Review moleculesCarbones and Carbon Atom as Ligands in Transition Metal Complexes Review Lili Zhao 1, Chaoqun Chai 1, Wolfgang Petz 2,* and Gernot Frenking 1,2,* Carbones and Carbon1 Institute Atomof Advanced as Synthesis, Ligands School of Chem inistry Transition and Molecular Engineering, Jiangsu National Metal Complexes Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China; [email protected] (L.Z.); [email protected] (C.C.) 2 Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, 1 1 2, 1,2, Lili Zhao , Chaoqun Chai , WolfgangD-35043 Marburg, Petz * andGermany Gernot Frenking * 1 Institute of Advanced Synthesis,* Correspondence: School of Chemistry [email protected] and Molecular Engineering, (W Jiangsu.P.); [email protected] National (G.F.) Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China; [email protected] (L.Z.);Received: [email protected] 23 August 2020; Accepted: (C.C.) 15 October 2020; Published: 23 October 2020 2 Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, D-35043 Marburg, Germany * Correspondence: [email protected]: This review (W.P.); [email protected] experimental and (G.F.) theoretical studies of transition metal complexes with two types of novel metal-carbon bonds. One type features complexes with Academic Editors: Yves Canac and Carlo Santini carbones CL2 as ligands, where the carbon(0) atom has two electron lone pairs which engage Received: 23 August 2020; Accepted: 15 October 2020; Published: 26 October 2020 in double (σ and π) donation to the metal atom [M]⇇CL2. The second part of this review Abstract: This review summarizesreports experimental complexes and which theoretical have a studies neutral of carbon transition atom metal C as complexes ligand. Carbido complexes with with two types of novel metal-carbonnaked carbon bonds. atoms One typemay featuresbe considered complexes as endpoint with carbones of the CL series2 as [M]-CR3 → [M]-CR2 → ligands, where the carbon(0) atom[M]-CR has → two [M]-C. electron This review lone pairs includes which some engage work in doubleon uranium (σ and andπ) cerium complexes, but it donation to the metal atom [M]does⇔CL not2. The present second a complete part of this coverage review reportsof actinide complexes and lanthanide which have complexes with carbone or a neutral carbon atom C as ligand.carbide Carbido ligands. complexes with naked carbon atoms may be considered as endpoint of the series [M]-CR [M]-CR [M]-CR [M]-C. This review includes some work 3 ! 2 ! ! on uranium and cerium complexes,Keywords: but itcarbone does not complexes; present a completecarbido complexes; coverage of transition actinide andmetal complexes; chemical lanthanide complexes with carbonebonding or carbide ligands. Keywords: carbone complexes; carbido complexes; transition metal complexes; chemical bonding 1. Introduction 1. Introduction Transition metal compounds with metal-carbon bonds are the backbone of organometallic Transition metal compoundschemistry. with metal-carbonMolecules with bonds M-C aresingle the bonds backbone are already of organometallic known since 1849 when Frankland chemistry. Molecules with M-Creported single bonds the accidental are already synthesis known since of diethyl 1849 when zinc Frankland while attempting reported to the prepare free ethyl radicals accidental synthesis of diethyl zinc[1,2]. while Molecules attempting with to a prepare[M]=CR free2 double ethyl radicalsbond (carbene [1,2]. Molecules complexes) with or a [M]≡CR triple bond a [M]=CR double bond (carbene(carbyne complexes) complexes) or a [M]wereCR synthesized triple bond much (carbyne later complexes) [3–6]. Two were types of compounds with 2 ≡ synthesized much later [3–6].metal-carbon Two types of compoundsdouble or withtriple metal-carbon bonds having double different or triple types bonds of bonds are generally having different types of bondsdistinguished, are generally which distinguished, are named which after arethe named people afterwho theisolated people them who first. Fischer-type carbene isolated them first. Fischer-typeand carbenecarbyne andcomplexes carbyne are complexes best described are best in describedterms of dative in terms bonds of following the Dewar– (–)(─) (+(+)) dative bonds following the Dewar–Chatt–DuncanChatt–Duncan (DCD) (DCD) model model [7,8] [7,8 ][M] [M]⇄CRCR2 2andand [M ] CR CR ,, whereas Schrock-type whereas Schrock-type alkylidenesalkylidenes and alkylidynes and alkylidynes are assumed are toassumed have electron-sharing to have electron-sharing double and double and triple bonds triple bonds [M]=CR and [M][M]=CRCR [9–211 and]. [M]≡CR [9–11]. 