Bonding between electron-deficient atoms: Strong Lewis-acid character preserved in X-Y-X (X = B, Al; Y = Be, Mg) bridges

Contribution from Maxime Ferrer, M. Merced Montero-Campillo, Otilia Mó, Manuel

Yáñez, Ibon Alkorta, José Elguero. Depto. de Química, Facultad de Ciencias, Módulo

13, and Institute of Advanced Chemical Sciences (IadChem). Universidad Autónoma de

Madrid, Campus de Excelencia UAM-CSIC, Cantoblanco, E-28049 Madrid, Spain and

Instituto de Química Médica (CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain. E-

mail: manuel.yanez@uam.es, ibon@iqm.csic.es, mm.montero@uam.es

Electronic Supplementary Information (A total of 5 pages)

Contents

Table S1. G4 optimized geometries of the different systems included in Scheme 1Table S2. Different parameters characterizing beryllium bis(diazaborolyl) and its Mg and Al containing analogues and Z2B-Be-BZ2 ( Z = H, NH2, OH, F, CN). Table S3. G4 optimized geometries of the different Z2B -Be-BZ2 compounds included in this study.Table S4. G4 optimized geometries of the different Complexes formed by attaching one and two molecules of trimethylamine to the compounds of Scheme 1.Figure S1. ELF plots of the Z2B-Be-BZ2 (Z = H, NH2, OH, F, CN) derivatives.Figure S2. G4-optimized structures of the beryllium and magnesium bis(diaza-aluminyl) compounds.Figure S3. G4-optimized structures of the complexes between beryllium and magnesium bis(diaza-boronyl) and one or two molecules of trimethylamine. Figure S4. G4MP2-optimized structures, ELF plots and molecular graphs for the complexes of trimethylamine with the magnesium bis(diazaborolyl) derivative.Figure S5. G4MP2-optimized structures and molecular graphs for the complexes of trimethylamine with beryllium and magnesium bis(diaza-aluminyl) derivatives.Figure S6. G4MP2-optimized structures for the complexes between beryllium and magnesium bis(diaza-aluminyl) derivatives and two molecules of trimethylamine.Figure S7.

S1

Electronic Supplementary Material (ESI) for New Journal of Chemistry.This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

Table S1. G4 optimized geometries of the different systems included in Scheme 1

System Coordinates (Å)HN

X

NH

NH

X

HN

Y

X = B, Y = Be C 4.08172600 0.48036800 -0.47897700 C 4.08189100 -0.48014500 0.47883300 H 4.92089300 0.96026000 -0.95746300 H 4.92122300 -0.95983900 0.95723100 N 2.76422800 -0.79007200 0.78787800 N 2.76395500 0.79004100 -0.78781500 H 2.55308700 -1.48539000 1.48126400 H 2.55257200 1.48535800 -1.48112800 B 1.84665200 -0.00019000 0.00002400 Be 0.00000000 -0.00030300 -0.00004700 C -4.08168600 0.47907400 0.48035400 C -4.08193100 -0.47876400 -0.48013000 H -4.92081400 0.95765900 0.96021800 H -4.92130200 -0.95709100 -0.95982500 N -2.76429400 -0.78790000 -0.79007700 N -2.76388900 0.78782100 0.79000800 H -2.55321000 -1.48128600 -1.48541300 H -2.55244800 1.48117100 1.48527000 B -1.84665200 -0.00021900 -0.00010400

X = Al, Y = Be C -4.84454600 -0.48061200 -0.48078600 C -4.84460200 0.48055200 0.48071900 H -5.74175200 -0.90878700 -0.90915500 H -5.74185600 0.90867600 0.90903700 N -3.58888200 0.90041600 0.90074600 N -3.58877800 -0.90041800 -0.90072400 H -3.57566000 1.61211200 1.61280300 H -3.57547100 -1.61213400 -1.61276000 C 4.84450700 -0.48084000 0.48066500 C 4.84464000 0.48070700 -0.48045700 H 5.74167900 -0.90924200 0.90887800 H 5.74192800 0.90902900 -0.90850600 N 3.58895400 0.90076400 -0.90039300 N 3.58870600 -0.90076200 0.90038500 H 3.57578900 1.61283700 -1.61207500 H 3.57534100 -1.61285900 1.61203800 Be 0.00000000 0.00009500 -0.00006800 Al -2.28315600 0.00002900 0.00005900 Al 2.28315600 0.00005900 -0.00013100

