Electronic Structure of Ligand-Bridged Complexes Containing the [Rh2]3+ Core: ESR Spectroscopy, MO Calculations, and X-ray Structures of the Three Redox Pairs [Rh2(CO)2LL′{μ-PhNC(Me)NPh} 2]z [z = 0, 1; L = L′ = PPh3, P(OPh)3; L = PPh3, L′ = P(OPh)3]

David C. Boyd, Neil G. Connelly, Gabriel Garcia Herbosa, Michael G. Hill, Kent R Mann, Carlo Mealli, A. Guy Orpen, Karen E. Richardson, Philip H. Rieger

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Abstract

The X-ray crystal structure analyses of the three redox pairs [Rh2(CO)2LL′{μ-PhNC(Me)NPh}2]z [z = 0, 1; L = L′ = PPh3 (1,1+); L = L′ = P(OPh)3 (2, 2+); L = PPh3, L′ = P(OPh)3 (3, 3+)] are reported and the molecular structures compared with those of [Rh2(CO)2(PPh3)2(μ-RNNNR)2]z [z = 0,1; R = p-tolyl (4,4+)]. These complexes have locally planar rhodium coordination with the planes splayed apart between 20 and 40°. The nonbonded Rh···Rh distance of ca. 2.9 Å in the neutral complexes is reduced by between 0.147(2) and 0.262(4) Å on oxidation. These and other changes in the molecular structure are consistent with the HOMO of the neutral complexes being delocalized, over the two metals, and having rhodium-rhodium σ* character. ESR spectroscopic analysis is reported on 1+-4+ and the related ligand-bridged [Rh2]3+ species [Rh2(CO)2(μ-dppm)(μ-RNNNR)2]+ [R = p-tolyl (5+)], [Rh2(μ-dimen)2(μ-dppm)2]3+ (6+, dimen = 1,8-diisocyanomenthane), [Rh2((μ-TM4)2((μ-dppm)2]3+ [TM4 = 2,5- diisocyano-2,5-dimethylhexane(7+)],[Rh2((μ-CNCH2CH2CH2NC)2((μ-dppm)2]3+(8+),[Rh2((μ-HTP5)2((μ-dppm)2]3+ (9+,HTP5 = 1,5-diisocyano-1,1,5-triphenylpentane),[Rh2(CO)2(PMe2Ph)2((μ-chp)2]+(10+), [Rh2(μ-mhp)2(COD)2]+ (11+), [Rh2((μ-chp)2(NBD)2]+ (12+), and [Rh2((μ-mhp)2(NBD)2]+ (13+). The spectroscopic parameters are shown to be dependent on the geometry of the cations and especially on the splay angle between the rhodium coordination planes. Quantitative interpretation of the ESR parameters suggests an orbital composition for the SOMO which is in good agreement with extended Hückel MO calculations on the model compounds [{Rh(H)2(CO)2}2]2- (14) and [Rh2(CO)4((μ-HNNNH)2] (15). The theoretical and ESR analyses are in full accord with the X-ray results and together provide a coherent picture of the electronic and geometric structures of ligand-bridged [Rh2]3+ complexes.

Original languageEnglish (US)
Pages (from-to)960-971
Number of pages12
JournalInorganic chemistry
Volume33
Issue number5
DOIs
StatePublished - Mar 1 1994

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