Showing papers by "Peter S. White published in 1999"
TL;DR: In this article, the authors investigated how similarity changes with distance in biological communities, and explored whether growth form, dispersal type, rarity, or support affected the rate of distance decay in similarity.
Abstract: Aim Our aim was to understand how similarity changes with distance in biological communities, to use the distance decay perspective as quantitative technique to describe biogeographic pattern, and to explore whether growth form, dispersal type, rarity, or support affected the rate of distance decay in similarity. Location North American spruce-fir forests, Appalachian montane spruce-fir forests. Methods We estimated rates of distance decay through regression of log-transformed compositional similarity against distance for pairwise comparisons of thirty-four white spruce plots and twenty-six black spruce plots distributed from eastern Canada to Alaska, six regional floras along the crest of the Appalachians, and six regional floras along the east‐west extent of the boreal forest. Results Similarity decreased significantly with distance, with the most linear models relating the log of similarity to untransformed distance. The rate of similarity decay was 1.5‐1.9 times higher for vascular plants than for bryophytes. The rate of distance decay was highest for berry-fruited and nut-bearing species (1.7 times higher than plumose-seeded species and 1.9 times higher than microseeded/spore species) and 2.1 times higher for herbs than woody plants. There was no distance decay for rare species, while species of intermediate frequency had 2.0 times higher distance decay rates than common species. The rate of distance decay was 2.7 times higher for floras from the fragmented Appalachians than for floras from the contiguous boreal forest. Main conclusions The distance decay of similarity can be caused by either a decrease in environmental similarity with distance (e.g. climatic gradients) or by limits to dispersal and niche width differences among taxa. Regardless of cause, the distance decay of similarity provides a simple descriptor of how biological diversity is distributed and therefore has consequences for conservation strategy.
1,529 citations
TL;DR: In this article, the reaction of [RuCl 2 ( p -cymene)] 2 with the tridentate N - N ′- N ligands, 2,6-pyridyl-diimines, led to substitution of p -cyclohexene.
Abstract: Reaction of [RuCl 2 ( p -cymene)] 2 with the tridentate N - N ′- N ligands, 2,6-pyridyl-diimines, led to substitution of p -cymene. The resulting complexes, believed to be coordinatively unsaturated, exhibit efficient activity for the epoxidation of cyclohexene in the presence of iodosobenzene (PhIO): the complexes formed initially take up donor molecules such as acetonitrile to achieve hexacoordination. The molecular structure for one of these, (acetonitrile){2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine}dichlororuthenium(II), 2 , has been determined by X-ray diffraction. The immediate coordination sphere is a distorted octahedron with trans chloride atoms and a short Ru–N(py) (1.906 A) bond.
93 citations
TL;DR: A series of [C5Me5Rh(CH2CHR)2] complexes (1a−e) have been prepared in which the olefin bears a bulky silyl substituent as discussed by the authors.
Abstract: A series of [C5Me5Rh(CH2CHR)2] complexes (1a−e) have been prepared in which the olefin bears a bulky silyl substituent, R = (a) SiMe3, (b) SiMe2OEt, (c) Si(OiPr)3, (d) SiMe(OSiMe3)2, (e) SiPh2OiPr The solid-state structure of 1c has been determined by X-ray crystallography When complex 1a is heated (50 °C) in deuterated solvents (C6D6, C6D5CD3, C6D5Cl, or (CD3)2CO), deuterium is incorporated into the olefinic sites Thermolysis at higher temperatures results in further H/D exchange and deuteration of both the SiMe3 and C5Me5 groups Heating 1a in C6D6 with added substrates (aniline, MeOtBu, MeOSiMe3, Cp2Fe, cyclopentene, or EtOAc) results in deuteration of these substrates via shuttling of deuterium from C6D6 to the olefinic sites and then into certain sites of the substrates Thermolysis of 1a in the presence of vinyltrimethylsilane at higher temperatures results in C−Si bond cleavage and generation of a silacyclopentadiene complex (6) whose structure was determined by X-ray analysis Thermolysis of 1c
91 citations
TL;DR: In this article, the synthesis and characterization of a series of complexes of the general formula [Cl(bpy)Ru(tppz)Ru (bpy‘)Cl](PF6)2, where tppz is tetra-2-pyridylpyrazine and bpy is 2,2‘-bipyridine or substituted 2.2
Abstract: The syntheses and characterization of a series of complexes of the general formula [Cl(bpy‘)Ru(tppz)Ru(bpy‘)Cl](PF6)2, where tppz is tetra-2-pyridylpyrazine and bpy‘ is 2,2‘-bipyridine or substituted 2,2‘-bipyridine, are described. Preparations with 2,2‘-bipyridine or 4,4‘-dimethyl-2,2‘-bipyridine (1 and 2, respectively) give mixtures of two isomers. The isomers of 2 were separated chromatographically and the X-ray crystal structure of the cis isomer was solved. Significant distortion is observed in both the coordination geometry around each ruthenium atom and in the tppz ligand resulting in short intramolecular Ru···Ru and Cl···Cl separations (6.558(1) and 5.880(2) A, respectively). Ligands containing two bipyridine groups linked through eight- and nine-atom bridges have also been synthesized. The length of the bridge between bipyridine groups allows for the formation of cis complexes 3−5 having the general formula shown above but prevents formation of a trans isomer.
