Since the publication of the last review in 1998, the transition-metal chemistry of boron has continued to raise unceasing interest. Boryl complexes, representing the most extensive subclass, have remained a focus of intense research, particularly for their implication in the metal-mediated functionalization of organic substrates. Absolute novelties such as borane complexes and terminal borylene complexes have been structurally authenticated. Upon further elaboration of these compounds, the known coordination modes of boron-based ligands have grown considerably. Combined structural and theoretical investigations have contributed to elucidate the fundamental electronic characteristics of the transition-metal-boron bond and are leading to applications of these compounds. The most useful synthetic strategies for the generation of transition-metal-boron bonds are highlighted here, and the most recent and intriguing compounds that have been reported are outlined and discussed.

Transition-metal-borylene complexes of the type [(OC)(5)M=BR] {M=Cr, Mo, W; R=N(SiMe(3))(2), 1a-3a, Si(SiMe(3))(3), 4a} and [(OC)(4)Fe=B=N(SiMe(3))(2)] (8) were prepared by salt elimination reactions. Synthesis of the latter complex was accompanied by the formation of substantial amounts of an unusual dinuclear iron complex [Fe(2){mu-C(2)O(2)(BN(SiMe(3))(2))}(2)(CO)(6)] (9). The aminoborylene complexes of Group 6 metals were converted to trans-[(Cy(3)P)(CO)(4)M=B=N(SiMe(3))(2)] (5a-7a) by irradiation in the presence of PCy(3). Structural and spectroscopic parameters were discussed with respect to the trans-effect of the borylene ligand and the degree of M-B d(pi)-p(pi)-backbonding. Computational studies were performed on Group 6-borylene complexes. The population and topological analyses as well as the molecular orbital composition are consistent with the presence of both sigma-and pi-type interactions. There are, however, indications that the d(pi)-p(pi)-backbonding in the silylborylene complex is significantly more pronounced than in the aminoborylene complexes.

Nahezu linear sind die drei Atome der zentralen W‐B‐N‐Einheit des Wolframkomplexes [(CO)5WBN(SiMe3)2] 1 im Kristall angeordnet (Bindungswinkel 177.9°; siehe Bild). Diese Verbindung und ihr Cr‐Analogon sind die ersten terminalen Borylenkomplexe mit zweifach koordiniertem Bor. Die ähnliche Geometrie des axialen und der äquatorialen CO‐Liganden in 1 spricht gegen einen trans‐Effekt des Borylenliganden.

A nearly linear arrangement is observed for the three atoms in the central W-B-N unit of the tungsten complex [(CO) WBN(SiMe ) ] (1) in the crystal (W-B-N 177.9°; see picture). This compound along with its Cr analogue represent the first examples of terminal borylene complexes with a two-coordinate metal-bound boron atom. The geometries of the axial and equatorial CO groups in 1 are similar, and thus indicate that there is no trans effect of the borylene ligand.

Seit der Veröffentlichung des letzten Aufsatzes im Jahre 1998 haben Übergangsmetallkomplexe des Bors anhaltendes Interesse ausgelöst. Borylkomplexe, die zahlenmäßig größte Klasse, wurden weiterhin intensiv erforscht, besonders in Hinblick auf die metallkatalysierte Funktionalisierung von organischen Substraten. Zusätzlich konnten neue Verbindungsarten wie Boran‐ und terminale Borylenkomplexe charakterisiert werden. Diese Studien erweiterten den Umfang der bekannten Koordinationstypen für borhaltige Liganden beträchtlich. Durch strukturanalytische und theoretische Untersuchungen konnten grundlegende elektronische Eigenschaften der Übergangsmetall‐Bor‐Bindung aufgeklärt werden, sodass sich Anwendungsmöglichkeiten für diese Komplexe abzeichnen. Hier werden die besten Methoden zur Bildung von Übergangsmetall‐Bor‐Bindungen vorgestellt, die zu neuen, faszinierenden Verbindungen geführt haben.

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Depending on the nature of the amino group bound to the boron atom, the reactions of various aminodichloroboranes R2NBCl2 with Na[(η5‐C5R′5)Fe(CO)2] yield either boryl or bridging borylene complexes of iron. The compounds [(C5R′5)(CO)2Fe{BCl(NR2)}] (1a, C5R′5 = C5H5, R = Me; 1b, C5R′5 = C5H4Me, R = Me; 1c, C5R′5 = C5Me5, R = Me) and [(η‐BNR2)(μ‐CO){(C5R′5)Fe(CO)}2] (2a, C5R′5 = C5H5, R = SiMe3; 2b, C5R′5 = C5H4Me, R = SiMe3) were isolated as orange (1a−c) or red (2a, b) crystalline solids, and characterized by multinuclear NMR methods and IR spectroscopy. The structures of 1c and 2b in the crystalline state were determined by single‐crystal X‐ray studies.

The reactivity of aminodihaloboranes R 2 NBX 2 (R = Me, be isolated from these mixtures as pure materials. In addition the novel boryl and borylene ruthenium complexes [(η 5 -SiMe 3 ; X = Cl, Br) towards transition metal complexes of the type Na[(η 5 -C 5 RЈ 5 )M(CO) 2 )] (M = Fe, Ru; RЈ = H, Me) was C 5 H 5 )(CO) 2 Ru{BX(NMe 2 )}] (X = Cl 12a; X = Br 12b), [(η 5 -C 5 H 5 )(CO) 2 Ru{BCl{NSiMe 3 {BClN(SiMe 3 ) 2 }}}] (13) and [µ-investigated. In the case of Me 2 NBBr 2 and M = Fe the borylcomplexes [(η 5 -C 5 RЈ 5 )(CO) 2 Fe{BBr(NMe 2 )}] (C 5 RЈ 5 = BN(SiMe 3 ) 2 (µ-CO){(η 5 -C 5 H 5 )Ru(CO)} 2 ] (14) were obtained by similar methods. All compounds were characterized by C 5 H 5 9a; C 5 RЈ 5 = C 5 H 4 Me 9b; C 5 RЈ 5 = C 5 Me 5 10) were obtained. The compounds 9a and 9b were formed together multinuclear NMR and IR spectroscopy. The structure of 13 in the solid state was determined by a single-crystal X-ray with the corresponding bridged borylene complexes [µ-BNMe 2 (µ-CO){(η 5 -C 5 RЈ 5 )Fe(CO)} 2 ] (C 5 RЈ 5 = C 5 H 5 11a; diffraction study. C 5 RЈ 5 = C 5 H 4 Me 11b) in a 1:1 ratio, the latter, however, could C 5 RЈ 5 )Fe(CO)} 2 ] (C 5 RЈ 5 ϭ C 5 H 5 3a; C 5 RЈ 5 ϭ C 5 H 4 Me 3b) [b]