Ken Wade

Contemporary boron chemistry

Applications to Polyolefin Catalysis Materials and Polymers Medicinal Applications Cluster Synthesis Carboranes Metallaboranes Metallaheteroboranes Organic and Inorganic Chemistry of Mono- and Di-Boron Systems Theoretical and Computational Studies Subject Index.

Evolving patterns in boron cluster chemistry

This paper outlines the development of our knowledge and understanding of the structures and bonding of boron cluster compounds, with particular reference to the evolving complementary roles localized bonding and molecular orbital treatments have played in providing simple rationalizations of their polyhedral molecules.

Metal–metal and metal–ligand bond strengths in metal carbonyl clusters

Abstract The limited experimental thermochemical information about metal carbonyl clusters, and the more extensive literature on structural studies of such compounds, provide a means of exploring trends in their stabilities. This review surveys that literature for selected metals, showing how the enthalpy of disruption of gaseous Mx(CO)y clusters into gaseous metal atoms and carbon monoxide can be partitioned into two components representing the strengths of metal–metal and metal–ligand bonds. In doing so, it is assumed that the bond enthalpies, E(Mue5f8M), of metal–metal bonds vary smoothly with their length, d(Mue5f8M), according to a relationship E(Mue5f8M)=A[d(Mue5f8M)]−4.6, for which a justification is provided. The structure of a cluster thus provides a means of determining the total metal–metal bond enthalpy of that cluster. Application of this method to thermodynamically characterised clusters demonstrates that the average metal–ligand bond enthalpy, E(Mue5f8CO), in carbonyl clusters Mx(CO)y varies slightly with the ligand to metal ratio, y/x; a carbonyl ligand binds more strongly to a metal when it is competing with few other ligands. We demonstrate that for binary osmium carbonyl clusters, Osx(CO)y, the distances d(Osue5f8C) and d(Cue5f8O) are also functions of the ligand to metal ratio, y/x, providing evidence for the familiar synergistic bonding of the carbonyl ligand, and that these distances are a function of the metal–ligand bond enthalpy, E(Osue5f8CO). Trends in cluster stability, as determined by the total metal–metal bond enthalpy, ΣE(Mue5f8M), for anionic and carbonyl hydride clusters of osmium, rhenium and rhodium, [Mx(CO)yHz]c−, are presented. Similar trends for clusters of rhenium and rhodium containing core or interstitial carbon, nitrogen or other atoms are also explored, and partition of the atomisation enthalpy of binary metal carbides, MC and M2C, into metal–metal and metal–carbon components is investigated to provide insight into the strength of binding of core carbon atoms surrounded by octahedral arrays of metal atoms.

Bonding with boron.

Long ago, a global search for borane superfuels led fortuitously to the discovery of carboranes. Ken Wade recalls his own undistinguished part in the space race, and notes how carboranes revitalized boron hydride chemistry and modified our ideas of chemical bonding.

SOME BORON-CONTAINING RING SYSTEMS

The synthesis and characterisation of series of new types of boron-containing ring systems are described. They include: (a) carborane systems containing C-H--X (X = O or N) hydrogen bonds; (b) systems in which Cn rings share C-C links with o-carborane units; (c) macrocycles containing 2,3 or 4 carborane icosahedra linked through benzene or pyridine rings, and (d) ‘new types of ‘carborazacycles’ containing one carbon, two boron and three nitrogen atoms in a single 6-membered ring system.

Insertion and cleavage reactions of [closo-3,1,2-Ta(NMe2)3(C2B9H11)] with nitriles, phenols and thiols; structural characterisation of N,N-dimethylamidinate ligands

The tantalum complex [closo-3,1,2-Ta(NMe2)3(C2B9H11)] underwent insertion into the NC bond of acetonitrile and p-fluorobenzonitrile to give the N,N-dimethylacetamidinate complex [closo-3,1,2-Ta{NC(Me)NMe2}3(C2B9H11)], and p-fluoro-N,N-dimethylbenzamidinate, [closo-3,1,2-Ta{NC(C6H4F)NMe2}3(C2B9H11)], respectively. Attempted re-crystallisation of the latter from chlorinated solvents led to [closo-3,1,2-Ta{NC(C6H4F)NMe2}2Cl(C2B9H11)], in which one amidinate ligand has been replaced by a chloride. [closo-3,1,2-Ta(NMe2)3(C2B9H11)] reacts with cyclohexylisocyanide to give [Ta(NMe2)2{η2-N(Cy)CNMe2}(C2B9H11)]. The structures of the novel N,N-dialkylamidinate complexes have been determined by single crystal X-ray diffraction, and reveal the extensive delocalisation and strong π-donor character of the amidinate ligands. The M–N bonds of [closo-3,1,2-Ta(NMe2)3(C2B9H11)] are cleaved by protic reagents, and it reacts with 2,6-dimethylphenol to give [closo-3,1,2-Ta(OC6H3Me2-2,6)3(C2B9H11)] and with benzenethiol to give the charge-compensated complex [closo-3-Ta(SC6H5)4(9-NHMe2-1,2-C2B9H10)] where the β-boron of the C2B3 face bears a NHMe2+ substituent. The structures of the last two compounds have also been determined.