73 citations
TL;DR: The pyrazine-bridged complexes have properties that place them in a new class of mixed-valence molecules, Class II-III having properties associated with both Class II and Class III in the Robin and Day classification scheme.
Abstract: The mixed-valence complexes [(bpy)2(Cl)OsIII(BL)OsII(Cl)(bpy)2]3+ and [(tpy)(bpy)OsIII(BL)OsII(bpy)(tpy)]5+ (bpy is bipyridine; tpy is 2,2‘:6‘,2‘ ‘-terpyridine; BL is a bridging ligand, either 4,4‘-bipyridine (4,4‘-bpy) or pyrazine (pz)) have been prepared and studied by infrared and near-infrared measurements in different solvents. For BL = 4,4‘-bpy, there is clear evidence for localized OsII and OsIII oxidation states in the appearance of the expected two interconfigurational dπ → dπ bands at OsIII and additional, broad absorption features in the near-infrared arising from intervalence transfer (IT) transitions. For [(bpy)2(Cl)Os(pz)Os(Cl)(bpy)2]3+ and [(tpy)(bpy)Os(pz)Os(bpy)(tpy)]5+, unusually intense ν(pz) bands appear in the infrared at 1599 cm-1 (e = 2600 M-1 cm-1) for the former and at 1594 cm-1 (e = 2020 M-1 cm-1) for the latter. They provide an oxidation state marker and evidence for localized oxidation states. A series of bands appear in the near-infrared from 2500 to 8500 cm-1 that can be assi...
54 citations
TL;DR: Investigation of other CoI chelate complexes with a single aldehyde group shows that the coordination mode of the aldealdehyde is dictated by the nature of the bonding of the other ligating group.
Abstract: A bidentate phthalaldehyde ligand with both σ and π coordination of the aldehyde groups is found in [(C5 Me5 )Co{(C(O)H)2 C6 H4 }] (structure depicted). This complex is the "resting state" of the catalyst in the ring closure of the dialdehyde to give the lactone. Interchange of coordination modes occurs with a barrier of 70 kJ mol-1 at 35°C. Investigation of other CoI chelate complexes with a single aldehyde group shows that the coordination mode of the aldehyde is dictated by the nature of the bonding of the other ligating group.
50 citations
41 citations
TL;DR: In this article, an extensive redox chemistry associated with high oxidation state Ru(IV) oxo complexes has been uncovered by kinetic and mechanistic studies, including pathways involving well-defined, multiple electron transfer such as O-atom or hydride ion transfer.
Abstract: An extensive redox chemistry associated with high oxidation state Ru(IV) oxo complexes has been uncovered by kinetic and mechanistic studies. This includes pathways involving well-defined, multiple electron transfer such as O-atom or hydride ion transfer. Large H{sub 2}O/D{sub 2}O solvent kinetic isotope effects have been identified in a series of reactions and interpreted as involving synchronous transfer of an electron and a proton (proton-coupled electron transfer). For example, k(H{sub 2}O)/k(D{sub 2}O) = 30 {+-} 1 for the oxidation of hydroquinone (H{sub 2}O) to benzoquinone (Q) by cis-[Ru{sup IV}(bpy){sub 2}(py)(O)]{sup 2+} at 20.0 {+-} 0.2 C.
29 citations
01 Jan 1999
26 citations
TL;DR: The authors compared the diversity, phytogeography, and plant communities in two mid-latitude alpine tundras with comparable aerial and elevational extents: Changbaishan Summit in eastern Asia and Indian Peaks in western North America.
