This study presents a complementary geochemical and isotopic database (Ca2+, Mg2+, Na, K+, HCO 3--, SO42--, Cl --, trace elements and isotopes of H, O, C, S and Sr) for water samples from the Indus River Basin. These results, as well as published data for precipitation and river discharges were used to address the following aspects of the Indus River Basin: (1) water budget, annual solute fluxes, and denudation rate; (2) whether the summer monsoon or delayed runoff from winter precipitation dominates the discharge of the Indus and where does the water vapor for precipitation originate; (3) what are the sources and processes that control the distribution of solutes; (4) estimate the contribution of major ions to river water from carbonate and silicate weathering. The long-term mean annual precipitation water flux into the Indus River Basin is 398 km3. Based on major ion chemistry of rain and snow, the annual precipitation flux of Total Dissolved Solids (TDS) to the Indus River Basin is ∼ 844,400 tons. The Indus River annually transports ∼ 18 million tons of TDS that translates into a chemical denudation rate of 21 tons km--2. Oxygen and deuterium isotopes in the Indus River at Sukkur barrage for the Water Year March-94 to February-95 define the relationship deltaD = 7.5 (delta18O) + 10. This implies that despite aridity, significant evaporative enrichment is limited due to the short residence time of water and due to the minor contribution of runoff from the and middle and lower parts of the basin. Hydrochemistry of the Indus River is dominated by Ca2+ > Mg2+ > (Na+ +K+) and HCO 3-- > (SO42-- +Cl--) > Si. Sediment weathering is the dominant source for major cations, silicate weathering is important only locally. Three end member compositions control the Sr-isotope systematics of the Indus River. These are: (a) weathering of old silicate (silicic) rocks with high 87Sr/86Sr ratios and represented by rivers draining the Precambrian high grade metamorphic rocks of the Nanga Parbat-Haramosh massif and the "Central Crystallines" of the Higher Himalayas; (b) young silicate (mafic) rocks with the lowest 87Sr/ 86Sr ratios of all the Indus tributaries and represented by rivers draining mafic-ultramafic units of the Cretaceous Kohistan-Ladakh arcs; and (c) weathering of sedimentary carbonates with intermediate 87 Sr/86Sr, represented by the lowland tributaries draining sedimentary carbonates and shales of the West Pakistan Fold Belt. (Abstract shortened by UMI.)

The overall objectives in the creation of this work were to investigate the structure and reactivity of the group 14 elements germanium and tin supported by amidinates and guanidinates. Chapter One outlines features of the supporting ligation used throughout the thesis. Chapter Two describes the first characterized amidinate complexes of Ge using alkylamidinates as ancillary ligands. Chapter Three represents the first comprehensive study of a heteronuclear intermetal two-electron sulfur atom transfer between Sn and Ge. Complete sulfur atom exchange between amidinate complexes of tin and germanium was achieved through the reaction of (CyNC(tBu)NCy) 2Sn=S with (CyNC(tBu)NCy)2Ge to form the corresponding germanium sulfide (Ge=S) and stannylene. Chapter Four is concerned with the cleavage of chalcogen-chalcogen bonds as routes to the formation of rare chalcogenolates of tin and germanium. Unusual cyclic tin tetrasulfido complexes {(CyNC(R)NCy)[N(SiMe3 )2]}MS4 (M = Sn, R = Me, tBu; M = Ge, R = Me) were prepared by the reaction of elemental sulfur with the divalent group 14 starting materials [CyNC(R)NCy]M[N(SiMe3)2] (M = Sn, R = Me, tBu; M = Ge, R = Me). In Chapter Five mixed amidinato amido complexes [Me 3SiNC(tBu)NSiMe3]M[N(SiMe 3)2] (M = Sn, Ge) were prepared by the reaction of [Me 3SiNC(tBu)NSiMe3]Li with SnCl2 and GeCl2(dioxane) in ether. The N(SiMe3)2 ligand in these compounds is derived from the rearrangement of the [Me 3SiNC(tBu)NSiMe3]- anion with extrusion of tBuCN. The focus of Chapter Six is on the catalytic cyclotrimerization of isocyanates to perhydro-1,3,5-triazine-2,4,6-triones (isocyanurates) at room temperature. In Chapter Seven, we investigate the use of monoanionic N,N',N″-trisubstituted guanidinates as well as the anion of 1,3,4,6,7,8-hexahydro-2H-pyramido[1,2-a]pyramidine (Hhpp) in the formation of both Sn(II) and Sn(IV) complexes. Chapter Eight deals with investigations into the formation of amidinate and guanidinate complexes of both Fe(II) and Fe(III). Presented in this Chapter are results regarding the use of N, N'-dicyclohexylformamidinate as a ligand with Fe(II) to yield Fe4(mu4-O)(mu-Br) 2(mu-CyNCHNCy)4. (Abstract shortened by UMI.)

