Isomerism and Biradical Character of Tetrapnictide Dianions: A Computational Study

Abstract

We present a computational study on tetrapnictide dianions Pn42– (Pn = P, As, Sb, Bi), using density functional theory (DFT), coupled‐cluster [DLPNO‐CCSD(T)] and complete active space self‐consistent field (CASSCF) methods. Environmental effects such as solvation and coordination of counterions are included. The calculations reveal that out of three isomers (square‐planar, butterfly and capped‐triangle), the square planar isomers are generally the most stable. The counterion (Li+ and Mg2+) used in the calculations have a substantial effect on the relative stabilities. The square planar isomers show considerable biradical character. Calculated reactions toward alkenes indicate that this unusual electronic structure has significant implications on the reactivity of the Pn42– dianions.

Phosphonium‐substituted Diphosphaindenylide (PPI): Exploration of Biradical Character and Ligand Properties

“…Here, a value of 12 % is found. Again, a similar value was calculated for the heterocycle N 2 S 2 (14 %) [32] . However, larger values were found for five‐membered 2‐aza‐1,3‐diphosphaindane‐1,3‐diyls (18 %) [34] which are structurally related to PPI , and the four‐membered biradicaloid [P( μ ‐NTer)] 2 (28 %) [35,34] .…”

“…Again, a similar value was calculated for the heterocycle N 2 S 2 (14 %). [32] However, larger values were found for five-membered 2-aza-1,3-diphosphaindane-1,3-diyls (18 %) [34] which are structurally related to PPI, and the four-membered biradicaloid [P(μ-NTer)] 2 (28 %). [35,34] Based on these comparisons, PPI can be described as a P-centered biradicaloid with modest biradical character comparable to N 2 S 2 .…”

“…For disulfur dinitride, N 2 S 2 , a textbook example of a p-block biradicaloid, [29,30] the total contributions of biradicaloid Lewis-structures amount to 46-48 % using a similar CASSCF approach. [31,32] When the CAS(4,3) calculation is carried out with natural orbitals, another commonly used measure of biradical character, the β-index, is obtained [see the SI, section 4.2 for results using CAS(2,2) and CAS (10,9)]. [33] Here, a value of 12 % is found.…”

“…The remaining Lewis‐structures PPI‐C , PPI‐C’ and PPI‐D involve a considerable amount of charge separation and thus only exhibit minor contributions to the overall wavefunction (14 % in total). For disulfur dinitride, N 2 S 2 , a textbook example of a p‐block biradicaloid, [29,30] the total contributions of biradicaloid Lewis‐structures amount to 46–48 % using a similar CASSCF approach [31,32] …”

Heteroatom-Based Diradical(oid)s

“…The diradical character of the cyclic tetrapnictide anions of the type [Pn(μ-Pn) 2 Pn] 2− , which are isovalence electronic to S 2 N 2 , was studied theoretically by Coburger et al utilizing CASSCF(6,4) calculations (Scheme 17). 229 All species have medium-low diradical character with n(LUNO) = 0.11 (P), 0.14 (As), 0.16 (Sb), and 0.19 (Bi). Moreover, the diradical character increases only slightly with heavy pnictogen atoms.…”