J. A. Connor

Bonding in some donor–acceptor complexes involving boron trifluoride. Study by means of ESCA and molecular orbital calculations

Core binding energies of a number of nitrogen bases and their adducts with BF3 are reported, and interpreted using both ab initio and semi-empirical molecular orbital calculations. The binding energies are found to reflect the change in molecular charge distribution which occurs on formation of the B—N σ bond. The correlation of the valence molecular orbitals of the complexes with those of the bases and BF3 is discussed.

High energy photoelectron spectroscopy of transition metal complexes. Part 1.—Bonding in substituted and unsubstituted first row carbonyls

Core electron binding energies of a series of substituted and unsubstituted transition metal carbonyls are reported and interpreted using both ab initio and approximate molecular orbital calculations. The ESCA chemical shifts observed upon substitution are well reproduced by such calculations, although the rather poor prediction of the change in CO core binding energies on complex formation, together with the observation of unusually intense satellite peaks in the photoelectron spectra of the carbonyls suggests that orbital relaxation may be greater in complexed than in free CO.

High energy photoelectron spectroscopy of transition metal complexes. Part 2.—Metallocenes

Metal and core electron binding energies of the metallocenes of the first row transition metals are reported and discussed with the aid of approximate molecular orbital calculations. In all cases it is found that the metal atom is positively charged. The measured splitting of the metal 3s photoelectron peaks is interpreted in terms of spin delocalization in these molecules.

Oxidation of a tetrakis-(tertiary alkylphosphine)molybdenum(0) complex

Abstract The complex cis-[Mo(CO)2(dmpe)2] (dmpe = Me2PCH2CH2PMe2) may be prepared directly from Mo(CO)6 and dmpe, either thermally or photochemically; in each case fac,-fac- [Mo2(CO)6(dmpe)3]is formed as an intermediate. Hydrogen chloride reacts with cis-[Mo(CO)2(dmpe)2]to give trans-[MoH(CO)2(dmpe)2]HCl2 which is stereochemically rigid in the temperature range 220–320 K. Iodine and also trifluoroiodomethane react with cis-[Mo(CO)2(dmpe)2]to give cis-[MoI(CO)2(dmpe)2]I which is fluxional in the same temperature range. When one mole of AgX(X = NO3,BF4) is added to cis-[Mo(CO)2 (dmpe)2]the product is trans-[Mo(CO)2(dmpe)2]X. Addition of a second mole of AgNO3 gives the nitrato complex, cis-[Mo(NO3)(CO)2(dmpe)2]NO3. Under rigorously anhydrous conditions, cis-[Mo(CO)2(dmpe)2]does not react with hydrogen, methyl iodide or with acetyl chloride.

High-energy photoelectron spectroscopy of some antimony compounds

High-energy photoelectron (p.e.) spectra have been obtained for a number of compounds of antimony. The antimony 3d(,5//2) binding energies range from 542.9, 533.3 eV for [Et4N][SbF6] to 538.6, 529.0 eV for Bun3Sb, but there is little correlation with the formal oxidation state. Assignment of the oxidation state of antimony on the basis of X-ray p.e. data alone is not possible with certainty and even gross structural differences may remain undetected by this method.

Ditertiary alkylphosphine derivatives of the Group VI metal carbonyls and their oxidation with iodine

The preparation and characterisation of the complexes fac,fac-[M2(CO)6(dmpe)3](M = Cr, Mo, W; dmpe is Me2PCH2CH2PMe2) and cis-[M (CO)2(dmpe)2] is reported. The oxidation of cis-[M(CO)4(dmpe)] with iodine gives [M(CO)4(dmpe)I]I which can be converted into [M(CO)3(dmpe)I2](M = Mo, W). Iodine oxidation of fac,fac-[M2(CO)6(dmpe)3] gives [M2(CO)4(dmpe)3I4](M = MO, W), which are also obtained from the reaction of [M (CO)3(dmpe)I2] with dmpe. Oxidation of cis-[M(CO)2(dmpe)2] gives [M(CO)2(dmpe)2I]I (M = Mo, W) which are also obtained in the reaction of [M2(CO)4(dmpe)3I4] with dmpe. The products of these oxidation reactions have been characterised by i.r. and n.m.r. (1H and 31P) spectroscopy. The cations [M(CO)2(dmpe)2I]+ are shown to be fluxional in the temperature range 220–320 K and a monocapped trigonal prismatic structure is indicated for the low-temperature form. Comparisons are made with analogous complexes containing Ph2PCH2CH2PPh2(dpe) and differences in their behaviour on oxidation are explained in terms of the steric effect of the substituents (methyl, phenyl) at phosphorus.