Mixed metal organometallic complexes with oxide co-ligands may serve as useful models for platinum metal catalysts on early transition metal oxide supports. Reaction of the electronically unsaturated hydride containing cluster [H2Os3(CO)10] with [W(C≡CPh)(O) 2(Cp*)] afforded the mu-oxo, mu-hydrido, mu-vinylidene cluster [Os3W(mu-H)(mu-eta1-C=CHPh)(CO)10(O)(mu-O)(Cp*)] 1 via a process involving acetylide and oxide coordination to the Os 3 framework and transfer of a hydride ligand to the unsaturated hydrocarbyl. Under thermal conditions, cluster 1 undergoes smooth decarbonylation concomitant with the formation of a novel mu3-oxo ligand to afford [Os3W(mu-H)(trans-mu-eta1-C=CHPh)(CO) 9(O)(Cp*)(mu3-O)] 2. Under photolysis, 2 isomerizes in high yield to a second mu3-oxo, mu-vinylidene complex [Os3W(mu-H)(cis-mu-eta1-C=CHPh)(CO) 9(O)(Cp*)(mu3-O)] 3. Under photolytic conditions, the mu-hydrido ligand of 1 is transferred over to the mu-vinylidene ligand resulting in the formation of a mu-eta1,eta2-vinyl ligand in [Os 3W(CO)10(O)(mu-eta1,eta2-CH=CHPh{ W-Os})(mu-O)(Cp*)] 4. Cluster 4 reacts under photolytic conditions with dihydrogen gas in a non-regioselective manner to produce two clusters, [Os3W(mu-H)(anti-mu-eta 1-CHCH2Ph)(CO)9(O)(Cp*)(mu3-O)] 5 and [Os3W(mu-H)(gauche-mu-eta 1-CHCH2Ph)(CO)9(O)(Cp*)(mu3-O)] 6, bearing a mu3-oxo ligand and a mu-alkylidene organic fragment. The complexes 1, 4, 5 and 6 resemble species proposed as intermediates on hydrogenation catalysts. A rational next step in such a catalyzed reduction process would be the elimination of the organic fragment from the metal framework. However in this study no evidence supporting such chemical behavior was observed. Nevertheless interesting clusters bearing no hydrocarbyl fragments were isolated from a related experiment. Thermolysis of 4 led to the isolation of two clusters, [Os 3W(mu-H)(CO)9(mu-O)2(Cp*)] 9 and [Os3W(CO)8(mu-O)2(Cp*)]2 12, without hydrocarbyl units. The metal framework of 12 consists of a rare Os6 rhombic raft and a packing diagram suggested the presence of a layered W(O)2|Os6|W(O)2 lattice. This reaction also led to the isolation of an additional four clusters demonstrating various bonding modes for both the [W(O)2(Cp*)] fragment and the vinyl ligand on a Os3 skeleton. As suggested by TLC analysis and infrared spectroscopy, the first complex formed upon thermolysis of 4 is [Os3W(CO)9(mu-O)2(mu-eta 1,eta2-CH=CHPh{W-Os})(Cp*)] 7 which further isomerizes to [Os3W(CO)9(mu-O) 2(mu-eta1,eta2-CH=CHPh{ Os···Os})(Cp*)] 8. The reaction of 8 with carbon monoxide liberated during previous reaction steps led to the isolation of [Os3W(CO)10(O)(mu-eta 1,eta2-CH=CHPh{Os-Os})(mu-O)(Cp*)] 10 which also reacts with one molecule of carbon monoxide to provide [Os3W(eta1-CH=CHPh)(CO)11(O)(mu-O)(Cp*)] 11.

