Comparisons (25 degrees C) are made of substitution reactions, X replacing H(2)O, at the tetrahedral Ni of the heterometallic sulfido cuboidal cluster [Mo(3)NiS(4)(H(2)O)(10)](4+), I = 2.00 M (LiClO(4)). Stopped-flow formation rate constants (k(f)/M(-)(1) s(-)(1)) for six X reagents, including two water soluble air-stable phosphines, 1,3,5-triaza-7-phosphaadamantane PTA (119) and tris(3-sulfonatophenyl)phosphine TPPTS(3)(-) (58), and CO (0.66), Br(-) (14.6), I(-) (32.3), and NCS(-) (44) are reported alongside the previous value for Cl(-) (9.4). A dependence on [H(+)] is observed with PTA, which gives an unreactive form confirmed by NMR as N-protonated PTA (acid dissociation constant K(a) = 0.61 M), but in no other cases with [H(+)] in the range 0.30-2.00 M. The narrow spread of rate constants for all but the CO reaction is consistent with an I(d) dissociative interchange mechanism. In addition NMR studies with H(2)(17)O enriched solvent are too slow for direct determination of the water-exchange rate constant indicating a value <10(3) s(-)(1). Equilibrium constants/M(-)(1) for 1:1 complexing with the different X groups at the Ni are obtained for PTA (2040) and TPPTS(3)(-) (8900) by direct spectrophotometry and from kinetic studies (k(f)/k(b)) for Cl(-) (97), Br(-) (150), NCS(-) (690), and CO (5150). No NCS(-) substitution at the Ni is observed in the case of the heterometallic cube [Mo(3)Ni(L)S(4)(H(2)O)(9)](4+), with tridentate 1,4,7-triazacyclononane(L) coordinated to the Ni. Substitution of NCS(-) for H(2)O, at the Mo's of [Mo(3)NiS(4)(H(2)O)(10)](4+) and [Mo(3)(NiL)S(4)(H(2)O)(9)](4+) are much slower secondary processes, with k(f) = 2.7 x 10(-)(4) M(-)(1) s(-)(1) and 0.94 x 10(-)(4) M(-)(1) s(-)(1) respectively. No substitution of H(2)O by TPPTS(3)(-) or CO is observed over approximately 1h at either metal on [Mo(3)FeS(4)(H(2)O)(10)](4+), on [Mo(4)S(4)(H(2)O)(12)](5+) or [Mo(3)S(4)(H(2)O)(9)](4+).

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There are numerous examples where animals or plants synthesize extracellular high-performance skeletal biocomposites consisting of a matrix reinforced by fibrous biopolymers. Cellulose and chitin are classical examples of these reinforcing elements, which occur as whisker-like microfibrils that are biosynthesized and deposited in a continuous fashion. In many cases, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free, with the consequence of axial physical properties approaching those of perfect crystals. Starch is another example of natural semicrystalline polymer that is produced by many plants and occurs as microscopic granules. It acts as a storage polymer in cereals and tubers. These abundant and natural polymers can be used to create high performance nanocomposites presenting outstanding properties. Aqueous suspensions of crystallites can be prepared by acid hydrolysis of the purified substrates. The object of this treatment is to dissolve away regions of low lateral order so that the water-insoluble, highly crystalline residue may be converted into a stable suspension by subsequent vigorous mechanical shearing action. For cellulose and chitin, these monocrystals appear as rod-like nanoparticles which dimensions depend on the biological source of the substrate. In the case of starch they consist of platelet-like nanoparticles. High reinforcing capability was reported resulting from the intrinsic chemical nature of these polymers and from their hierarchical structure. During the last decade, many works have been devoted to mimic biocomposites by blending cellulose whiskers from different sources with polymer matrices.

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Three heterometallic Mo 3 MЈS 4 derivatives (MЈ = Pt, Rh, Re) of the incomplete single-metal depleted cube [Mo 3 S 4 (H 2 O) 9 ] 4ϩ have been prepared by reactions with [PtCl 4 ] 2Ϫ (ϩ H 3 PO 2 reductant), RhCl 3 and [Re(CO) 5 Br], respectively. With [PtCl 4 ] 2Ϫ , the initial product gives, on standing for 2-3 days, the edge-linked double cube [{Mo 3 PtS 4 (H 2 O) 9 } 2 ] 8ϩ , which is difficult to elute in Dowex cation-exchange chromatography. In the reaction with RhCl 3 , chloro products, e.g. [Mo 3 RhCl 3 S 4 (H 2 O) 9 ] 4ϩ , precede formation of [Mo 3 RhS 4 (H 2 O) 12 ] 7ϩ , which is also difficult to elute, and with [Re(CO) 5 Br], the product [Mo 3 Re(CO) 3 S 4 (H 2 O) 9 ] 5ϩ is obtained. Analyses are consistent with oxidation state assignments Pt 0 , Rh III and Re I , where the reactions proceed by addition of these forms to [Mo 3 S 4 (H 2 O) 9 ] 4ϩ . The analyses and X-ray crystal structure of (Mo 2 NH 2 )[Mo 3 Re(CO) 3 S 4 (NCS) 4 ] are consistent with the formation of an Mo 3 ReS 4 single cube. Yields are high in the first two cases, but much lower in the Re case. Alternative preparations are described in the case of Rh and Re. All three products decay on heating, with reformation of trinuclear [Mo 3 S 4 (H 2 O) 9 ] 4ϩ , which can be recovered by Dowex chromatography. As compared to heterometallic derivatives so far considered, those with MЈ = Pt, Rh, Re are much more substitution (and redox) inert, and are air stable. Analogues from [W 3 S 4 (H 2 O) 9 ] 4ϩ with MЈ = Pt, Rh have been prepared using similar procedures. DALTON