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Investigating CN− cleavage by three-coordinate M[N(R)Ar]3 complexes

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Christian, G and Stranger, R and Yates, BF and Cummins, CC (2008) Investigating CN− cleavage by three-coordinate M[N(R)Ar]3 complexes. Dalton Transactions. pp. 338-344. ISSN 1477-9226

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Abstract

Three-coordinate Mo[N(tBu)Ar]3 binds cyanide to form the intermediate [Ar(tBu)N]3Mo–CN–Mo[N(tBu)Ar]3 but, unlike its N2 analogue which spontaneously cleaves dinitrogen, the C–N bond remains intact. DFT calculations on the model [NH2]3Mo/CN− system show that while the overall reaction is significantly exothermic, the final cleavage step is endothermic by at least 90 kJ mol−1, accounting for why C–N bond cleavage is not observed experimentally. The situation is improved for the [H2N]3W/CN− system where the intermediate and products are closer in energy but not enough for CN− cleavage to be facile at room temperature. Additional calculations were undertaken on the mixed-metal [H2N]3Re+/CN−/W[NH2]3 and [H2N]3Re+/CN−/Ta[NH2]3− systems in which the metals ions were chosen to maximise the stability of the products on the basis of an earlier bond energy study. Although the reaction energetics for the [H2N]3Re+/CN−/W[NH2]3 system are more favourable than those for the [H2N]3W/CN− system, the final C–N cleavage step is still endothermic by 32 kJ mol−1 when symmetry constraints are relaxed. The resistance of these systems to C–N cleavage was examined by a bond decomposition analysis of [H2N]M–L1≡L2–M[NH2]3 intermediates for L1≡L2 = N2, CO and CN− which showed that backbonding from the metal into the L1≡L2 pi* orbitals is significantly less for CN− than for N2 or CO due to the negative charge on CN− which results in a large energy gap between the metal d pi and the pi* orbitals of CN−. This, combined with the very strong M–CN− r interaction which stabilises the CN− intermediate, makes C–N bond cleavage in these systems unfavourable even though the C≡N triple bond is not as strong as the bond in N2 or CO.

Item Type: Article
Journal or Publication Title: Dalton Transactions
Page Range: pp. 338-344
ISSN: 1477-9226
Identification Number - DOI: 10.1039/b713757e
Date Deposited: 30 Mar 2008 21:38
Last Modified: 18 Nov 2014 03:31
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