This thesis has been divided into four major sections. The first section deals with the reactions of CrR4 (R is neopentyl or trimethylsilylmethyl) with the surfaces of partially dehydroxylated silicas, leading to the formation of discrete mononuclear surface organometallic fragments. The reaction stoichiometry was found to depend on the density of surface hydroxyl groups. On silica subjected to prior dehydroxylation at 500°C, one Cr is grafted per hydroxyl group and one equiv. of alkane is evolved, leaving ≡SiOCrR3 on the surface. On silica dehydroxylated at 200°C, each Cr is grafted onto two hydroxyl groups and two equiv. of alkane are evolved, giving (≡SiO) 2CrR2. When CrR4 reacts with deuterated hydroxyl groups, monodeuteroalkane is liberated. The chemisorbed Cr species retain the oxidation state (IV) and nuclearity (mono-) of their parent molecular precursors. The second section examines the thermal transformations of silicasupported (≡SiO)2Cr(CH2CMe3)2, which undergoes a clean reaction at 69°C to generate a supported alkylidene complex, (≡SiO)2Cr=CHCMe3, with concurrent liberation of CMe4. The reaction is quantitative and kinetically first-order. Isotope-labeling and kinetics experiments support a mechanistic assignment of intramolecular alpha-H abstraction. The temperature dependence of the first-order rate constants is consistent with a two-step mechanism of reversible (alkyl)(alkyhdene)Cr(VI) hydride formation followed by reductive elimination of alkane. Reactions with acetone and Br2 are consistent with the alkylidene formulation. A related alkylidene complex was prepared by thermolysis of silicasupported (≡SiO)2Cr(CH2SiMe3) 2 at 150°C. The third section describes the reactions of supported alkylidenes with various olefins. In contrast to silica-supported bis(neopentyl)Cr(IV), the surface alkylidene complexes are efficient room temperature alpha-olefin polymerization catalysts. Based on kinetic and isotope effects, a mechanism of agostic-assisted migratory insertion is proposed for the propagation step. The overall oxidation state of the Cr remains (IV) during the reaction. The last part of this thesis deals with the study of silica-supported amido and alkoxo chromium complexes. The reactions of Cr(NEt2) 4 and Cr(OtBu)4 with the partially dehydroxylated silica surface at 200°C give exclusively (≡SiO)2Cr(NEt 2)2 and (≡SiO)2Cr(OtBu) 2, respectively. Both surface organometallic complexes undergo ligand exchange reactions with alcohols. Thermal treatment of (≡SiO)2 Cr(OtBu)2 followed by addition of benzyl alcohol shows evidence for oxidation to benzoic acid. However, the "Cr=O" intermediate has yet to be detected.

The chiral (P,N)-ligated dichloropalladium (II) complexes, cis-dichloro[(+) O-diphenylphosphino-N-methylephedrine] palladium dichloride (2.15a), cis-dichloro[(S)-Prophos] palladium dichloride (2.15b), and cis-dichloro[(S)-1-methyl-2-(diphenylphosphinomethyl)pyrrolidine] palladium dichloride (2.15c) complexes, have been synthesized and completely characterized in the solid state. The synthetic procedure involves a simple one pot reaction of equivalent amounts of (CH3CN) 2PdCl2 and the corresponding ligand. These complexes are active precatalysts for the cycloaddition reactions of aziridines and heterocumulenes, yielding imidazolidinimines (2.34, X=NAr) for the case of carbodiimides. The cycloaddition reactions of aziridines and isocyanates are dependent on the catalyst used. When 2.15b is the catalyst, one product (imidazolidinones 2.34, X=O) is formed. However, when either 2.15(a,c) are used, two products (imidazolidinones 2.34, X=O and oxazolidinimines 2.35, X=O) are formed. Catalysts 2.15(a--c) failed to provide stereoselectivity. The one pot reaction of (CH3CN) 2PdCl2 and two equivalents of the chiral aminophosphinite ligand L*, [L* = (+)O-diphenylphosphino-N-methylephedrine (2.2), (S)-1-methyl-2-(diphenylphosphinomethyl) pyrrolidine (2.8)] resulted in the formation of [Ph2P(O)PdL*Cl] 2.18(a,c). A mechanism was proposed explaining the formation of the phosphine oxide moiety [Ph2P(O)] in these complexes. The structure of 2.18a was confirmed by an X-Ray crystal structure determination. The geometry of the compound is square planar with a slight distortion caused by the "trans-influence" of the phosphorus groups, causing them to be cis to each other.* *Please refer to dissertation for diagrams.

