| Abstract
| - Substitution reactions X for H2O at theNi of the cuboidal heterometallic cluster[Mo3NiS4(H2O)10]4+give rate constants in the range 9.4−44 M-1s-1 with X = Cl-,Br-, I-, and NCS-, consistentwith an Id dissociative interchange mechanism.Reactions with the two water soluble phosphines PTA (119M-1 s-1) andTPPTS3- (58) and reaction with CO (0.66M-1 s-1) lie at thetwo extremes of this range. Special conditions relating to sizeand competitiveness of the ligands for the Ni are considered. Anadditional feature is the protonation of PTA, which gives an unreactiveform. The tridentate 1,4,7-triazacyclononane ligand L in thecomplex[Mo3(NiL)S4(H2O)9]4+blocks substitution at the Ni.
- Comparisons (25 °C) are made of substitution reactions, Xreplacing H2O, at the tetrahedral Ni of theheterometallicsulfido cuboidal cluster[Mo3NiS4(H2O)10]4+,I = 2.00 M (LiClO4). Stopped-flowformation rate constants (kf/M-1 s-1) for six Xreagents, including two water soluble air-stable phosphines,1,3,5-triaza-7-phosphaadamantanePTA (119) and tris(3-sulfonatophenyl)phosphineTPPTS3- (58), and CO (0.66),Br- (14.6), I- (32.3), andNCS-(44) are reported alongside the previous value for Cl-(9.4). A dependence on [H+] is observed withPTA,which gives an unreactive form confirmed by NMR as N-protonated PTA(acid dissociation constant Ka =0.61M), but in no other cases with [H+] in the range0.30−2.00 M. The narrow spread of rate constants for allbutthe CO reaction is consistent with an Id dissociativeinterchange mechanism. In addition NMR studies withH217O enriched solvent are too slow for directdetermination of the water-exchange rate constant indicatingavalue <103 s-1. Equilibriumconstants/M-1 for 1:1 complexing with thedifferent X groups at the Ni are obtainedfor PTA (2040) and TPPTS3- (8900) by directspectrophotometry and from kinetic studies(kf/kb) forCl- (97),Br- (150), NCS- (690), and CO (5150).No NCS- substitution at the Ni is observed in thecase of the heterometalliccube[Mo3Ni(L)S4(H2O)9]4+,with tridentate 1,4,7-triazacyclononane(L) coordinated to the Ni.Substitution ofNCS- for H2O, at the Mo's of[Mo3NiS4(H2O)10]4+and[Mo3(NiL)S4(H2O)9]4+are much slower secondaryprocesses, with kf = 2.7 ×10-4 M-1s-1 and 0.94 ×10-4 M-1s-1 respectively. No substitution ofH2O byTPPTS3- or CO is observed over ∼1h ateither metal on[Mo3FeS4(H2O)10]4+,on[Mo4S4(H2O)12]5+or [Mo3S4(H2O)9]4+.
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