X = B, Y = Mg C 4.49995100 0.47940000 -0.47889200 C 4.50017500 -0.47903000 0.47861500 H 5.33871200 0.95925300 -0.95823800 H 5.33916100 -0.95865600 0.95779600 N 3.17980800 -0.78912200 0.78842700

S2

N 3.17943800 0.78918100 -0.78839300 H 2.97063200 -1.48381500 1.48248500 H 2.96993600 1.48385500 -1.48237100 B 2.26111300 -0.00014100 0.00006600 C -4.49998600 0.47894500 0.47921700 C -4.50014000 -0.47859600 -0.47918000 H -5.33878300 0.95834000 0.95895800 H -5.33909000 -0.95776100 -0.95888400 N -3.17975000 -0.78841000 -0.78917300 N -3.17949600 0.78845400 0.78908700 H -2.97052200 -1.48245000 -1.48386800 H -2.97004600 1.48249000 1.48371900 B -2.26111300 -0.00017000 -0.00000400 Mg 0.00000000 -0.00039400 0.00015700

X = Al, Y = Mg C -5.23922600 -0.47959200 -0.48048500 C -5.23924300 0.47938300 0.48067300 H -6.13576400 -0.90816000 -0.90991800 H -6.13579600 0.90782800 0.91019800 N -3.98111700 0.89916800 0.90126500 N -3.98108600 -0.89920700 -0.90120200 H -3.96732500 1.61018800 1.61392600 H -3.96726800 -1.61023800 -1.61385100 C 5.23913100 -0.48074100 0.47967200 C 5.23933800 0.48044700 -0.47927300 H 6.13558500 -0.91030800 0.90828300 H 6.13597500 0.90986000 -0.90765300 N 3.98129600 0.90120600 -0.89914300 N 3.98090800 -0.90128000 0.89921400 H 3.96764500 1.61386500 -1.61016600 H 3.96694800 -1.61395200 1.61022000 Al -2.67331000 0.00006100 -0.00002500 Al 2.67331000 0.00007000 -0.00014100 Mg 0.00000000 0.00025100 -0.00027900

S3

Table S2. Different parameters characterizing beryllium bis(diazaborolyl) and its Mg and Al containing analogues and BZ2-Be-BZ2 ( Z = H, NH2, OH, F, CN).

RXYa QX

b QYb QN

b HBDEc PXYd

HN

X

NH

NH

X

HN

Y

X = B, Y = Be 1.847 -0.02 +1.11 -0.82 369 2.34X = Al, Y = Be 2.283 +0.66 +0.67 -1.05 310 2.31X = B, Y = Mg 2.261 +0.03 +1.12 -0.84 274 2.34X = Al, Y = Mg 2.673 +0.60 +0.88 -1.06 255 2.30 BZ2-Be-BZ2 RBeB

e QBf QBe

f QZf HBDEg PBeB

h

Z = H 1.876 -0.35 +1.04 -0.08 346 2.07Z = NH2 1.884 +0.13 +1.04 -0.33 334 2.13Z = OH 1.884 +0.40 +1.00 -0.45 353 2.10Z = F 1.886 +0.52 +0.91 -0.49 361 2.11Z = CN 1.878 -0.12 +1.20 -0.24 351 2.01a Internuclear X-Y distance in Åb Natural atomic charges on atoms X, Y and N, respectivelyc X-Y Homolytic Bond dissociation enthalpies (kJ·mol-1)d Population associated to the XY disynaptic basin e Internuclear Be-B distance in Åf Natural atomic charges on atoms B, Be and ligand Zg Be-B Homolytic Bond dissociation enthalpies (kJ·mol-1)h Population associated to the BeB disynaptic basin

Table S3. G4 optimized geometries of the different BZ2-Be-BZ2 compounds included in this study BZ2-Be-BZ2 Coordinates (Å)Z = H Be 0.00000000 0.00000000 0.00000000