Abstract Not Available.

Reactions of trans-TiCl$\rm\sb2(TMEDA)\sb2$ with two equivalents of RONa gave the paramagnetic linear trimeric (Ti$\rm\sb3(PhO)\sb9(TMEDA)\sb2\rbrack$ (2.1), dimeric (Ti(OR)Cl(TMEDA)) $\sb2(\mu$-OR)$\sb2$ (2.2) (R = 3,5-(t-Bu)$\rm\sb2C\sb6H\sb3\rbrack$ or monomeric (Ti(OR)$\rm\sb4\rbrack\lbrack Li(TMEDA)\sb2\rbrack$ (2.7) (R = 2,6-(iPr)$\rm\sb2C\sb6H\sb3$), indicating that the role of steric hindrance not only controls the geometry and the nuclearity of the complex, but also stabilizes the +3 oxidation state. Conversely, in the case of vanadium, it was possible to isolate and characterize monomeric and neutral V(II) aryloxides with a variety of coordination geometries like square-planar and saddle-shaped. Less bulky phenols give monomeric or dimeric products after disproportionation and oxidation. Similar to the case of titanium, vanadium aryloxides do not show any interaction with dinitrogen. Reaction of V ((2-OMe)C$\rm\sb6H\sb4O\rbrack\sb2$(TMEDA) (4.7) with Me$\rm\sb3SiCHN\sb2$ gave $\{$ ((2-OMe)C$\rm\sb6H\sb4O\rbrack\sb2V\}\sb2 \lbrack\mu$-NNCHSiMe$\sb3\rbrack\sb2$ co-crystallized with $\{$ ((2-OMe)C$\rm\sb6H\sb4O\rbrack\sb2V\}\sb2 \lbrack\mu$-(2-OMe)$\rm C\sb6H\sb4O\rbrack\sb2$ (4.14). The structures of these complexes were determined by X-ray analysis. The synthesis and reactions of ionic Ti(III) amides with RLi are discussed. Sterically demanding alkyl groups (R = CM$\rm\sb2CMe\sb3,\ CH\sb2SiMe\sb3,\ CH\sb2CMe\sb2Ph\rbrack$ led to disproportionation and isolation of Ti(IV) complexes of the formulation ((Cy$\rm\sb2N)\sb2TiR\sb2\rbrack$ whereas with MeLi & BzLi, (Cy$\rm\sb2N)\sb2TiR\sb2$Li(TMEDA) (R = Bz (3.4a), Me (3.4b)) was formed, thus retaining the +3 oxidation state of titanium. Thermolysis of (Cy$\rm\sb2N)\sb2TiNf\sb2$ gave (Cy$\rm\sb2N)\sb2TiCH\sb2C(Me)\sb2C\sb6H\sb4$ (3.10) after losing a molecule of neophane. The ability of three-center chelating ligands (such as formamidinates or benzamidinates) to form dinuclear complexes with a short M-M contact was studied in order to understand the role of bridging ligands in promoting or disfavoring the dinuclear aggregation and to determine the extent of intermetallic separation in such species. The reaction of Li amidinates with trans-VCl$\rm\sb2(TMEDA)\sb2$ gave dinuclear ((CyNC(H)NCy)$\rm\sb2V\rbrack\sb2$ (5.3), monomeric (CyNC(Me)NCy) $\rm\sb2V(THF)\sb2$ (5.7) and (Me$\rm\sb3SiNC(Ph)NSiMe\sb3\rbrack\sb2V(THF)\sb2$ resulted in the formation of V(III) complex (CyNC(Me)NCy) $\sb3$V (5.6) whereas in the case of (Me$\sb3$SiNC(Ph)NSiMe$\sb3\rbrack\sb2$V(THF)$\sb2$ a novel dinitrogen complex ($\{\rm Me\sb3SiNC(Ph)NSiMe\sb3\}\sb2V\rbrack\sb2(N\sb2)$ (5.12) was formed. All these complexes could also be synthesized via reduction of V(III) amidinates. Reactions of LiNR$\sb2$ with SmCl$\sb3$(THF)$\sb3$ in a 2:1 molar ratio gave Sm(III) amides with different formulations and structures depending upon the R group of the amide. Dimeric ((Cy$\rm\sb2N)\sb2Sm(THF)(\mu$-Cl)) $\sb2$ (6.1) was obtained