Simulations Suggest Possible Triply Bonded Phosphorus≡E13 Molecules (E13 = B, Al, Ga, In, and Tl) Simulations Suggest Possible Triply Bonded Phosphorus º E13 Molecules (E13 = B, Al, Ga, In, and Tl)

The effect of substitution on the potential energy surfaces of RE13 ☰ PR (E13 = B, Al, Ga, In, Tl; R = F, OH, H, CH3, SiH3, SiMe(Si t Bu3)2, Si i PrDis2, Tbt, and Ar* is studied using density functional theory (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ + dp). The theoretical results demonstrate that all triply bonded RE13 ☰ PR compounds with small substituents are unstable and spontaneously rearrange to other doubly bonded isomers. That is, the smaller groups, such as R 〓 F, OH, H, CH3 and SiH3, neither kinetically nor thermo-dynamically stabilize the triply bonded RE13 ☰ PR compounds. However, the triply bonded R ’ E13 ☰ PR´ molecules, possessing bulkier substituents (R´ = SiMe(Si t Bu3)2, Si i PrDis2, Tbt and Ar*), are found to have a global minimum on the singlet potential energy surface. In partic- ular, the bonding character of the R ’ E13 ☰ PR´ species is well defined by the valence-electron bonding model (model [II]). That is to say, R ’ E13 ☰ PR´ molecules that feature groups are regarded as R 0 -E13 P-R 0 . The theoretical evidence shows that both the electronic and the steric effects of bulkier substituent groups play a prominent role in rendering triply bonded R 0 E13 ☰ PR 0 species synthetically accessible and isolable in a stable form.


Introduction
Phosphorus is an interesting element, but many chemists have a poor comprehension of its bonding properties. Even though phosphorus and nitrogen belong to the same group in the periodic table, molecular nitrogen is a triply bonded diatomic molecule, but elemental white phosphorus is a tetrahedral compound wherein each atom is connected by three single bonds to the other atoms in the molecule. Phosphorus is usually connected to other elements by a single chemical bond, which has been verified by lot of experimental evidences [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Also, molecules that feature a phosphorus double bond have been the subject of many experimental and theoretical studies of structure and reactivity [15][16][17][18][19][20][21][22][23][24][25][26][27]. However, little is known about the molecules that feature a phosphorus triple bond [28][29][30][31][32]. In particular, whether it is possible to anticipate the stability of the R-E13 ☰ phosphorus-R (E13 = B, Al, Ga, In, and Tl) species based on the effects of substituents, since the R-E13 ☰ phosphorus-R systems are isoelectronic to the R-E14 ☰ E14-R (E14 = C, Si, Ge, Sn, and Pb) compound from the valence electron viewpoints.
This study uses the heavier acetylene analogue, R-E13 ☰ P-R as a model molecule to determine the possibility of generating stable RE13PR species that feature the E13 ☰ P triple bond. In order to understand the effects of substituents on the stability of triply bonded RE13 ☰ PR molecules, both small and bulky groups are chosen in this work. A better understanding of the bonding character and the structure of triply bonded RE13 ☰ PR species will allow experimental chemists to discover novel and stable molecules that feature the E13 ☰ P triple bond.

General considerations
This section uses a simple valence-electron bonding model to demonstrate the bonding nature of substituted triply bonded RE13☰PR compounds.
First, the RE13☰PR species is separated into two units: R-E13 and R-P. Figure 1 shows that these two fragments represent two types of valence-electron bonding model (model [I] and model [II]). Therefore, the R-E13 moiety and the R-P component have two and four valence electrons, respectively. The computational results show that the ground states of these two units are a singlet for R-E13 ([R-E13] 1 ) and a triplet for R-P ([R-P] 3 ). Therefore, model [I] in Figure 1 is considered as [R-E13] 1 + [R-P] 1 ! [R-E13☰P-R] 1

and model [II] is given as
If the excitation energy (ΔE1) from the triplet ground state to the singlet excited state for R-P is smaller than that for R-E13, then model [I] can be used to interpret the bonding character of RE13☰PR. That is, model [I] demonstrates that the triple bond in RE13☰PR is a single donoracceptor (E13 ! P) σ bond and two donor-acceptor (E13 P) π bonds. Therefore, the bonding character of RE13☰PR can be viewed as RE13 PR. However, if the promotion energy (ΔE2) from the singlet ground state to the triplet excited state for R-E13 is smaller than that for R-P, then model [II] can be used to explain the bonding character of RE13☰PR. Namely, model [II] shows that the triple bond in RE13 ☰ PR is a single traditional σ bond, a single traditional π bond and a single donor-acceptor (E13 P) π bond, so its bonding character can be viewed as RE13 PR. Figure 1, two points need to be emphasized here. First, it is experimentally known that the covalent radius decreases as: Tl (148 pm) > In (142 pm) > Ga (122 pm) > Al (121 pm) > P(107 pm) > B (84 pm) [33]. Therefore, a large difference in the atomic radius results in a significant reduction in the overlap populations between E13 and phosphorus. Consequently, the bonding strength between phosphorus and the E13 element in the heteroatomic analogues of acetylene (RE13☰PR) should be weak. Second, the π bond in the RE13 ☰ PR species is also attributed to the lone pair of the R-P moiety, which is donated into the empty p-π orbital of the R-E13 unit. Since the lone pair of the R-P component contains the s valence orbital of phosphorus and the p valence orbital of phosphorus is not the same size as that of the E13 atom, the overlap in the orbital populations between the P and E13 elements is small. In other words, on the basis of the bonding models that are shown in Figure 1, the triple bond between E13 and phosphorus is predicted to be very weak.
The computational results that are shown in Figures 2-6 show that regardless of the type of small substituent that is chosen, the triply bonded RE13 ☰ PR compound cannot be stabilized on the 1,2-migration energy surfaces. That is to say, it is easy for the RE13PR species to migrate to the corresponding doubly bonded R2E13 = P: or: E13 = PR2 isomers rather than to the triply bonded RE13 ☰ PR molecules. The theoretical evidence strongly suggests that the experimental detection of RE13☰PR that features small groups is very unlikely so they are not discussed in this section [28][29][30][31][32].

2.
The computed reaction enthalpies (ΔH1 and ΔH2) that are shown in Scheme 1 and Tables 1-5 show that regardless of the bulky ligand that is chosen, the energy of the triply bonded R'E13 ☰ PR´species is much lower than those of its corresponding doubly bonded R´2E13 = P: or: E13 = PR´2 isomers. This computational evidence indicates that sterically congested ligands kinetically stabilize the triply bonded R'E13 ☰ PRć ompound. Tables 1-5 show that the R 0 -E13 moiety has a singlet ground state, but the R 0 -P component has a triplet ground state. The production of the triply bonded     R 0 E13 ☰ PR 0 compound at the singlet ground state constitutes a combination of two triplet units, [R 0 -E13] 3 and [R 0 -P] 3 . Therefore, using the information in Figure 1, the bonding nature of the E13 ☰ P triple bond in R 0 E13 ☰ PR 0 can be regarded as RE13 PR. The natural charge density on the boron atom. 2 The natural charge density on the phosphorus atom. 3 ΔEB 0 (kcal mol À1 ) = E(triplet state for R 0 -B)-E(singlet state for R 0 -B). 4 ΔEP 0 (kcal mol À1 ) = E(triplet state for R 0 -P)-E(singlet state for R 0 -P). 5 BE (kcal mol À1 ) = E(triplet state for R 0 -B) + E(triplet state for R 0 -P)-E(singlet for R 0 B ☰ PR 0 ). 6 See Scheme 1.