I first facilitate analysis as to how the number of these multiple bonds affects the strength of CI··N XB to NH3 Then, electron-withdrawing F and C≡N substituents are placed on the opposite end of the chain, and their effects on the XB properties are monitored as a function of their distance from I These same EWSs are added to the ortho, meta, and para positions of aromatic iodobenzene It is found that the XB grows in strength as more triple bonds are added to the alkyne, but there is little change caused by elongating an alkene The cyano group has a much stronger effect than does F While F strengthens the XB, its effects are quickly attenuated as it is moved further from I The consequences of C≡N substitution are stronger and extend over a longer distance Placement of an EWS on the phenyl ring diminishes with distance: o > m > p, and the effects of disubstitution are nearly additive These trends apply not only to energetics but also to geometries, properties of the wave function, σ-hole depth, and NMR shielding

Proximity Effects of Substituents on Halogen Bond Strength.

Many coordination complexes and organometallic compounds with the 4d6 and 5d6 valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d6 complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr0, MnI, FeII, and CoIII compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

The ability of B atoms on two different molecules to engage with one another in a noncovalent diboron bond is studied by ab initio calculations. Due to electron donation from its substituents, the trivalent B atom of BYZ2 (Z=CO, N2 , and CNH; Y=H and F) has the ability to in turn donate charge to the B of a BX3 molecule (X=H, F, and CH3 ), thus forming a B⋅⋅⋅B diboron bond. These bonds are of two different strengths and character. BH(CO)2 and BH(CNH)2 , and their fluorosubstituted analogues BF(CO)2 and BF(CNH)2 , engage in a typical noncovalent bond with B(CH3 )3 and BF3 , with interaction energies in the 3-8 kcal/mol range. Certain other combinations result in a much stronger diboron bond, in the 26-44 kcal/mol range, and with a high degree of covalent character. Bonds of this type occur when BH3 is added to BH(CO)2 , BH(CNH)2 , BH(N2 )2 , and BF(CO)2 , or in the complexes of BH(N2 )2 with B(CH3 )3 and BF3 . The weaker noncovalent bonds are held together by roughly equal electrostatic and dispersion components, complemented by smaller polarization energy, while polarization is primarily responsible for the stronger ones.

Diboron Bonds Between BX3 (X=H, F, CH3 ) and BYZ2 (Y=H, F; Z=CO, N2 , CNH).

The ability of the F atom of HC≡CF, H2C=CHF and H3CCH2F to serve as an electron donor to the triel (Tr) atom of TrR3 in the context of a triel bond is assessed by ab initio calculations. The triel bond formed by Csp3-F is strongest, as high as 30 kcal/mol, followed by Csp2-F, and then by Csp-F whose triel bonds can be as small as 1 kcal/mol. The noncovalent bond strength diminishes in the order Tr = Al > Ga > B, consistent with the intensity of the π-hole above the Tr atom in the monomer. The triel bond strength of the Al and Ga complexes increases along with the electronegativity of the R substituent but is largest for R=H when Tr=B. Electrostatics play the largest role in the stronger triel bonds, but dispersion makes an outsized contribution for the weakest such bonds.

Effect of carbon hybridization in C-F bond as an electron donor in triel bonds.

A series of homoleptic Cr(III) complexes containing substituted anionic 1,3-bis(pyridin-2-ylimino)isoindolin-2-ide ligands are phosphorescent with λmax in the 777-970 nm range in degassed fluid solutions. The energy gap law has been successfully applied to the doublet excited states of Cr(III) complexes.

The energy gap law for NIR-phosphorescent Cr(III) complexes.

The binary complexes of MCO3···HF/XF (M = Zn and Cd; X = F, Cl, Br and I) have been studied using ab initio calculations. The carbonyl O atom is inclined to engage in a stronger H-bond or X-bond, with interaction energy up to −17 kcal/mol. The CdCO3 complex brings out a stronger H-bond or X-bond than does the ZnCO3 analogue. The H-bond is stronger than the X-bond for X = F and Cl, while an opposite result is obtained for X = Br and I. If the M atom in MCO3···HF/XF again participates in a spodium bond with three N-bases (HCN, CH2NH and NH3), the H-bond or X-bond is strengthened and the largest interaction energy amounts to −24 kcal/mol. In addition, the introduction of a spodium bond has a prominent effect on the geometries, orbital interaction and nature of the H-bond and X-bond. GRAPHICAL ABSTRACT

Qingqing Yang

Papers

Weak σ-Hole Triel Bond between C5H5Tr (Tr=B, Al, Ga) and Haloethyne: Substituent and Cooperativity Effects

Effect of carbon hybridization in C-F bond as an electron donor in triel bonds.

Diboron Bonds Between BX3 (X=H, F, CH3 ) and BYZ2 (Y=H, F; Z=CO, N2 , CNH).

The energy gap law for NIR-phosphorescent Cr(III) complexes.

Hydrogen and halogen bonds formed by MCO3 (M = Zn, Cd) and their enhancement by a spodium bond