from the reaction of SmCl$\sb3$(THF)$\sb3$ with Cy$\sb2$NLi whereas ((i-Pr$\sb2\rm N)\sb2SmCl\sb3(LiTMEDA)\sb2$) (6.2) was obtained from the amide, iPr$\sb2$NLi under similar reaction conditions but in the presence of TMEDA. The reactivity of these amides was studied which gave another variety of Sm(II) amides like ((Cy$\rm\sb2N)\sb6Sm\sb4Cl\sb6(THF)\sb2$) (6.3) and ((Cy$\rm\sb2N)\sb4$SmLi(THF)) (6.4). Attempts to reduce (6.1) gave either metallic samarium or Sm(Cy$\rm\sb2N)\sb3$(THF) (6.5). The direct synthesis of a Sm(II) amide was possible only with Ph$\sb2$N, the salt having no $\alpha$-hydrogen. Using Ph$\sb2$N as the ligand, both ionic (Sm(Ph$\rm\sb2N)\sb4Na\sb2(TMEDA)\sb2$) (6.6) and neutral (Sm(Ph$\rm\sb2N)\sb2(THF)\sb4\rbrack$ (6.7) complexes were obtained. The structures of all these complexes are demonstrated by X-ray analysis.

The compounds MN(SiMe$\rm\sb3)\sb2Cl\sb2\cdot$base (1: M = Ga, base = thf; 2: M = In, base = pyridine) were synthesised. Compound 1 disproportionated to gallium trichloride while 2 did not show any thermal reactivity. The compounds (Ga(R)$\sb2$N(SiMe$\sb3)\sb2\rbrack\sb2$ (3: R = $\rm\sp{n}$Bu, 4: R = $\rm\sp{t}$Bu) were synthesised. Complex 4 decomposed into several products at 400$\sp\circ$C, while 3 thermolysed to form hexagonal GaN at 400$\sp\circ$C, after annealing at 900$\sp\circ$C. Phosphido analogues of these, $\rm\lbrack Ga(\sp{n}Bu)(R)P(SiMe\sb3)\sb2\rbrack\sb2$ (5: R = $\rm\sp{n}$Bu, 6: R = Cl) as well as (($\rm\sp{n}Bu)\sb2InP(SiMe\sb3)\sb2\rbrack\sb2$ (7) were synthesised. Complex 5 thermally rearranged to GaP at 250$\sp\circ$C. NMR studies determined that although the side product $\rm Me\sb3SiCl$ was formed in the thermolysis of 6, several products resulted and the desired phosphinido was isolated. The thermolysis of 7 at 400$\sp\circ$C produced both InP and In$\sp0.$ A series of primary amido gallium alkyl complexes: $\rm\lbrack\sp{t}Bu\sb2Ga(\mu$-N(H)$\rm\sp{t}Bu)\rbrack\sb2$ (8), $\rm\lbrack\sp{n}Bu\sb2Ga(N(H)\sp{t}Bu)\rbrack\sb2$ (9), $\rm\sp{n}Bu\sb2Ga\lbrack NH$(2.6-$\rm Me\sb2C\sb6H\sb3)1py$ (10) and $\rm\sp{n}Bu\sb2Ga\lbrack NH$(2,6-$\rm Me\sb2C\sb6H\sb3)\rbrack\sb2\lbrack Li(Et\sb2O)\rbrack$ (11) were synthesised.. Thermolysis of 8 at 120$\sp\circ$C gave a compound which $\sp1$H NMR characterisation suggested was $\rm\lbrack\sp{t}BuGaN\sp{t}Bu\rbrack\sb{x},$ but the extreme air sensitivity of this compound precluded characterization. Compound 9 was robust to thermolysis. Compounds 10 and 11 formed many products upon thermolysis at 170$\sp\circ$C and 155$\sp\circ$C respectively. The siloxide compounds M(OSiMe$\sb3)\sb{3-x}Cl\sb{x}\cdot$py (12: M = Ga, X = 0; 13: M = Ga, X = 1, 14: M = In, X = 0; 15: M = In, X = 1) were synthesised. All demonstrated elimination of $\rm(Me\sb3Si)\sb2O$ as determined by $\sp1$H NMR and MS, with only 12 forming any undesirable side-products. Thermolysis products were not isolated. A series of trimethylsilylamido-siloxo complexes: $\rm Ga(N(SiMe\sb3)\sb2)(OSiMe\sb3)\sb2py$ (17), $\rm\lbrack Li(thf)\sb2\rbrack\lbrack Ga(N(SiMe\sb3)\sb2)(OSiMe\sb3)\sb2Cl\rbrack$ (16), $\rm\lbrack Li(py)\sb2\rbrack\lbrack In(N(SiMe\sb3)\sb2)(OSiMe\sb3)\sb2Cl\rbrack$ (18), and $\rm\lbrack Li(py\sb2\rbrack\lbrack In(N(SiMe\sb3)\sb2)(OSiMe\sb3)\sb3\rbrack$ (19) were synthesised. 