B 0.00000000 0.00000000 1.87614200 H 0.70639500 0.70561900 2.54367000 H -0.70639500 -0.70561900 2.54367000 B 0.00000000 0.00000000 -1.87614200 H 0.70639600 -0.70561900 -2.54367000 H -0.70639600 0.70561900 -2.54367000

Z = NH2 Be -0.00001200 0.00001800 0.00003500 B -1.88434500 -0.00001900 0.00004700 B 1.88435900 -0.00010900 -0.00011800 N 2.62095300 0.85208300 -0.86948500 H 2.19011300 1.48669400 -1.51707200 H 3.62998400 0.89050800 -0.90862300 N 2.62100400 -0.85207200 0.86948600 H 2.19023000 -1.48646000 1.51733500 H 3.63005200 -0.89013400 0.90878100 N -2.62100000 0.87067400 0.85091700 H -3.63008000 0.90974500 0.88893600

S4

H -2.19047600 1.51941100 1.48456600 N -2.62092500 -0.87065300 -0.85094700 H -3.62997600 -0.90992600 -0.88920000 H -2.19009400 -1.51948800 -1.48430700

Z = OH Be 0.00000100 0.00072000 0.00070000 B 1.88420000 0.00012200 0.00002400 B -1.88419900 0.00024800 0.00025700 O -2.55402100 -0.84845300 0.84648900 H -3.51823000 -0.79921900 0.79680500 O -2.55404500 0.84834100 -0.84657400 H -3.51825300 0.79894500 -0.79701900 O 2.55346700 -0.84818700 -0.84705000 H 3.51770900 -0.79905100 -0.79790300 O 2.55459900 0.84791200 0.84670500 H 3.51877300 0.79768700 0.79735000

Z = F Be 0.00000000 0.00000000 0.00000000 B 0.00000000 0.00000000 -1.88618700 B 0.00000000 0.00000000 1.88618700 F -0.68921500 0.87600900 -2.59730900 F 0.68921500 -0.87600900 -2.59730900 F 0.68921500 0.87600900 2.59730900 F -0.68921500 -0.87600900 2.59730900

Z = CN Be 0.00000000 0.00000000 0.00000100 B 0.00000000 0.00000000 -1.87796100 B 0.00000000 0.00000000 1.87796300 C 0.00000000 1.30398800 -2.65092900 N 0.00105600 2.34633900 -3.16452500 C 0.00000000 -1.30398800 -2.65092900 N -0.00105600 -2.34633900 -3.16452500 C 1.30395400 -0.00925700 2.65093200 N 2.34629000 -0.01556700 3.16452100 C -1.30395400 0.00925700 2.65093200 N -2.34629000 0.01556700 3.16452100

Table S4. G4 optimized geometries of the different Complexes formed by attaching one and two molecules of trimethylamine to the compounds of Scheme 1.

Y

NMe3

XX

HN

HN

HN

NH

Coordinates (Å)

X = B, Y = Be C 3.34900100 -2.02209800 0.54275900

S5

C 3.78323100 -1.37114500 -0.56159900 H 3.85198800 -2.77895800 1.12469600 H 4.71644700 -1.48234600 -1.09174200 N 2.79587600 -0.46807200 -0.95272200 N 2.07734500 -1.54943700 0.85626200 H 2.91261400 0.06813900 -1.79384000 H 1.57927600 -1.91429800 1.64849600 B 1.63585800 -0.53145100 -0.07925400 Be -0.05501500 0.32711100 -0.07950900 C -3.98284800 -1.22902000 -0.15491800 C -3.20990300 -2.34081000 -0.08084900 H -5.05826600 -1.15309500 -0.20232500 H -3.51560100 -3.37548500 -0.05388500 N -1.88163500 -1.94330700 -0.04043100 N -3.14730100 -0.11632900 -0.16078100 H -1.13652200 -2.61675000 -0.00925300 H -3.54370100 0.80304100 -0.23468700 B -1.74751300 -0.50100300 -0.09122000 N 0.03231700 2.13196400 0.09057000 C 1.01532100 2.69378200 -0.86201000 H 1.11709100 3.77925100 -0.72508100 H 1.97853500 2.20720900 -0.70580500 H 0.67969500 2.49790000 -1.88391800 C 0.49650100 2.36512500 1.47783900 H 0.57698200 3.44058000 1.68643300 H -0.21840800 1.91945300 2.17444600 H 1.46746400 1.88712100 1.61477800 C -1.27581800 2.79343200 -0.10508800 H -1.64373400 2.57468700 -1.10937100 H -1.98533900 2.39638400 0.62162000 H -1.18930700 3.88167500 0.02275500

