Further studies of the Enhanced Structural Carborane Effect: tricarbonylruthenium and related derivatives of benzocarborane, dihydrobenzocarborane and biphenylcarborane

Powley, Samuel L.; Rosair, Georgina M.; Welch, Alan J.

doi:10.1039/c6dt01888b

Cited by 11 publications

(2 citation statements)

References 25 publications

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“…Briefly, in a carborane ligand the frontier molecular orbitals of the carborane are localised on the boron atoms in the open face, causing the cage carbon to have a weaker trans influence than the cage boron. In bis(phosphine) metallacarboranes this results in longer M–P distances trans to boron and in [3,3,3‐(CO) 3 ‐3,1,2‐ closo ‐MC 2 B 9 H 11 ] metallacarboranes to longer M–CO distances trans to boron (see Table of ref …”

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confidence: 99%

Balancing Steric and Electronic Effects in Carbonyl–Phosphine Molybdacarboranes

et al. 2017

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Analysis of the literature structures [(CO)(PPh3)2MC2B9H11] and [(CO)2(PPh3)MC2B9H11] suggests that in [L3MC2B9H11] metallacarboranes the trans influence of CO is greater than that of PPh3. Extending this study to the [L4MC2B9H11] system the new molybdacarboranes [3,3,3‐(CO)3‐3‐PPh3‐3,1,2‐closo‐MoC2B9H11] (2), [1,2‐Me2‐3,3,3‐(CO)3‐3‐PPh3‐3,1,2‐closo‐MoC2B9H9] (3) and trans‐[3,3‐(CO)2‐3,3‐(PPh3)2‐3,1,2‐closo‐MoC2B9H11] (4) were prepared and fully characterised. Consideration of the exopolyhedral ligand orientations (ELO) in 2 confirms that, in terms of trans influence, CO > PPh3 in [L4MC2B9H11] also. The ELO is effectively reversed in 3 through intramolecular steric crowding between the cage CH3 groups and the PPh3 ligand. The dicarbonylbis(triphenylphosphine) compound 4 has effective Cs symmetry with one CO ligand trans and the other CO ligand cis to the cage C–C connectivity. Unexpectedly the Mo–CO bond lengths are equal. DFT calculations on 4 reproduce this unusual result, but suggest that in the less‐crowded PH3 analogue, the Mo–CO bond length trans to cage C would be about 0.2 Å shorter than that trans to cage B. To test this prediction, the analogous PEt3 complex was prepared as cis and trans structural isomers 5 and 6. The cis isomer 5 is quantitatively converted into the trans isomer 6 when heated to reflux in THF. In 6 the Mo–CO bond more trans to cage C is about 0.2 Å shorter than that which is more trans to cage B, in line with the DFT prediction.

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Section: Introductionmentioning

confidence: 99%

Balancing Steric and Electronic Effects in Carbonyl–Phosphine Molybdacarboranes

et al. 2017

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“…(CO) 3 -closo-3,1,2-MC 2 B 9 H 11 ] x− structures[31], showing a general inverse relationship between |θ| and the M-CO distance.Figure 5a, the Co−Ccage connectivities are actually significantly shorter that the three Co−B distances even though the former are the weaker bonds[27]. On the other hand, clear, albeit indirect, evidence for weaker Co−Ccage bonding relative to Co−B bonding is evident in the Co−CCp distances, those trans to cage C being significantly shorter than those trans to B.…”

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What Can We Learn from the Crystal Structures of Metallacarboranes?

Welch

2017

Crystals

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The determination of the molecular structures of metallacarboranes by X-ray diffraction remains critical to the development of the field, in some cases being the only viable way in which the overall architecture and the isomeric form of the molecule can be established. In such studies one problem frequently met is how to distinguish correctly {BH} and {CH} vertices, and this review begins by describing two relatively new methods, the Vertex-Centroid Distance (VCD) and Boron-Hydrogen Distance (BHD) methods, that have been developed to overcome the problem. Once the cage C atoms are located correctly, the resulting metallacarborane structure can frequently be analysed on the basis that cage B has a greater Structural Trans Effect (STE) than does cage C. In the absence of significant competing effects this gives rise to unequal M-L distances for a homogeneous ligand set and to a preferred Exopolyhedral Ligand Orientation (ELO) for a heterogeneous ligand set. ELO considerations can be used, amongst other things, to rank order the STEs of ligands and to identify suspect (in terms of cage C atom positions) metallacarborane structures.

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