17 thermolised to a grey powder, which evolves into hexagonal GaN upon heating at 900$\sp\circ$C. Thermolysis experiments of compounds 16, 18, and 19 indicate a more complex rearrangement process than for 17; they do not proceed smoothly to yield the III-V nitride. Lastly, $\rm\lbrack In(N(SiMe\sb3)\sb2)(\mu\sp2$-$\rm O)\rbrack\sb{x},$ (20) was prepared from hexane in the absence of coordinating base. It was characterised by $\sp1$H NMR, elemental analysis and MS, but its crystallinity precluded a single-crystal XRD to establish the extent of oligomerisation.

This thesis deals with the syntheses and characterization of a novel series of Cr(II), as well as V(II) complexes. The strength of Cr-Cr quadruple bonds has been investigated experimentally. The Cr-Cr multiple bond of the dimeric (TAACr) $\sb2$ (TAA = tetramethyldibenzotetraaza (14) annulene) (2.1) is reversibly cleaved by pyridine to form monomeric paramagnetic octahedral (TAACrpy$\sb2\rbrack$.py (2.2). The synthesis, characterization and stability properties of a novel series of Cr(II) alkylchromates together with their transformation into unprecedented alkylidene (Schrock-type) Cr(III) species, and Cr(II) and Cr(III) metallacycles is described in chapter 4. The ability of three center chelating ligands to form dichromium units and to enforce short and very short Cr-Cr contacts is examined in a consistent series of cyclohexyl amidinate chromium complexes, with the aim of understanding the factors intrinsic in the nature of the bridging ligands which are able to promote or disfavor dinuclear aggregation and to determine the extent of intermetallic separation. The synthesis and characterization of a new series of mono-, di- and trinuclear Cr(II) borohydride compounds is described in chapter 7. The reaction of CrCl$\sb2$(TMEDA) with two equivalents of NaBH$\sb4$ afforded the thermally unstable (TMEDA)Cr(BH$\sb4)\sb2$ (7.1) which was converted by treatment with pyridine into the octahedral monomeric (Py)$\sb4$Cr(BH$\sb4)\sb2$ (7.2). Reaction of V(II) and V(III) salts with lithium amidinates formed a series of compounds where both the nuclearity and the oxidation state of the final complex were determined by the steric bulk of the substituents of the amidinate ligand. While dinuclear compounds with and without short V-V contacts have been obtained in the case of formamidinate anions, monomeric complexes and a dinuclear end-on dinitrogen complex were formed with the more bulky acetamidinate and trimethylsilyl benzamidinate. The reaction of VCl$\sb2$(TMEDA)$\sb2$ and of VCl$\sb3$(THF)$\sb3$ with two equivalents of formamidinate lithium salts respectively yielded dimeric $\rm \{\lbrack CyNC(H)NCy\rbrack\sb2V\}\sb2$ (8.2), with a very short