Organometallic Complexes of Nickel, Palladium and Platinum

We have examined the action of organomercury reagents on complexes of the nickel group in an attempt to find a cleaner route to the organotransition-metal compounds, and in the hope of clarifying the mechanisms involved in these reactions. The reactions between R2Hg (R = Ph or Me) and (R3P)2MCl2(M = Ni, Pd or Pt) yield RHgCl and trans- (R31P)2 MRCl. The reactions with R = Ph proceed more readily than those with R = Me. Reverse reactions between HgCl2 and (R3 1P)2 MR2 produce the same products. R2Hg and the carbonyl complexes (R3 1P) (CO)PtCl2 react to give RHgCl and the previously unknown chlorine-bridged compounds (R3 1P)2 Pt2 (COR)2Cl2. No carbonyl insertion is observed when RLi replaces R2Hg, and Grignard reagents lead to a complex mixture of products. Possible mechanisms of the mercurial reactions are discussed. It seems that the synthetic value of the reactions is small, except for those involving carbonyl insertion. The halogen-bridged binuclear acyl complexes are produced in good yield and can be converted into other derivatives by bridge-splitting reactions using tertiary phosphines or cyclopentadienyl thallium, for example. Till now, very few cyclopentadienyl complexes of palladium or platinum have been reported. We investigated the possibility of using dicyclopentadienyl mercury as an agent to introduce the ligand to complexes of the nickel group. Although we have spectroscopic evidence that (C5H5)2Hg can be so utilised, no organometallic complexes could be isolated from the reaction mixtures due to decomposition of the organomercury by-products. We find, however, that the binuclear halogen-bridged complexes (R3P)2Pd2X4. (R = Et, iPr or Ph; X = Cl, Br or I) react with cyclopentadienyl thallium to produce (pi-C5H5) (R3P)PdX. Yields are poor when analogous platinum complexes are used, but halogen-bridged organoplatinum compounds (R3P)2Pt2R21X2 give good yields of (pi -C5H5) (R3P)PtR1 (R = Et; R1 = Ph or o-tolyl: R = Bu n; R1 = COPh). The complexes (pi-C5H5)(R3P)MX (M = Pd or Pt) react with tertiary phosphines to give either ionic pi-bonded derivatives [(pi -C5H5) (R3P)2M]+X- or unstable d-bonded compounds (R3P)2M(D-C5H5)X, depending on the nature of the solvent and the tertiary phosphine. The same complexes can be produced from (R3P)2MX2and C5H5T1. The nickel complexes (R3P)2iNiX2, on the other hand, react with C5H5-T1 or (C5H5)2Hg to yield (pi -C5H5) (R3P)NiX. The diarsine complex [C6H4(AsEt2)2] Pt (D-C5H5)Cl, produced from [C6H4(AsEt2)2] PtCl2 and C5H5T1, is stable in air to 130. Less stable D-bonded cyclopentadienyl complexes can be prepared from the reactions of (pi-C5H5) (R3P)PtR1 with R3P or CO, or cis-(R3P)(CO)PtCl2 with C5H5T1. The palladium complex (pi -C5H5)(Et3P)PdBr reacts with C5H5T1 to produce the novel derivative (pi-C5H5) ( D-C5H5)Pd(PEt3). Reaction between hydrazine hydrate and (pi-C5H5) (R3P)PdBr (R = Et or iPr) gives the phosphido-bridged complexes [(pi-C5H5 )PdPR2]2. The physical techniques of 1H n. m. r. , i. r. and mass spectroscopy were applied in the characterisation of all the complexes. The platinum derivative C20H20Pt2, first reported by German workers in 1967, has been re-investigated by X-ray analysis. The complex contains a platinum-platinum bond, with a pi-cyclopentadienyl ring bound to each metal atom. The most interesting feature of the structure is the presence of the hitherto unsuspected 5-(5 1-cyclopentadienyl)-cyclopentadiene unit bridging the two metal atoms. Finally, we note that facile pi→ D rearrangement of C5H5 rings is a dominant feature in the chemistry of the cyclopentadienyl complexes of nickel, palladium and platinum, and this interconversion is discussed at several points in the text

The ecosystem and evolutionary contexts of allelopathy

Plants can release chemicals into the environment that suppress the growth and establishment of other plants in their vicinity, a process known as ‘allelopathy’. However, chemicals with allelopathic functions have other ecological roles, such as plant defense, nutrient chelation, and regulation of soil biota in ways that affect decomposition and soil fertility. These ecosystem-scale roles of allelopathic chemicals can augment, attenuate or modify their community-scale functions. In this review we explore allelopathy in the context of ecosystem properties, and through its role in exotic invasions consider how evolution might affect the intensity and importance of allelopathic interactions

The use of chronosequences in studies of ecological succession and soil development