X = Al, Y = Be C 3.73953300 -2.41948000 0.28104900 C 4.46733100 -1.38765700 -0.21693500 H 4.15423900 -3.38825000 0.53113300 H 5.52946500 -1.43737300 -0.42261900 N 3.74761700 -0.22051700 -0.46031200 N 2.38683000 -2.15614000 0.46170300 H 4.27810400 0.53998300 -0.85322600 H 1.83805300 -2.91062000 0.84055900 C -4.52046000 -1.34988600 -0.00039500 C -3.72847400 -2.44305400 -0.14628900 H -5.60096000 -1.38699500 0.06330200 H -4.10275500 -3.45691600 -0.21748500 N -2.36583900 -2.18101500 -0.20236800 N -3.84389800 -0.13484500 0.06619100 H -1.76626700 -2.98103400 -0.32306700 H -4.42791000 0.67806200 0.17733600 N 0.03854600 2.52214600 0.03457800 C 1.31470800 3.08759000 -0.48012200 H 1.31561500 4.18065900 -0.37970800 H 2.14837800 2.66423000 0.08042600 H 1.42848300 2.82224700 -1.53284700 C -0.07891400 2.83484000 1.48435500 H -0.04796200 3.91958800 1.64388100

S6

H -1.02163300 2.43531500 1.86301400 H 0.74879300 2.36515000 2.02037200 C -1.08789100 3.16022600 -0.69695000 H -1.02941300 2.89562100 -1.75448600 H -2.03298600 2.79339200 -0.29817200 H -1.03961100 4.25197900 -0.59381200 Be -0.01211200 0.75997700 -0.05651600 Al 1.97253900 -0.44075100 -0.03366700 Al -2.02646100 -0.38446500 -0.06366900

X = B, Y = Mg C 4.26701500 -1.50727400 0.67691900 C 4.26705200 -1.50752300 -0.67658400 H 5.07039500 -1.74980700 1.35514300 H 5.07046700 -1.75031100 -1.35467600 N 3.00146800 -1.12185700 -1.11340600 N 3.00142000 -1.12140500 1.11353000 H 2.79416000 -1.08834100 -2.09524500 H 2.79407000 -1.08748800 2.09534600 B 2.10837800 -0.86099400 -0.00000900 C -4.63858100 -0.82181900 -0.00011300 C -4.19627700 -2.10226200 -0.00021100 H -5.65331300 -0.45493900 -0.00007100 H -4.76932600 -3.01656000 -0.00028800 N -2.80591600 -2.08344300 -0.00022600 N -3.53119800 0.02227400 0.00008700 H -2.28427500 -2.94165900 -0.00031000 H -3.65731500 1.01784000 0.00028700 B -2.29077700 -0.72886100 -0.00016800 N 0.12946600 2.14882500 0.00005400 C 0.89638200 2.50674600 -1.20506800 H 1.10550200 3.58735600 -1.24375800 H 1.83983100 1.95579800 -1.20950200 H 0.32506800 2.23432900 -2.09787300 C 0.89609500 2.50649900 1.20543500 H 1.10520100 3.58710100 1.24440400 H 0.32457200 2.23389500 2.09805000 H 1.83954500 1.95555300 1.20998600 C -1.17020000 2.83737400 -0.00003500 H -1.73843500 2.54616200 -0.88691000 H -1.73867800 2.54596600 0.88662000 H -1.04827300 3.93221100 0.00010200 Mg -0.08023300 -0.16158500 -0.00023000

X = Al, Y = Mg C -5.24824000 -0.83101800 -0.31216800 C -4.88478000 -2.03874700 0.18741500 H -6.27101800 -0.54211900 -0.51970200 H -5.58365200 -2.82840800 0.43391200 N -3.51745100 -2.21518700 0.37514400 N -4.19564500 0.04922800 -0.55544000 H -3.23437300 -3.10828800 0.74409000 H -4.45501300 0.93653000 -0.95472700 C 5.24756600 -0.83111100 -0.31262100 C 4.88470000 -2.03866700 0.18779900 H 6.27019400 -0.54191100 -0.52047900 H 5.58394000 -2.82786800 0.43472200