First structural characterisation of a 2,1,12-MC2B9 metallacarborane, [2,2,2-(NMe2)3-closo-2,1,12-TaC2B9H11]. Trends in boron NMR shifts on replacing a {BH} vertex with a metal {MLn} vertex in icosahedral carboranes

Reactions of M(NMe2)5 (Mxa0=xa0Ta or Nb) with nido-2,9-C2B9H13 and the salt [Me3NH][nido-7,9-C2B9H12] gave the isomeric dicarbollide complexes [2,2,2-(NMe2)3-closo-2,1,12-MC2B9H11] (Mxa0=xa0Ta 1, Nb 4) and [2,2,2-(NMe2)3-closo-2,1,7-MC2B9H11] (Mxa0=xa0Ta 2, Nb 5) respectively. The structures of 1 and 2 were determined by single crystal X-ray diffraction and 1 represents the first structurally characterised example of a 2,1,12-MC2B9 metallacarborane. Comparison of 11B NMR data of the tantalum complexes, along with the isomeric [3,3,3-(NMe2)3-closo-3,1,2-TaC2B9H11] 3, with that of 1,2-, 1,7- and 1,12-C2B10H12, reveals that the metal vertex {Ta(NMe2)3}, on replacing a {BH} vertex, influences significantly the boron NMR shifts of the neighbouring and antipodal cage atoms. Based on this observation the assignments of the reported peaks in the boron NMR data for the seven isomers of (η5-C5H5)CoC2B9H11 are tentatively predicted.

Unprecedented electron deficient bridging between zinc atoms by boron atoms of nido-carborane anions: preparation, crystal and molecular structure of the dimer [(nido-C2B9H11)ZnNMe3]2

The alkane elimination reaction between ZnMe2 and [(NMe3H)+(nido-C2B9H12)–] gives the macropolyhedral dimer [(nido-C2B9H11)ZnNMe3]2, containing an unprecedented planar diamond-shaped Zn2B2 ring at its core.

Transition metal dicarbollide complexes: synthesis, molecular, crystal and electronic structures of [M(C2B9H11)(NMe2)3] (M = Nb or Ta) and their insertion reactions with CO2 and CS2†

The homoleptic amides [M(NMe2)5] (Mxa0=xa0Nb 1 or Ta 2; the latter is characterised by a structural study) reacted with the carborane nido-C2B9H13 to eliminate two equivalents of HNMe2 and generate the dicarbollide half-sandwich tris(dimethylamide) complexes [M(C2B9H11)(NMe2)3] (Mxa0=xa0Nb 3 or Ta 4). The crystal structures of isomorphous 3 and 4 have been determined and reveal two NMe2 ligands in a vertical orientation and the third one in a horizontal orientation with respect to the η5-co-ordinated face of the C2B9H11 ligand. The electronic factors responsible for the amide ligand orientations in these complexes are explored using qualitative MO arguments. Complexes 3 and 4 reacted with CO2 and CS2 to yield the tris(carbamate) [M(C2B9H11)(O2CNMe2)3] (Mxa0=xa0Nb 5 or Ta 7) and tris(dithiocarbamate) [M(C2B9H11)(S2CNMe2)3] (Mxa0=xa0Nb 6 or Ta 8) complexes, respectively. The crystal structures of 6 and 7 show two (dithio)carbamate ligands in horizontal and one in vertical orientation, demonstrating the similarity between the σ,π-donor frontier orbitals of the ligands NMe2 and X2CNMe2 in 4 and 6 or 7 respectively.

Structural and bonding trends in osmium carbonyl cluster chemistry:metal–metal bond lengths and calculated strengths in the anions[Osx(CO)y]2-, hydrides[Osx(CO)yHz]and hydride anions[Osx(CO)yHz]c-

The metal–metal bond distances [d(M–M)] in the nknown structurally characterised osmium carbonyl anions, n[Os n x n(CO) n y n] n 2- n, neutral ncarbonyl hydrides, [Os n x n(CO) n y nH n z n] and ncarbonyl hydride anions, n[Os n x n(CO) n y nH n z n] n c- n, have nbeen used to calculate bond enthalpy terms E(Os–Os) using nthe relationship nE(Os–Os) = 1.928 × n10 n 13 n [d(Os–Os)] n -4.6 n, itself nderived from published structural and enthalpy data. Summation of the nmetal–metal bond enthalpy terms, to give the total nmetal–metal bond enthalpy, ΣE(Os–Os), has nrevealed the varying efficiency with which these compounds use their nelectrons for metal–metal bonding. There is a strong correlation nbetween the total metal–metal bond enthalpy per metal atom, nΣE(Os–Os)/x, and the number of ligand nelectrons per metal atom, the data falling on a curve which includes nbulk osmium metal and [Os(CO) n 5 n] at the extremes. Correlations nare also noted between ΣE(Os–Os) and the number of nskeletal electron pairs (polyhedral skeletal electron pair theory) or nnumber of formal two-centre two-electron (2c2e) bonds (18-electron nrule). These correlations show that the electrons are used more nefficiently for metal–metal bonding in larger clusters with fewer nligands. Thus, the metal–metal bond enthalpy per electron pair navailable (using the 18-electron rule) increases as the cluster becomes nlarger, indicating the error in models based on assigning fixed energies nto notional 2c2e Os–Os bonds. Trends in nΣE(Os–Os) were explored as Os(CO) n 4 n, nOs(CO) n 3 n or Os(CO) n 2 n fragments are added to clusters nin cluster build-up processes, as CO ligands are replaced by nH n - n, and on oxidative addition of H n 2 n to nclusters, the latter leading to a prediction of limiting values of nOs–H bond enthalpy terms. Trends in nΣE(Os–Os) were examined for series of closely nrelated clusters, including those derivable from n[Os n 4 n(CO) n 14 n] by replacing CO by H n - n nor H n 2 n, and a series of clusters derived from n[Os n 6 n(CO) n 18 n]. The sum nΣE(Os–Os) is shown to be a single parameter which nquantifies the overall effect of small changes in metal–metal ndistances in osmium carbonyl clusters.