Kinetic Studies of Water Exchange and Substitution by NCS– on the Sulphur-capped Triangular Ion [Μο3(μ3-S)(μ-O)3(ΟΗ2)9]4+

Abstract

A kinetic study of water exchange and NCS complexation on the mixed oxo-sulphido aqua complex [Μο3(μ3-5)(μ-0)3(ΟΗ2)9] 4+ is reported along with structural (Mo Kedge EXAFS) and electronic ( 95 MO NMR) data. The complex possesses nonequivalent water ligands with those labelled 'd' (two per Mo), located approximately trans to μ-οχο, undergoing exchange ~3 χ 10 4 times faster than those labelled 'c' (one per Mo), located approximately trans to the capping μ3-βυ1ρ1^ο group. The faster rate for the former arises, as with the all μ-sulphido cluster, [MO3^3-S)^-S)3(OH2)9] 4+ , from a conjugate-base labilising pathway involving the monohydroxy species, [Μο3(μ3-5)(μ-0)3(ΟΗ2)8(ΟΗ)] 3+ (iCaM = 0.58M estimated from the [Η + ] dependence of the anation reaction with NCS -). Deprotonation at an adjacent water'd' on the same Mo atom is suggested. Taken together, the substitution and water exchange data gathered on this family of cluster ions is consistent with dissociative ligand substitution preferentially at a water'd' on the monohydroxy species in all cases. Substitution of sulphido for oxo in the two core positions affects mainly the exchange at water'd'; substitution at the μ3^3ρρϊ^ position retarding the rate of exchange by a factor of ~4 (no effect on water 'c') whereas similar substitution at all three μ-bridging positions increases the rate of the water 'd' exchange ~200-fold but that on water 'c' only ~ll-fold. These along with the 95 Mo NMR with Mo Kedge EXAFS data suggest that the observed kinetic * Corresponding author. effects stemming from changes to the capping μ3-ligand may be largely structural in nature whereas a direct electronic influence (affecting most noticeably water'd') is implied for the μ-bridging ligands.

Enhanced Kinetic Lability of Ru(III) Centers in Oxo-Centered Mixed-Metal Ru2M Trinuclear Complexes (M = Zn and Mg)

“…The present mixed-metal Ru III 2 M II complexes exhibit site-distinctive kinetics in terminal ligand exchange reactions due to the nonequivalent metal centers, e.g., inert Ru(III) and labile M(II) ions. Few reports on multinuclear metal complexes exhibiting varied kinetic rates depending on the nature of metal centers are available at present …”

Ligand Substitution Reactions at Inorganic Centers

“…Because of the two kinds of bridging X ligand now present, two distinct substitution sites c and d are defined, Figure a, with distinctly differing reactivity. When the apical μ 3 -X ligand is S or Se with μ-X = O, substitution rates at the more labile d site are slower by a factor of 6 (X = S) and by 10 (X = Se) versus when μ 3 -X is O . However, a significant rate acceleration is seen when the bridging μ-O ligands are replaced by X = S (factor of ∼600); X = Se (factor of ∼2500).…”

“…What is also apparent from Figure is that the points representing water exchange on the various homometal aqua clusters (open circles) mentioned in this review lie distinctly away from the correlation line either significantly to the left, e.g., [Nb 3 (μ 3 -Cl)(μ-O) 3 (H 2 O) 9 ] 4+ A , [W 3 (μ 3 -O)(μ-O 2 CCH 3 ) 6 (H 2 O) 3 ] 2+ B , and [W 3 (μ 3 -O) 2 (μ-O 2 CCH 3 ) 6 (H 2 O) 3 ] 2+ C 149 , or significantly to the right, e.g., [Mo 3 (μ 3 -S)(μ-S) 3 (H 2 O) 8 OH] 3+ D , [Mo 3 (μ 3 -S)(μ-O) 3 (H 2 O) 8 OH] 3+ E ,262a [Ru 4 (μ-O) 6 (H 2 O) 12 ] 4+ F , [Mo 3 (μ 3 -O) 2 (μ-O 2 CCH 3 ) 6 (H 2 O) 3 ] 2+ G , and [Ru 2 (μ-OH) 2 (H 2 O) 8 ] 4+ H . Given the arguments presented above, it is interesting to speculate as to whether Figure indicates extreme dissociative D mechanisms for water exchange on clusters D (Δ S ⧧ = +107 ± 15 J K -1 mol -1 ), E (Δ S ⧧ = +57 ± 14), F (Δ S ⧧ = +69 ± 47), G (Δ S ⧧ = +77 ± 30), and H (Δ S ⧧ = +141 (cis to OH) and +168 (trans to OH)), while indicating extreme associative A mechanisms for water exchange on clusters A (Δ S ⧧ = −72 ± 7 J K -1 mol -1 ), B (Δ S ⧧ = −131 ± 11), and C (Δ S ⧧ = −164 ± 25).…”

Structural parameters for the incomplete cuboidal cluster cation: [Nb3(μ3-Cl)(μ-O)3(OH2)9]4+ in solutions of acid hydrolysed trivalent niobium from Nb K edge EXAFS

“…The findings have provided compelling evidence for the existence of the cluster [Nb 3 (µ 3 -Cl)(µ-O) 3 -(OH 2 ) 9 ] 4ϩ in these solutions following comparisons with a corresponding molybdenum K edge EXAFS study recently carried out on the structurally analogous incomplete cuboidal trinuclear cluster [Mo 3 (µ 3 -S)(µ-O) 3 (OH 2 ) 9 ] 4ϩ under similar conditions. 5 A concentrated sample of the green niobium cation (0.06 mol dm Ϫ3 per Nb 3 ) in 3.0 mol dm Ϫ3 CF 3 SO 3 H was prepared as described previously 1,2 following chromatographic elution with aqueous HCl, evaporation of the HCl eluates under vacuum and dissolution of the resulting chloride salt in aqueous 3.0 mol dm Ϫ3 CF 3 SO 3 H. Samples were then loaded into a specially designed Perspex sample cell contructed with polyester (Mylar) windows for EXAFS measurements. 5, 6 The EXAFS spectra were collected on the Wiggler 1 beam line station 9.2 at the Synchrotron Radiation Source at the UK CLRC Daresbury Laboratory operating at 2 GeV (eV ≈ 1.602 × 10 Ϫ19 J) and 200 mA.…”