S7

N 3.51747800 -2.21554200 0.37594700 N 4.19455700 0.04853500 -0.55627200 H 3.23481800 -3.10865900 0.74517400 H 4.45347700 0.93564300 -0.95628000 N 0.00062200 2.34253400 0.18240900 C 0.00063400 2.52014500 1.64917000 H 0.00091300 3.58563400 1.92208400 H -0.88979800 2.04955200 2.07556500 H 0.89079500 2.04906700 2.07559000 C -1.20629000 2.96815100 -0.39459400 H -1.22620400 4.04895900 -0.19064400 H -1.21796900 2.81632500 -1.47724000 H -2.09992000 2.50743300 0.03220800 C 1.20793400 2.96746100 -0.39451100 H 2.10127100 2.50607700 0.03219200 H 1.21948900 2.81580400 -1.47718300 H 1.22854300 4.04821800 -0.19038500 Mg -0.00019700 0.08740200 -0.11302000 Al -2.58458100 -0.72078300 -0.12564900 Al 2.58387200 -0.72191500 -0.12574400

Be

NMe3

F2B BF2

Be -0.00016500 -0.37970500 -0.15424400 B -1.74814600 -1.11451900 0.03638900 B 1.74754800 -1.11525400 0.03646300 F -2.68667400 -0.48087000 0.76766300 F -2.19672000 -2.25113000 -0.47688500 F 2.68669500 -0.48182600 0.76710300 F 2.19523000 -2.25242200 -0.47638200 N 0.00035700 1.39632000 -0.19856100 C 1.21132800 1.97533900 -0.83259100 H 2.09890300 1.57370500 -0.34543400 H 1.22981000 1.70332300 -1.89055600 H 1.20312300 3.06957400 -0.74683100 C -1.21063800 1.97606600 -0.83188600 H -1.22987200 1.70411800 -1.88985400 H -2.09817300 1.57491000 -0.34426300 H -1.20177700 3.07029300 -0.74608500 C 0.00087300 1.71738500 1.25428200 H -0.89260200 1.28889800 1.71388300 H 0.89434100 1.28835100 1.71337600 H 0.00124100 2.80377200 1.40609000

Y

NMe3

XX

HN

HN

HN

NHNMe3

Coordinates (Å)

X = B, Y = Be C -2.96556500 -2.62287600 -0.24500000 C -3.76228800 -1.69794500 0.33824600 H -3.20830900 -3.62926100 -0.55061900 H -4.80197600 -1.77654300 0.61727600

S8

N -3.00748100 -0.54174800 0.54749300 N -1.69958800 -2.06538400 -0.40652400 H -3.44103400 0.26178400 0.96591300 H -0.95313900 -2.58722400 -0.83216800 B -1.63165900 -0.69570600 0.08697100 Be 0.00498800 0.34031700 -0.00099800 C 3.74035800 -1.77041500 -0.19447000 C 2.88253300 -2.70805100 0.27088200 H 4.79650600 -1.85828000 -0.39912200 H 3.08186200 -3.73571800 0.53432400 N 1.61981900 -2.13072300 0.36945200 N 3.02684500 -0.58625800 -0.39048300 H 0.83350000 -2.65140400 0.71754300 H 3.50638000 0.23145700 -0.72251800 B 1.61682200 -0.73489400 -0.04427500 N -0.12760300 1.40770400 -1.63655900 C -1.41302600 2.12452700 -1.77103100 H -1.51781200 2.56269600 -2.77491800 H -2.22878400 1.42501400 -1.58473700 H -1.46902200 2.93062700 -1.03729800 C -0.06366400 0.37825000 -2.70140100 H -0.16690500 0.84585100 -3.69177100 H 0.89045700 -0.14512600 -2.63393100 H -0.86745800 -0.34297700 -2.54763600 C 0.98101800 2.35597300 -1.84705300 H 0.92627600 3.16414200 -1.11441900 H 1.92920300 1.82940000 -1.72963800 H 0.94072300 2.80314000 -2.85186000 N 0.16148200 1.48926800 1.57563000 C -0.89085200 2.51233700 1.71175100 H -0.86295800 2.98718900 2.70413300 H -0.75187900 3.29263200 0.96029100 H -1.86739200 2.04854700 1.56550500 C 0.01834300 0.52185100 2.68995300 H -0.97077800 0.06611700 2.64409200 H 0.76708900 -0.26326400 2.57502200 H 0.15508500 1.02917400 3.65647400 C 1.48390300 2.13779100 1.70204600 H 1.58708100 2.63645500 2.67756500 H 2.26091500 1.38128400 1.58829300 H 1.60774200 2.88735900 0.91811300

