Retention of Pt→B Bonding in Oxidative Addition Reactions of the Platinaboratrane [Pt(PPh3){B(mt)3}](Pt→B)10 (mt = Methimazolyl)

Heterobimetallic μ2-carbido complexes of platinum and tungsten

The tungsten–platinum μ-carbido complex [WPt(μ-C)Br(CO)₂(PPh₃)₂(Tp*)] (Tp* = hydrotris(dimethylpyrazol-1-yl)borate) undergoes facile substitution of both bromide and phosphine ligands to afford a diverse library of μ-carbido complexes.

Relative hemilabilities of H2B(az)2 (az = pyrazolyl, dimethylpyrazolyl, methimazolyl) chelates in the complexes [M(η-C3H5)(CO)2{H2B(az)2}] (M = Mo, W)

The question of B–H–Mo hemilability in a range of dihydrobis(azolyl)borate scorpionate ligands is discussed with reference to η³-allyl complexes [Mo(η³-C₃H₅)(CO)₂{H₂B(az)₂}] [az = pyrazolyl (pz), dimethylpyrazolyl (pz*), mercaptoimidazolyl (mt)].

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Synthesis and ligand substitution reactions of κ4-B,S,S′,S′′-ruthenaboratranes

Ruthenaboratranes of the form [Ru(CO)L{κ⁴-B(mt)₃}] (mt = N-methimazolyl) arise via substitution of the PPh₃ ligand in [Ru(CO)(PPh₃){κ⁴-B(mt)₃}] by L (L = PMe₂Ph, PMe₃, P(OMe)₃, P(OEt)₃, P(OPh)₃) or reactions of [RuCl(R)(CO)L_n] (R = Ph, CHCHPh; n = 2, L = PCy₃; n = 3, L = P(OMe)₃, PMe₂Ph) with Na[HB(mt)₃].

Methimazolyl based diptych bicyclo-[3.3.0]-ruthenaboratranes

The reactions of [RuCl(R)(CO)(PPh₃)₂] (R = CHCHPh, Ph) with Na[H₂B(mt)₂] (mt = N-methyl-2-mercaptoimidazolyl) transiently provide [Ru(R)(CO)(PPh₃){κ³-H,S,S′-H₂B(mt)₂}] which each evolve to the ruthenaboratrane [Ru(CO)(PPh₃)₂{κ³-B,S,S′-BH(mt)₂}](Ru→B)⁸.

Zero-valent isocyanides of nickel, palladium and platinum as transition metal σ-type Lewis bases

Transition metal complexes that contain metal-to-ligand retrodative σ-bonds have become the subject of increasing studies over the last decade. Lewis acidic "Z-type ligands" can modulate the electronic structure of their resultant complexes in a manner distinct from 2e^- donor ligands, and can also engage in cooperative reactivity with a Lewis basic transition metal. In this Feature article, we summarize our work with transition metal isocyanide complexes of group 10 metals that have exploited metal-based σ-type Lewis basicity. While the complexes Ni(CNAr^Mes2)₃, Pd(CNAr^Dipp2)₂ and Pt(CNAr^Dipp2)₂ were initially targeted as analogues to unstable, low-coordinate metal carbonyls, it soon became apparent that these zero-valent metal centers bore appreciable Lewis basic qualities due largely to the enhanced σ-donor/π-acid ratio of isocyanides compared to CO. Detailed spectroscopic and structural studies of metal-only Lewis pairs (MOLPs) formed from these complexes have furthered our understanding of the electronic structure perturbations effected by Z-type ligand binding. In addition, the platinum (boryl)iminomethane (BIM) complex Pt(κ²-N,B-^Cy₂BIM)(CNAr^Dipp2) has illuminated a general ligand design strategy that can engender significant reverse-dative interactions with buttressed Lewis acids, and also has expanded the known scope of cooperative reactivity that can be realized at a transition metal-borane linkage.

An Efficient Method for the Synthesis of Boratrane Complexes of Late Transition Metals

