Metallaaromatics can be broadly defined as aromatic compounds in which one of the ring atoms is a transition metal. The metallabenzenes are one important class of these compounds that has undergone extensive study recently. Closely related species such as fused-ring metallabenzenes, heterometallabenzenes, π-coordinated metallabenzenes and metallabenzynes have also attracted considerable attention. Although many metallaaromatics can be considered as metalla-analogues of classic organic aromatic compounds, this is not always the case. Recent seminal studies have shown that metallapentalenes and metallapentalynes, which are metalla-analogues of the anti-aromatic compounds pentalene and pentalyne, are in fact aromatic and highly stable. Very unusual spiro-metallaaromatic compounds have also recently been isolated. In this concepts article, key features of all these intriguing metallaaromatic compounds are discussed with reference to the structural, spectroscopic, reactivity and theoretical studies that have been undertaken. These compounds continue to generate much interest, not only because of the contributions they make to fundamental chemical understanding, but also because of the promise of possible practical applications.

Metalla-analogues of archetypal aromatic molecules are attracting ever increasing interest. Although metallabenzenes (which fall within this class) have been well studied, fused-ring metallabenzenes are rare and of the linear polycyclic metallaaromatic hydrocarbons, only metallanaphthalene is known. Herein we report the first metallaanthracene, [Ir(C H {CH CO Me-5})Cl(PPh ) ]O SCF (5), which represents the next member of this series of polycyclic compounds. Structurally, 5 has a number of features in common with anthracene including fused-ring planarity and bond-length alternation. In analogues of classic reactions of anthracene, 5 forms a Diels-Alder adduct with maleic anhydride and on oxidation the unprecedented fused-ring metallaanthraquinone, [Ir(C H O{Br-6}{OMe-7}{=O-8}{=O-15})Br(PPh ) ], is obtained.

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Regioselective electrophilic substitution reactions of the iridabenzofurans [Ir(C 7 H 5 O{OMe-7})(CO)(PPh 3 ) 2 ]-[OTf] (1) and IrCl(C 7 H 5 O{OMe-7})(PPh 3 ) 2 (2) provide a convenient route to mononitro-, dinitro-, and mixed nitro-/halosubstituted derivatives. Treatment of cationic 1 with copper(II) nitrate in acetic anhydride ("Menke" nitration conditions) gives the mononitrated iridabenzofuran [Ir(C 7 H 4 O{NO 2 -2}{OMe-7})(CO)(PPh 3 ) 2 ][O 3 SCF 3 ] (3). Under the same conditions neutral 2 undergoes dinitration to form IrCl(C 7 H 3 O{NO 2 -2}{NO 2 -6}{OMe-7})(PPh 3 ) 2 (5). Simple substitution of the carbonyl ligand in 3 with chloride gives the neutral mononitro derivative IrCl(C 7 H 4 O{NO 2 -2}{OMe-7})(PPh 3 ) 2 (4). Depending on the conditions employed, treatment of the iridabenzofurans 1 and 2 with Cu(NO 3 ) 2 and either lithium chloride or lithium bromide in acetic anhydride gives either the mixed nitro-/halo-substituted iridabenzofurans IrCl(C 7 H 3 O{NO 2 -2}{Cl-6}{OMe-7})(PPh 3 ) 2 (6) and IrCl(C 7 H 2 O{NO 2 -2}{NO 2 -4}{Cl-6}{OMe-7})(PPh 3 ) 2 (7) or the simple halo-substituted iridabenzofurans [Ir(C 7 H 4 O-{Cl-6}{OMe-7})(CO)(PPh 3 ) 2 ][OTf] (8), [Ir(C 7 H 4 O{Br-6}{OMe-7})(CO)(PPh 3 ) 2 ][OTf] (9), and IrBr(C 7 H 3 O{Br-2}{Br-6}{OMe-7})(PPh 3 ) 2 (10). Bromination of 4 with pyridinium tribromide gives IrCl(C 7 H 3 O{NO 2 -2}{Br-6}{OMe-7})(PPh 3 ) 2 (11). The molecular structures of 3−7 and 11 have been obtained by X-ray crystallography.

The lithiocarbyne [W(CLi)(CO) 2 (Tp*)] (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) reacts with [PtCl 2 -(L 2 )] (L 2 = 1,5-cyclo-octadiene,n orbornadiene) to furnish ditungsten ethanediylidyne complexes,[W 2 {m-C 2 Pt(L 2 )}(CO) 4 -(Tp*) 2 ], wherein at rigonal platinum(0) center unsymmetrically ligates one WCb ond in the solid state but rapidly shimmies between the two WCb onds in solution. The h 4dienes are displaced by monodentate CO or isocyanide ligands to provide derivatives where both W Cb onds coordinate to as ingle Pt 0 center,a ttended by significant distortion of the WCCW spine. Scheme 1. Valence bond localization of m-dicarbido complexes. Scheme 2. Ethanediylidyne synthesis and novel coordination modes.

The Pd0/AuI mediated [C1+C2] cross‐coupling reactions of [W(≡CBr)(CO)2(Tp*)] (Tp*=hydrotris(dimethylpyrazolyl)borate) and trimethylsilylethynyl‐substituted arenes afford new polycyclic aromatic hydrocarbon propargylidynes [W(≡CC≡CR)(CO)2(Tp*)] (R=9‐anthracenyl, 1‐pyrenyl). The strategy extends to the first bis(propargylidyne) and bis(pentadiynylidyne) complexes bridged by phenyl or anthracenyl spacers, and to a tetrakis(propargylidyne) connected through a pyrene core.

The chemistryo ft ransitionm etal carbynes, L n M CR, has historically been dominated by speciesb earing hydrocarbyl or amino 'R' substituents, with other elements appearing only sporadically.I nr ecent years, carbynes and related 'C 1 's pecies bearing other main-group substituents, particularly heaviere lements of the p-block, have begun to emerge. This review details the chemistry of heavier pnicto-gen-functionalised C 1 ligands, MCAR n (A = P, As, Sb, Bi; n = 0-3), including their syntheses, properties andr eactivities,a nd how thesea re distinguished from more traditional carbyne complexes.R ecent developments in the closely related phospha-isonitrile L n M(CPR), cya-phosphide and cya-arside ligands,L n M(C A) (A = P, As), are also discussed.

Boron complexes of calix[4]phyrins (1.1.1.1) were prepared by reacting the free-base ligands with BF·EtO. The reaction conditions can be efficiently tailored to produce mono- or di-boron calixphyrins. Mono-BF calixphyrins with boron coordinating to either the dipyrrin, BF[H(Calix)], or dipyrromethane, BF[H(Calix)] and BF[H(Calix)], bonding sites were isolated. The dipyrromethane isomer, BF[H(Calix)], isomerises into BF[H(Calix)] which kinetic studies and DFT calculations indicate is an intramolecular process. Two isomers of BOF(Calix) were isolated, one isomer bonding via the dipyrrin sites with the FBOBF moiety in cisoid geometry, and the second isomer bonding via the dipyrromethane sites with the FBOBF moiety in transoid geometry. Although the cisoid/dipyrrin isomer was calculated to be most energetically favourable for BOF(Calix), the isolation of the transoid/dipyrromethane isomer is postulated to occur via the presumed intermediate (BF)(Calix), for which DFT indicated a preference for transoid/dipyrromethane geometry.