V-V multiple bond, and $\rm \{\lbrack CyNC(H)NCy\rbrack\sb2VCl\}\sb2$ (8.4) which is also dimeric. The dinuclear structure was reversibly cleaved by treatment with pyridine forming the monomeric (CyNC(H)NCy) $\sb2$V(Py)$\sb2$ (8.3). Conversely, similar reactions with acetamidinate anion gave only the monomeric (CyNC(Mc)NCy) $\sb2$V- (THF)$\sb2$ (8.5a) and (CyNC(Me)NCy) $\sb2$VCl (8.7) respectively. Attempts to form a dinuclear structure by either removal of THF from 8.5a or reduction of 8.7 gave only the V(III) compound (CyNC(Me)NCy) $\sb3$V (8.6). Finally, reaction of VCl$\sb2$(TMEDA)$\sb2$ with two equivalents of carboxylic acid in THF in the presence of a slight excess of TMEDA at room temperature, afforded the large scale preparation of the linear V(II) trimer V$\sb3$(R$\sb2$CHCOO)$\sb6$(TMEDA)$\sb2$ (R = Ph$\sb2$CH (9.1), PhCH$\sb2$ (9.2)). Cleavage of the trimeric unit and formation of a high-spin monomeric species ((R$\sb2$CHCOO)$\sb2$V(pyridine)$\sb4\rbrack$ (9.3) was achieved upon simple treatment of 9.1 with pyridine at room temperature. The structures of 9.1 and 9.2 are demonstrated by X-ray analysis. (Abstract shortened by UMI.)

A study has been made of cycloaddition reactions of 3- and 4-membered ring nitrogen-containing heterocycles with heterocumulenes in the presence of Pd(II) catalyst. Bis(benzonitrile)palladium dichloride, Pd(PhCN)$\sb2$Cl$\sb2$, smoothly catalyzes the completely regioselective reactions of 1,2-disubstituted aziridines with N,N-diarylcarbodiimides to give imidazolidinimines in high yield. Similar cycloaddition reactions involving 1,2,3-trisubstituted aziridines and heterocumulenes (carbodiimides, isocyanates and isothiocyanates) provide regio- and stereospecific routes to imidazolinimines, imidazolidinones, and thiazolidinimines. Pd(PhCN)$\sb2$Cl$\sb2$-catalyzed cycloaddition reactions of chiral, optically active 1,2-disubstituted aziridines with heterocumulenes proceed with retention of configuration affording the corresponding five-membered ring heterocycles in high yield and in optically pure form. The study of the reaction of aziridines with N,N-diarylsulfurdiimide in the presence of a Pd(II) catalyst resulted in the discovery of a new and unusual cyclization furnishing imidazolidinethiones. Cycloaddition reactions of azetidines with heterocumulenes (carbodiimides and isothiocyanates) are also catalyzed by Pd(PhCN)$\sb2$Cl$\sb2$ and afford tetrahydropyridin-2-imines and tetrahydro-1,3-thiazine-2-imines, respectively, in a regio- and stereospecific manner in excellent yield. On the basis of results obtained, including spectroscopic studies and X-ray analysis, a mechanism has been proposed for the cycloaddition reactions of the N-heterocycles with heterocumulenes. A bis(azetidine)palladium complex was isolated and shown to be catalytically active for the reaction of azetidines with heterocumulenes. Numerous heterocyclic compounds have been prepared for the first time, some of them having the potential for pharmacological activity.