Abstract: We compared the diversity, phytogeography, and plant communities in two mid-latitude alpine tundras with comparable aerial and elevational extents: Changbaishan Sum- mit in eastern Asia and Indian Peaks in western North America. Despite wide separation, the two areas shared 72 species. In all, 43 % of the species on Changbaishan Summit are also distributed in the alpine zones of western North America, while 22 % of the species on Indian Peaks are also distributed in the alpine zones of eastern Asia. Almost all the shared species also occur in the Beringian region. Phytogeographical profiles of species and genera showed that 69 % of species and over 90 % of genera in both alpine tundras belong to the three phytogeographical categories: cosmopolitan, circumpolar, and Asian-North American. We attributed the current floristic relationship between these widely separated areas to the peri- odic past land connection between the two continents during the Tertiary and Pleistocene. Indian Peaks has a closer floristic relationship with the Arctic tundra than does Changbaishan Summit. Indian Peaks also has 45 % higher species richness and lower vegetation cover than Changbaishan Summit. Plant communities from the two areas were completely separated in the two-way indicator species analysis and non- metric multidimensional scaling on floristic data at both species and generic levels, whereas ordination of communities by soil data produced a greater overlap. The plant communities on Changbaishan Summit in general have lower alpha diversity, higher beta diversity (lower between-community floristic simi- larity), and more rare species than does Indian Peaks. Mosaic diversity does not differ in the two alpine tundras, although the analysis suggests that Changbaishan Summit communities are more widely spaced on gradients than the Indian Peaks com- munities.
25 citations
TL;DR: A series of chiral tungsten(IV) oxo−alkyne complexes of the type TpW(O)(I)(RC⋮CR‘) (R = R‘ = H (1); R = H, R' = Me (2), R = R´ = ME (3); R= Me, R` = Ph (4); Tpw(O) = hydridotris(3,5-dimethylpyrazolyl)borate) has been prepared.
TL;DR: In this paper, an asymmetric synthesis of C2-symmetric fused diferrocenyl diamine and diol ligands is described. The synthesis is based on the Cu(II)-mediated coupling of the diastereomerically and enantiome...
TL;DR: In this article, a procedure for converting titanium diethylamidos to the corresponding dihalides (Cl, Br, I) and a complementary method for converting Titanium dichlorides into dibromides and diiodides are reported.
TL;DR: There is an extensive reactivity chemistry of the thionitrosyl group in [Os(II)(tpm)(Cl)(2)(NS)](PF(6)) (3b).
Abstract: Reaction between [OsVI(tpm)(Cl)2(N)](PF6) (tpm = tris(1-pyrazolyl)methane) (1) or OsVI(Tp)(Cl)2(N) (Tp = hydrotris(1-pyrazolyl)borate anion) (2) and CS2 + N3- in acetone gives the corresponding thionitrosyl complexes, -SCN, and N2. There is an extensive reactivity chemistry of the thionitrosyl group in [OsII(tpm)(Cl)2(NS)](PF6) (3b). Reaction between 3b and PPh3 occurs with S-atom transfer to give [OsIV(tpm)(Cl)2(NPPh3)]+ and SPPh3. 3b undergoes chemical or electrochemical reduction to give the corresponding OsII ammine complex and H2S. O-atom transfer from ONMe3 to 3b occurs to give OsIII(tpm)(Cl)2(NSO). Competitive NO+/NS+ exchange and S2- transfer occur in the reaction between [OsII(tpm)(Cl)2(NS)](BF4) (3c) and NO+ to give a mixture of [OsVI(tpm)(Cl)2(N)]+ and [OsII(tpm)(Cl)2(NO)]+.
TL;DR: Solid-state (X-ray) and solution ((1)H NMR) structural analysis indicates that the phenyl groups alpha to the sulfonamide nitrogen adopt an axial conformation, as characteristic (3)J(Pt)(-)(H) coupling constants are observed for equatorial, but not axially disposed hydrogens in the heterocycle.
Abstract: Reaction of (dppe)Pt(CO(3)) with (1S,2S)-HN(R)CHPhCHPhNH(R) (R = SO(2)CF(3), 1a; SO(2)-4-(t)BuC(6)H(4), 1b) and (1S,2R)-HN(R)CHPhCHPhOH (R = SO(2)CF(3), 3a; SO(2)-4-(t)BuC(6)H(4), 3b) leads to well-behaved chiral (dppe)Pt[N(R)CHPhCHPhN(R)] (R = SO(2)CF(3), 2a; SO(2)-4-(t)BuC(6)H(4), 2b) and (dppe)Pt[N(R)CHPhCHPhO] (R = SO(2)CF(3), 4a; SO(2)-4-(t)BuC(6)H(4), 4b) complexes. X-ray structural analysis of 2a and 4b reveal that the Pt-N bond lengths are long with the triflamide bond lengths being slightly longer than the aryl sulfonamide bond lengths ( approximately 2.12 and 2.06 A, respectively). Solid-state (X-ray) and solution ((1)H NMR) structural analysis indicates that the phenyl groups alpha to the sulfonamide nitrogen adopt an axial conformation, as characteristic (3)J(Pt)(-)(H) coupling constants are observed for equatorial, but not axially disposed hydrogens in the heterocycle. Relative binding affinities of 1a/b and 3a/b to the (dppe)Pt fragment were determined through pairwise ligand exchange reactions: Tf > SO(2)Ar and NN > NO.