X = Al, Y = Be C 2.62369300 -3.28673900 0.35795200 C 3.71462800 -2.57511000 -0.01599400 H 2.63015600 -4.35376900 0.54621200 H 4.69905900 -3.00182600 -0.16606600 N 3.49183400 -1.20820800 -0.20442200 N 1.45496800 -2.54260700 0.49226600 H 4.30151000 -0.67398200 -0.47586500 H 0.62207400 -3.04441500 0.75598800 Be 0.06469900 0.93474200 -0.00959800 C -3.98583400 -2.08961800 0.10353700 C -3.04310800 -2.91016500 -0.41907800 H -4.99798300 -2.39599100 0.33880800 H -3.20982300 -3.95442900 -0.65487600

S9

N -1.80430600 -2.31419700 -0.64435200 N -3.56525200 -0.77580900 0.33060400 H -1.07845500 -2.90583600 -1.01577000 H -4.26613300 -0.15992000 0.71018400 N 0.01546600 1.92874800 1.58025400 C 1.32820500 2.51774900 1.95507000 H 1.25531200 3.03268800 2.92254500 H 2.06928800 1.72034600 2.01483800 H 1.64795600 3.23723900 1.20149000 C -0.35753300 0.97875500 2.66577500 H -0.32365500 1.49271800 3.63562600 H -1.36151700 0.59350000 2.48690700 H 0.34361400 0.14228200 2.66804100 C -0.99892900 3.01023000 1.56741300 H -0.69207000 3.80065000 0.87968600 H -1.95688800 2.60053300 1.24315600 H -1.10885800 3.45308500 2.56624700 N 0.22836800 1.96327100 -1.56905800 C 1.36869100 2.91088100 -1.53336800 H 1.52335100 3.37130400 -2.51826500 H 1.16742000 3.70874600 -0.81583200 H 2.27204600 2.37626600 -1.23576400 C 0.47566800 1.00678900 -2.68399900 H 1.42277200 0.49211300 -2.52158800 H -0.32660300 0.26716700 -2.70650700 H 0.50552900 1.54879000 -3.63852100 C -1.00275400 2.72162800 -1.91718000 H -0.87305400 3.24364500 -2.87478400 H -1.83955100 2.02581600 -1.98470400 H -1.22200000 3.46047700 -1.14644800 Al 1.73476500 -0.76187400 0.12960400 Al -1.80755000 -0.53665900 -0.17223200

X = B, Y = Mg C -3.97986300 2.41891600 -0.00060000 C -4.51716000 1.17690200 -0.00018200 H -4.48295300 3.37380400 -0.00085000 H -5.55665900 0.88692700 -0.00005300 N -3.47223000 0.25238800 0.00017600 N -2.59290700 2.29492400 -0.00053700 H -3.67398400 -0.73115400 0.00039100 H -2.01140500 3.11354500 -0.00080600 B -2.16797500 0.90565700 -0.00017400 C 4.51731800 1.17687000 -0.00014800 C 3.97993100 2.41884000 -0.00069100 H 5.55684000 0.88697400 0.00010100 H 4.48295400 3.37376300 -0.00099200 N 2.59297800 2.29473600 -0.00084600 N 3.47245600 0.25227600 0.00006400 H 2.01140400 3.11330400 -0.00122100 H 3.67429100 -0.73124800 0.00037200 B 2.16816700 0.90543600 -0.00029600 N 0.00000600 -1.31553200 1.91298500 C -1.20812400 -2.14441700 2.02483200 H -1.26106100 -2.65714200 2.99914400 H -2.09011400 -1.51109500 1.90489900