2 ≡ This review deals with transitionThis metalreview complexes deals with with transition metal-carbon metal complexes bonds to with two typesmetal-carbon of bonds to two types ligands, which have only recentlyof ligands, been isolatedwhich have and only theoretically recently been studied. isolated One and type theoretically of ligand are studied. One type of ligand carbones CL2 [12], which are carbon(0)are carbones compounds CL2 [12], which with two are dativecarbon(0) bonds compounds to a carbon with atom two in dative the bonds to a carbon atom excited 1D state L C¯ L wherein the the excited carbon 1 atomD state retains L→C← itsL four where valence the carbon electrons atom as tworetains lone its pairs four valence electrons as two ! that can serve as four-electronlone donors pairs [13 that,14 ].can Thus, serve carbones as four-electron CL2 are four-electrondonors [13,14]. donor Thus, ligands carbones CL2 are four-electron whereas carbenes CR2 are two-electrondonor ligands donors. whereas Carbenes carbenes have a formallyCR2 are two-electron [15] vacant p( donors.π) orbital Carbenes that have a formally [15] can accept electrons in donor-acceptorMolecules 2020 complexes, 25, x; doi: FOR M PEERCR2 REVIEWwhereas carbones are double (σ and π) www.mdpi.com/journal/molecules donors in complexes [M]CL2. A good Lewis acid acceptor fragment A for a carbene complex has a vacant σ orbital and an occupied π orbital whereas a suitable acceptor for a carbone is a double Lewis acid with vacant σ and π orbitals as shown in Figure1a,b. If the Lewis acid A has an occupied π orbital, it would lead to π repulsion with the π lone pair of the carbone CL2, whereby the repulsive interaction is reduced if L is a good π acceptor (Figure1c). The two electron lone pairs of a carbone may bind Molecules 2020, 25, 4943; doi:10.3390/molecules25214943 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 54 vacantMolecules p(2020π), 25orbital, x FOR PEERthat canREVIEW accept electrons in donor-acceptor complexes M⇄CR2 whereas2 of 54 carbones are double (σ and π) donors in complexes [M]⇇CL2. A good Lewis acid acceptor fragmentvacant p( πA) fororbital a carbene that cancomplex accept has electrons a vacant in σ donor-acceptororbital and an occupied complexes π orbitalM⇄CR whereas2 whereas a suitablecarbones acceptor are double for a( σcarbone and π is) donorsa double in Lewis complexes acid with [M] ⇇vacantCL2. A σ andgood π Lewisorbitals acid as shownacceptor in Figurefragment 1a,b. A Iffor the a carbeneLewis acid complex A has hasan occupied a vacant πσ orbital,orbital andit would an occupied lead to π π repulsion orbital whereas with the a πsuitable lone pair acceptor of the for carbone a carbone CL2 ,is whereby a double the Lewis repulsive acid with interaction vacant σ is and reduced π orbitals if L asis ashown good inπ MoleculesacceptorFigure2020 ,1a,b.25 ,(Figure 4943 If the 1c).Lewis The acid two A haselectron an occupied lone pairs π orbital, of a itcarbone would leadmay to bind π repulsion to one withor two the2 of 48 monodentateπ lone pair of Lewis the carbone acids A CL or 2protons, whereby or tothe a repulsivesingle bidentate interaction Lewis is acid reduced as shown if L isin aFigure good 1.π Theacceptor large (Figure second 1c).proton The affinity two electron is a characteristic lone pairs featureof a carbone of carbones, may bindwhich to distinguishes one or two to onethemmonodentate or two from monodentate carbenes Lewis acids[16]. Lewis Ex A amplesor acidsprotons of A all or cases to protons a single are known or bidentate to a singleand areLewis bidentate described acid as Lewis shownbelow. acid in Figure as shown 1. in FigureThe1. Thelarge large second second proton proton affinity a ffi nityis a ischaracteristic a characteristic feature feature of carbones, of carbones, which which distinguishes distinguishes themthem from from carbenes carbenes [16]. [16]. Examples Examples of allof casesall cases are are known known and and are are described described below. below. (a) (b) (a) (b) (c) (d) Figure 1. Schematic representation(c) of the most important orbital(d) interactions between carbene ligands CR2 and carbones CL2 with Lewis acids A.(a) Carbene complex with a monodentate Figure 1. Schematic representation of the most important orbital interactions between carbene ligands LewisFigure acid;1. Schematic (b) Carbone representation with a bidentate of the mostLewis impo acid;rtant (c) Carboneorbital interactions with a monodentate between carbene Lewis CR2 and carbones CL2 with Lewis acids A.(a) Carbene complex with a monodentate Lewis acid; acid;ligands (d ) CRCarbone2 and carboneswith two CLmonodentate2 with Lewis Lewis acids acids. A.(a ) Carbene complex with a monodentate (b) Carbone with a bidentate Lewis acid; (c) Carbone with a monodentate Lewis acid; (d) Carbone with Lewis acid; (b) Carbone with a bidentate Lewis acid; (c) Carbone with a monodentate Lewis two monodentate Lewis acids. Itacid; is important (d) Carbone to with realize two thatmonodentate the two electronLewis acids. lone-pairs of a carbone CL2 may additionally Itengage is important in π-backdonation to realize that to the the ligands two electron L whose lone-pairs strength of depends a carbone on CLthe availabilitymay additionally of vacant engage 2 in π-backdonationπ orbitalsIt is important of the to ligands the to ligands realize L.