In a quest for efficient precursors for the synthesis of boratrane complexes of late transition metals, we have developed a useful synthetic method using [L'M(μ-Cl)Cl_x ]₂ as precursors (L'=η⁶ -p-cymene, M=Ru, x=1; L'=COD, M=Rh, x=0 and L'=Cp*, M=Ir or Rh, x=1; COD=1,5-cyclooctadiene, Cp*=η⁵ -C₅ Me₅ ). For example, treatment of Na[(H₃ B)bbza] or Na[(H₂ B)mp₂ ] (bbza=bis(benzothiazol-2-yl)amine; mp=2-mercaptopyridyl) with [L'M(μ-Cl)Cl_x ]₂ yielded [(η⁶ -p-cymene)RuBH{(NCSC₆ H₄ )(NR)}₂ ] (2; R=NCSC₆ H₄ ), [{N(NCSC₆ H₄ )₂ }RhBH{(NCSC₆ H₄ )(NR)}₂ ] (3; R=NCS-C₆ H₄ ), [(η⁶ -p-cymene)RuBH(L)₂ ] (5; L=C₅ H₄ NS), and [Cp*MBH(L)₂ ] (6 and 7; L=C₅ H₄ NS, M=Ir or Rh). In order to delineate the significance of the ligands, we studied the reactivity of [(COD)Rh(μ-Cl)]₂ with Na[(H₃ B)bbza], which led to the formation of the isomeric agostic complexes [(η⁴ -COD)Rh(μ-H)BHRh(C₁₄ H₈ N₃ S₂ )₃ ], 4 a and 4 b, in parallel to the formation of 16-electron square-pyramidal rhodaboratrane complex 3. Compounds 4 a and 4 b show two different geometries, in which the Rh-B bonds are shorter than in the reported Rh agostic complexes. The new compounds have been characterized in solution by various spectroscopic analyses, and their structural arrangements have been unequivocally established by crystallographic analyses. DFT calculations provide useful insights regarding the stability of these metallaboratrane complexes as well as their M→B bonding interactions.

Synthesis and Oxidation of a Paddlewheel-Shaped Rhodium/Antimony Complex Featuring Pyridine-2-Thiolate Ligands

The paddlewheel-shaped complex [Sb(μ-pyS)₄ Rh]₂ (1) (pyS^- = 2-S-C₅ H₄ N^- ) was synthesized from [Rh(pyS)(cod)]₂ (cod=1,5-cyclooctadiene) and Sb(pyS)₃ . Upon oxidation with ONMe₃ , the complex [(μ-O)Sb(μ-pyS)₃ Rh(κ² -pyS)]₂ (2) is formed. Both 1 and 2 form dimers and feature short Rh-Sb bonds and bridging pyS ligands. ¹²¹ Sb Mössbauer spectro- scopy and computational studies were employed to elucidate the Rh-Sb bonding in 1 and 2. Both covalent (Rh-Sb, X-type Sb ligand) and dative (Rh→Sb, Z-type; Rh←Sb L-type Sb ligand) interactions have to be considered for the description of their bonding situations.

Donor ligands based on tricoordinate boron formed by B–H-activation of bis(phosphine)boronium salts

The (R₃P)₂HB-moiety in a palladium(ii) pincer complex is demonstrated to be a L-type ligand and compared with other bonding types.

Complexes of ambiphilic ligands: reactivity and catalytic applications

This review focuses on the way Lewis acids of ambiphilic ligands influence the reactivity of transition metal complexes and on the new perspectives this opens in catalysis.

Air-stable cationic gold(I) catalyst featuring a Z-type ligand: promoting enyne cyclizations

An air-stable cationic Au(I) complex featuring a Z-type ligand (boron atom) as a σ-acceptor was developed for elucidating the effect of B on catalytic reactions. An enyne cyclization in the presence of either [Au→B](+) or [Au](+) showed that [Au→B](+) promotes the reactivity, which enabled the effective construction of not only five- and six-membered rings, but also seven-membered rings.

Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane

Reversible, heterolytic addition of H2 across an iron-boron bond in a ferraboratrane with formal hydride transfer to the boron gives iron-borohydrido-hydride complexes. These compounds catalyze the hydrogenation of alkenes and alkynes to the respective alkanes. Notably, the boron is capable of acting as a shuttle for hydride transfer to substrates. The results are interesting in the context of heterolytic substrate addition across metal-boron bonds in metallaboratranes and related systems, as well as metal-ligand bifunctional catalysis.

Acyclic boron-containing π-ligand complexes: η2- and η3-coordination modes

Cyclic boron-containing π-ligands such as boratabenzenes and borollides are well established, in particular as supporting ligands. By contrast, the chemistry of acyclic boron-containing π-ligands has remained relatively unexplored, presumably in part due to the higher reactivity of acyclic π-ligands relative to cyclic analogues. This perspective is focused on the synthesis, structures and reactivity of isolated transition metal complexes bearing η(n)-coordinated (n = 2 or 3) acyclic boron-containing ligands. Both monometallic and multimetallic compounds are included, and are discussed with an emphasis on metal-ligand and intraligand bonding and parallels with hydrocarbon π-ligand complexes.