S10

H -1.20980000 -2.90523200 1.23846000 C 0.00007300 -0.29645500 2.97846500 H 0.00000500 -0.76312400 3.97649400 H 0.88836200 0.33206600 2.87690800 H -0.88817300 0.33214500 2.87690000 C 1.20809000 -2.14448600 2.02474900 H 1.20965000 -2.90533900 1.23843200 H 2.09011200 -1.51120800 1.90481100 H 1.26108200 -2.65716700 2.99908700 N -0.00013000 -1.31668500 -1.91225500 C -1.20828000 -2.14559100 -2.02389500 H -1.26133800 -2.65832500 -2.99819300 H -1.20985900 -2.90641900 -1.23753000 H -2.09027200 -1.51229000 -1.90385400 C -0.00017400 -0.29780000 -2.97794500 H -0.88845000 0.33077400 -2.87649000 H 0.88809700 0.33077200 -2.87651500 H -0.00021000 -0.76466100 -3.97587800 C 1.20790400 -2.14571700 -2.02410000 H 1.26077900 -2.65834700 -2.99847300 H 2.08999100 -1.51252400 -1.90415500 H 1.20946500 -2.90666100 -1.23786200 Mg 0.00006500 0.06877200 -0.00006600

X = Al, Y = Mg C 4.70100400 -2.48150900 0.14830600 C 5.19510900 -1.23124200 -0.02516700 H 5.31341500 -3.37262500 0.21412700 H 6.24987000 -1.00042100 -0.11173600 N 4.23390300 -0.22187500 -0.08640200 N 3.31382900 -2.56168400 0.23977200 H 4.59500700 0.71136800 -0.20166700 H 2.93404600 -3.48606100 0.36424400 C -5.19495500 -1.23127600 0.02618200 C -4.70090800 -2.48181900 -0.14544000 H -6.24974000 -1.00019500 0.11175100 H -5.31332800 -3.37297300 -0.21063300 N -3.31366400 -2.56228600 -0.23582600 N -4.23364700 -0.22197400 0.08674300 H -2.93393000 -3.48687500 -0.35886500 H -4.59464000 0.71139400 0.20134000 N -0.12485700 1.55050200 1.87388500 C 1.02517100 2.46534400 1.99213300 H 1.01099000 3.00280300 2.95257100 H 1.95456500 1.89563700 1.91649900 H 0.99589800 3.20403400 1.18647200 C -0.09027400 0.58136200 2.99031500 H -0.15188500 1.09772700 3.95984300 H -0.93298100 -0.10884200 2.89931500 H 0.83978400 0.00923400 2.94715200 C -1.38414500 2.31600800 1.93985600 H -1.41850600 3.04704800 1.12773600 H -2.23179400 1.63451200 1.83514800 H -1.47137000 2.85512800 2.89540600 N 0.12459600 1.54626400 -1.87632800 C 1.38367300 2.31199200 -1.94408100

S11

H 1.47082800 2.84871600 -2.90098100 H 1.41771500 3.04511100 -1.13381600 H 2.23151400 1.63101000 -1.83755400 C 0.09026700 0.57464000 -2.99061400 H 0.93324000 -0.11504700 -2.89817100 H -0.83956900 0.00225600 -2.94608600 H 0.15157900 1.08888200 -3.96128600 C -1.02569500 2.46052800 -1.99658100 H -1.01163000 2.99589500 -2.95818700 H -1.95494700 1.89075500 -1.91971800 H -0.99661300 3.20096500 -1.19251800 Al 2.53015100 -0.90639400 0.09640500 Al -2.52981800 -0.90701100 -0.09375400 Mg 0.00012900 0.18650300 0.00023200

