Understanding the Uniqueness of 2p Elements in Periodic Tables

Abstract

The Periodic Table, and the unique chemical behavior of the first element in a column (group), were discovered simultaneously one and a half centuries ago. Half a century ago, this unique chemistry of the light homologs was correlated to the then available atomic orbital (AO) radii. The radially nodeless 1s, 2p, 3d, 4f valence AOs are particularly compact. The similarity of r(2s)≈r(2p) leads to pronounced sp‐hybrid bonding of the light p‐block elements, whereas the heavier p elements with n≥3 exhibit r(ns) ≪ r(np) of approximately −20 to −30 %. Herein, a comprehensive physical explanation is presented in terms of kinetic radial and angular, as well as potential nuclear‐attraction and electron‐screening effects. For hydrogen‐like atoms and all inner shells of the heavy atoms, r(2s) ≫ r(2p) by +20 to +30 %, whereas r(3s)≳r(3p)≳r(3d), since in Coulomb potentials radial motion is more radial orbital expanding than angular motion. However, the screening of nuclear attraction by inner core shells is more efficient for s than for p valence shells. The uniqueness of the 2p AO is explained by this differential shielding. Thereby, the present work paves the way for future physical explanations of the 3d, 4f, and 5g cases.

B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO2, Isocyanates, or Carbodiimides

“…According to 1 HNMR spectroscopy on the reaction mixture, K[4H] and K [5]a re produced in as toichiometric ratio of 1.5:1. Apart from an ecessary symmetry break due to the switch from an NCN to an NCO bridge,the NMR data of K [5] do not differ significantly from those of K [3]. Importantly,the resonances of H-1/1' (7.74/7.73 ppm) again appear below 8ppm, in agreement with a9 ,9'-difluorene-type molecule.…”

“…Thea ddition of iPrNCNiPr (1 equiv) to K[1H] in [D 8 ]THF resulted in the selective formation of K [3](Scheme 2a). The 1 HNMR spectrum shows only one set of tBu(C 6 H 3 ) and iPr signals with an overall integral ratio suggesting an equimolar reaction between the two starting materials.A resonance assignable to aB-bonded Hatom is not detectable, even upon 11 Bd ecoupling.I nstead, we observe as inglet resonance at d( 1 H) = 8.07 ppm (1 H), showing across-peak to as ignal at d( 13 C) = 158.8 ppm in the 1 H- 13 CH SQC experiment.…”

“…The resulting B(sp 3 )-B(sp 3 )d iborane salts M [3]( M + = Li + -K + ) featuring ac entral five-membered BNCNB heterocycle are accessible in almost quantitative yields, [41] thus adding new members to apromising, [42] but experimentally little explored diborane class. [43] In terms of electronic structure and steric demand, iPrNCO occupies an intermediate position between the two previously addressed substrate molecules.When it reacts with K[1H],t wo isomeric products are formed in competing reactions (Scheme 3a): K[4H] is related to the CO 2 adduct K[2H] (cf.S cheme 4), whereas K [5]i sa na nalog of K [3]. According to 1 HNMR spectroscopy on the reaction mixture, K[4H] and K [5]a re produced in as toichiometric ratio of 1.5:1.…”

“…Experimental insight into the unique properties of the central BHB unit has been hampered by the lack of synthetically accessible (m-hydrido)diborane(4) derivatives,i nw hich the BHB core is not perturbed by electron-donating or sterically protecting substituents.O ur representative K[1H] with planarized dibenzo[g,p]chrysene-type scaffold comes close to this ideal and reacts straightforwardly with various substrates, such as the heterocumulenes CO 2 , iPrNCO,and iPrNCNiPr in either B-B or B-H bond activation reactions.T he actual activation scenario is governed by two factors,( i) the steric demand of the substrate molecule and (ii)t he ready isomerization of [1H] À with bridging H À ligand to its isomer [1aH] À with terminal BÀHb ond:T he small CO 2 molecule can still undergo af ormal [2+ +2] cycloaddition with the sterically loaded B À Bb ond of [1H] À ,t hereby forming aB OCBH heterocycle (K[2H]). Theb ulky substrate iPrNCNiPr most easily interacts with the BÀHb ond of [1aH] À to give the B(sp 3 )-B(sp 3 )d iborane salt K [3]w ith aB NCNB ring. Both B-B and B-H bond activations are observed for the hybrid Angewandte Chemie Forschungsartikel substrate iPrNCO,l eading to the two isomeric and almost isoenergetic products K[4H] and K [5].…”

Understanding Periodic and Non-periodic Chemistry in Periodic Tables

“…Due to the radial node effect (meaning the correlation of small radius and no radial node of atomic orbitals, reviewed by Kaupp: Kaupp, 2006;Huheey et al, 2014;Wang et al, 2020), the 5f contraction along the series of elements occurs more slowly than 4f, so that f still contributes to the covalence of protactinium, uranium, neptunium and plutonium, and in special cases of thorium (potentially) and americium, too, as a rule of thumb (Morss et al, 2010;Neidig et al, 2013;Ortu et al, 2016;Liu et al, 2017;Vitova et al, 2017;Wilson et al, 2018). From the common chemical empirical point of view, the elements thorium, protactinium, uranium, neptunium, and plutonium are more akin to the lighter outer transition elements hafnium, tantalum, tungsten, rhenium, and osmium, than to their officially homologous inner transition elements cerium, praseodymium, neodymium, promethium, and samarium.…”