1. Chronosequences and associated space-for-time substitutions are an important and often necessary tool for studying temporal dynamics of plant communities and soil development across multiple time-scales. However, they are often used inappropriately, leading to false conclusions about ecological patterns and processes, which has prompted recent strong criticism of the approach. Here, we evaluate when chronosequences may or may not be appropriate for studying community and ecosystem development. 2. Chronosequences are appropriate to study plant succession at decadal to millennial time-scales when there is evidence that sites of different ages are following the same trajectory. They can also be reliably used to study aspects of soil development that occur between temporally linked sites over time-scales of centuries to millennia, sometimes independently of their application to shorter-term plant and soil biological communities. 3. Some characteristics of changing plant and soil biological communities (e.g. species richness, plant cover, vegetation structure, soil organic matter accumulation) are more likely to be related in a predictable and temporally linear manner than are other characteristics (e.g. species composition and abundance) and are therefore more reliably studied using a chronosequence approach. 4. Chronosequences are most appropriate for studying communities that are following convergent successional trajectories and have low biodiversity, rapid species turnover and low frequency and severity of disturbance. Chronosequences are least suitable for studying successional trajectories that are divergent, species-rich, highly disturbed or arrested in time because then there are often major difficulties in determining temporal linkages between stages. 5. Synthesis. We conclude that, when successional trajectories exceed the life span of investigators and the experimental and observational studies that they perform, temporal change can be successfully explored through the judicious use of chronosequences

Linking vegetation change, carbon sequestration and biodiversity

1. Despite recent interest in linkages between above- and belowground communities and their consequences for ecosystem processes, much remains unknown about their responses to long-term ecosystem change. We synthesize multiple lines of evidence from a long-term ‘natural experiment’ to illustrate how ecosystem retrogression (the decline in ecosystem processes due to long-term absence of major disturbance) drives vegetation change, and thus aboveground and belowground carbon (C) sequestration, and communities of consumer biota. 2. Our study system involves 30 islands in Swedish boreal forest that form a 5000 year fire-driven retrogressive chronosequence. Here, retrogression leads to lower plant productivity and slower decomposition, and a community shift from plants with traits associated with resource acquisition to those linked with resource conservation. 3. We present consistent evidence that aboveground ecosystem C sequestration declines, while belowground and total C storage increases linearly for at least 5000 years following fire absence. This increase is driven primarily by changes in vegetation characteristics, impairment of decomposer organisms and absence of humus combustion. 4. Data from contrasting trophic groups show that during retrogression, biomass or abundance of plants and decomposer biota decreases, while that of aboveground invertebrates and birds increases, due to different organisms accessing resources via distinct energy channels. Meanwhile, diversity measures of vascular plants and aboveground (but not belowground) consumers respond positively to retrogression. 5. We show that taxonomic richness of plants and aboveground consumers are positively correlated with total ecosystem C storage, suggesting that conserving old growth forests simultaneously maximizes biodiversity and C sequestration. However, we find little observational or experimental evidence that plant diversity is a major driver of ecosystem C storage on the islands relative to other biotic and abiotic factors. 6. Synthesis. Our study reveals that across contrasting islands differing in exposure to a key extrinsic driver (historical disturbance regime and resulting retrogression), there are coordinated responses of soil fertility, vegetation, consumer communities, and ecosystem C sequestration, which all feed back to one another. It also highlights the value of well replicated natural experiments for tackling questions about aboveground-belowground linkages over temporal and spatial scales that are otherwise unachievable

Drivers of inter-year variability of plant production and decomposers across contrasting island ecosystems

Despite the likely importance of inter-year dynamics of plant production and consumer biota for driving community- and ecosystem-level processes, very few studies have explored how and why these dynamics vary across contrasting ecosystems. We utilized a well characterized system of 30 lake islands in the boreal forest zone of northern Sweden across which soil fertility and productivity vary considerably, with larger islands being more fertile and productive than smaller ones. In this system we assessed the inter-year dynamics of several measures of plant production and the soil microbial community (primary consumers in the decomposer food web) for each of 9 years, and soil microfaunal groups (secondary and tertiary consumers) for each of 6 of those years. We found that for measures of plant production and each of the three consumer trophic levels, inter-year dynamics were strongly affected by island size. Further, many variables were strongly affected by island size (and thus bottom-up regulation by soil fertility and resources) for some years but none in others, most likely due to inter-year variation in climatic conditions. For each of the plant and microbial variables for which we had 9 years of data, we also determined the inter-year coefficient of variation (CV), an inverse measure of stability. We found that CVs of some measures of plant productivity were greater on large islands while those of other measures were greater on smaller islands; CVs of microbial variables were unresponsive to island7 size. We also found that the effects of island size on the temporal dynamics of some variables were related to inter-year variability of macroclimatic variables. As such, our results show that the inter year dynamics of both plant productivity and decomposer biota across each of three trophic levels, as well as the inter-year stability of plant productivity, differs greatly across contrasting ecosystems, with potentially important but largely overlooked implications for community and ecosystem processes