Be

Me3N

F2B BF2

NMe3 Be -0.00026200 -0.00006100 -0.18479700 B -0.25527700 1.63109800 -1.19454000 B 0.25492100 -1.63113100 -1.19454200 F -1.46691400 2.11604300 -1.53814900 F 0.71022800 2.43953100 -1.66530000 F -0.71055600 -2.43974200 -1.66503500 F 1.46656800 -2.11584600 -1.53844600 N -1.59173600 -0.20364200 0.84034100 N 1.59196900 0.20368600 0.84016800 C -2.65297400 -0.49170900 -0.16579700 H -2.40748200 -1.40448000 -0.70447300 H -3.62107200 -0.60756100 0.34075600 H -2.70844100 0.33733900 -0.86962700 C -1.54654000 -1.33546400 1.79088500 H -2.53516300 -1.51255300 2.23746100 H -1.22443400 -2.23578000 1.26546600 H -0.84392000 -1.11836000 2.59810200 C -1.99700000 1.02121000 1.56992400 H -1.25678300 1.26942100 2.33192700 H -2.07498500 1.84617100 0.86249400 H -2.96794100 0.87486600 2.06377700 C 1.54696600 1.33538300 1.79083500 H 0.84450700 1.11821700 2.59817500 H 2.53567400 1.51244500 2.23724300 H 1.22474700 2.23576900 1.26559300 C 1.99743600 -1.02125100 1.56949200 H 2.96853300 -0.87498900 2.06306500 H 1.25744600 -1.26953700 2.33169500 H 2.07517400 -1.84613800 0.86195000 C 2.65291100 0.49186200 -0.16622700 H 2.40720900 1.40460600 -0.70484600 H 3.62113500 0.60777800 0.34007200 H 2.70825800 -0.33717000 -0.87008900

S12

Figure S1. ELF plots of the Z2B-Be-BZ2 (Z = H, NH2, OH, F, CN) derivatives. Yellow lobes represent disynaptic basins corresponding to bonds between heavy atoms and hydrogen atoms, green lobes to disynaptic basins between heavy atoms and red lobes to monosynaptic basins associated to lone-pairs. The population of the Be-B and B-X disynaptic basins is given in e.

Figure S2. G4-optimized structures of the beryllium and magnesium bis(diaza-aluminyl) complexes with bond lengths in Å, and natural atomic charges at the Be, Mg and Al atoms in red. The corresponding molecular graphs are included in the second column with the electron densities at the BCPs in a.u.

S13

Figure S3. G4-optimized structures of the complexes between beryllium and magnesium bis(diaza-boronyl) and one and two molecules of trimethylamine. Analogues in which B is replaced by Al are also shown. Bond lengths are in Å, and bond angles (in red) are in degrees.

Figure S4. G4MP2-optimized structures, ELF plots and molecular graphs for the complexes of trimethylamine with the magnesium bis(diazaborolyl) derivative. Bond lengths are in Å and bond angles in degrees, the electron densities at the BCPs in a.u.

S14

For the ELF plot the same conventions as in Figure S1 have been used. The bond dissociation enthalpy (BDE) of the Be–N (green) and Be–B (magenta) linkages is also given (in kJ/mol).

Figure S5. G4MP2-optimized structures and molecular graphs for the complexes of trimethylamine with beryllium and magnesium bis(diaza-aluminyl) derivatives. Bond lengths are in Å and bond angles in degrees. The electron densities at the BCPs are in a.u.

Figure S6. G4MP2-optimized structures for the complexes of two molecules of trimethylamine with beryllium and magnesium bis(diaza-aluminyl) derivatives. Bond lengths are in Å and bond angles in degrees.

S15

Figure S7. Zoom view of ELF basins (isovalue 0.85) around beryllium for beryllium bis(diazaborolyl) and its complexes with one and two molecules of trimethylamine.

Figure S7 shows in detail the ELF basins (highlighted in yellow) around beryllium for the beryllium bis(diazaborolyl) complex (coordination number = 2), and the complexes in which one and two molecules of trimethylamine are also attached to the Be atom (coordination numbers = 3 and 4, respectively). With n = 2 both basins have exactly the same oval shape, but rotated according to the symmetry axis. With n = 3, the two beryllium-boron basins have evolved to a similar but slightly deformed shape, still rotated one with respect to the other. The third basin (Be-N) is mostly round and pretty close to N, in agreement with what expected for a N donor base. With n = 4, the two Be-B basins are again equal, more pyramidal-like. These two basins and the other two Be-N basins, more spherical-like, conform the tetrahedral coordination described in the manuscript.

All in all, the bulbs are conserved, big and flat, but its shape is modified from oval to pyramidal. The pyramidal shape is not easy to appreciate in n = 4, as we tried to show the four basins around Be, but can be guessed more easily from the n = 3 picture, where a peak is starting to appear.

S16