TL;DR: In this article, the structural changes with electron content including a shortening of the Os−N bond and an increase in the angle from Os(IV) to Os(VI) can be rationalized by an electronic structural model.
Abstract: Crystallographic data are presented for a series of Os(IV−VI) hydrazido complexes. The structural changes with electron content including a shortening of the Os−N bond and an increase in the Os−N−N angle from Os(IV) to Os(VI) can be rationalized by an electronic structural model. This model predicts an increase in electron donor character of the hydrazido ligand from four to six electrons from Os(IV) to Os(VI). The structures of trans-[OsVI(tpy)(Cl)2(NN(CH2)4O)]2+ and cis-[OsIV(tpy)(Cl)(NCCH3)(NN(CH2)4O)]+ are illustrated.
TL;DR: The crystal and molecular structures of OsIII(tpy)(Cl)3 (1) (tpy=2,2′:6′,2″-terpyridine) and (PPN)[OsIII(Tp)(CL)3] (2)(PPN+=bis-(triphenylphosphoranylidene) ammonium cation, Tp=hydrido tris(1-pyrazolyl)borate anion) are reported as mentioned in this paper.
TL;DR: Through pairwise exchange reactions a qualitative ranking of relative bidentate ligand binding strengths to the Lewis acid were obtained and a ranking is readily rationalized using hard-soft electronic arguments except for tmeda, which requires that unfavorable steric interactions be invoked.
Abstract: A series of monomeric Lewis acid-base adducts of the Diels-Alder catalyst Ti(O-2,6-Me(2)C(6)H(3))(2)Cl(2) have been synthesized from bidentate diphosphines and diamines, Ti(O-2,6-Me(2)C(6)H(3))(2)Cl(2)L(2) (L(2) = dmpe, depe, dpeda, and dmeda). X-ray crystal structures of Ti(O-2,6-Me(2)C(6)H(3))(2)Cl(2)(dmpe) and Ti(O-2,6-Me(2)C(6)H(3))(2)Cl(2)(dpeda) establish a distorted octahedral coordination environment with trans-chloride ligands. Bidentate ligands that were also studied but did not form isolable complexes with the Ti(IV) Lewis acid include dppe, tmeda, and binam. Through pairwise exchange reactions a qualitative ranking of relative bidentate ligand binding strengths to the Lewis acid were obtained (dmeda >/= dpeda > dmpe >/= depe > tmeda > binam > dppe). The ranking is readily rationalized using hard-soft electronic arguments except for tmeda, which requires that unfavorable steric interactions be invoked.
TL;DR: In this article, a remarkable reaction between trans-[Os{sup VI}(tpy)(Cl){sub 2}(NN(CH{sub 2]){sub 4}O)]{sup 2+} (2) has been characterized crystallographically.
Abstract: In recent experiments, the authors demonstrated that in the Os-hydrazido complexes, trans-[Os{sup VI}(L{sub 3})(Cl){sub 2}(NN(CH{sub 2}){sub 4}O)]{sup 2+} (L{sub 3} = 2,2{prime}:6{prime},2{double{underscore}prime}-terpyridine or tris(1-pyrazolyl)-methane and N(CH{sub 2}){sub 4}O{sup {minus}} = morpholide), there are four interconvertible oxidation states with Os(VI), Os(V), Os(IV), and Os(III) accessible within the solvent limit in CH{sub 3}CN. Examples of Os(VI), Os(V), and Os(IV) have been characterized by X-ray crystallography. The authors report here a remarkable reaction between trans-[Os{sup VI}(tpy)(Cl){sub 2}(NN(CH{sub 2}){sub 4}O)]{sup 2+} (2), has been characterized crystallographically. An extraordinary electrophilic substituent effect of Os(VI) on the tpy ligand and the ability of Os(VI) to undergo reversible intramolecular Os(VI {yields} IV) electron transfer appear to play essential roles in these reactions.
TL;DR: In this paper, the influence of changes in the X groups on the physical properties of the Fe3(CO)9 cluster core is examined, and the Hammett linear free energy relationship is used to quantify the observed changes in cluster properties.