Electrochemical oxidation of organometallic complexes. Carbene and Lewis base complexes of chromium, molybdenum, and tungsten carbonyls

Tha oxidative one-electron transfer reactions of a wide variety of Group VI metal carbonyl derivatives have been detected by voltammetry. The compounds studied are of the type M(CO)6 –nLn or M(CO)6 – 2n(LL)n(M = Cr, Mo, W; L = monodentate Lewis base, n= 1 or 2; LL = bidentate Lewis base, n= 1 or 2) and carbene complexes of the type M(CO)5C(X)Y. The value of the potential, E½, is influenced by each of the variables M, L(or LL), n, X, and Y. The order of E½-values follows closely the apparent π-acceptor/σ-donor ratio of the ligand, but comparison with the results of molecular orbital calculations suggests that the influence of L (or of {C(X)Y}) upon the redox orbital is indirect. Steric effects are shown to influence the value of E½, especially where bidentate ligands are present. The oxidation potentials are related to synthetic chemistry in these systems.

Preparation and some reactions of Group VI metal monodentate bis-phosphine carbonyl complexes. Mechanistic aspects of chelate-ring formation

A series of complexes LM(CO)5[M = Cr, Mo, or W; L = Me2PCH2CH2PMe2(dmpe), Ph2PCH2PPh2(dpm), Ph2PCH2CH2PPh2(dpe), Ph2PCH2CH2CH2PPh2(dpp), or Ph2PCH2CH2AsPh2(ape)] has been prepared and characterised by analysis, and i.r., mass, and n.m.r. spectroscopy. The complexes can be methylated with Me3OBF4 to give [(MeL)M(CO)5]BF4. The acid-assisted nucleophilic substitution reaction used in the formation of the complexs can also be applied to their conversion into bridged complexes, L[M(CO)5]2. Variations in geminal complexes coupling constants 2JPCH and aromatic solvent induced shifts for the series of complexes(dmpe)M(CO)n(M = Cr, Mo, or W; n= 4 or 5) are discussed and compared with those in (Me2PXCH2)2(X = O or S) and [(Me3PCH2)2](BF4)2, whose syntheses are also reported. Changes in the 31P chemical shift are used to demonstrate that the chelation shift is comparable to the co-ordination shift. Kinetic studies of the rate of the chelation reaction, LM(CO)5→ LM(CO)4+ CO, have been used to elucidate details of its mechanism. The reaction follows first-order kinetics, as required for the intramolecular process. The magnitude of the enthalpy of activation (ca. 140 kJ mol–1) and the similarity in rate between phosphorus and arsenic nucleophiles, suggest a large dissociative component in the activation. The positive entropy of activation and particularly its large variation (+6 to +71 J K–1 mol–1) suggest a concerted process in the transition state. The smaller the potential chelate ring, the faster the reaction in the series L = Ph2P(CH2)nPPh2(n= 1, 2 or 3); this appears to be largely an entropy effect.

Voltammetric oxidation of arene, cycloheptatriene, and cycloheptatrienyl tricarbonyl complexes of chromium

The series of π-arene chromium tricarbonyl complexes [(π-C6HnMe6–n)Cr(CO)3](n= 0–6), [(π-C6H5R)Cr(CO)3](R = Pri, NH2, NHMe, Nme2, OMe, CO2Me, and Cl), and [(π-C6H4RR′)Cr(CO)3](R = Me, R′= Pri; R = Me, R′= NH2) undergo, voltammetrically, two one-electron oxidation processes. The E½ value for the oxidation waves are dependent on the nature of the arene ring substituents but are independent of ring substituent positional isomerism. There is a linear correlation between E½(complex) and E½(free arene), and between E½(complex) and the ionisation potentials of the complexes [(π-C6HnMe6–n)Cr(CO)3] and of the free arenes. The cycloheptatriene species [(π-C7H6RR′)Cr(CO)3](R = H, R′= H, exo- or endo-Ph, and exo-CN; R = R′= OMe) may also be oxidised to [(π-C7H6RR′)Cr(CO)3]+ and the species where R = H, R′=exo-CN and R = R′= OMe may be reduced in a one-electron step. The cycloheptatrienyl complexes [(π-C7H7R)Cr(CO)3]+(R = H or OMe), undergo an irreversible one-electron reduction process.

Rearrangement of metal-stabilised carbocations containing cyclopropyl and other alkyl groups to give cyano- and isocyano-complexes: reactions of Group VI metal isocyano-complexes with nucleophiles

The preparation of a series of cyclopropylcarbene complexes [(CO)5Cr{C(X)C3H5}](X = OH, OMe, or NH2) is reported. The spectroscopic properties (particularly proton n.m.r.) of these complexes show no evidence of stabilisation of the carbocationic site by the cyclopropyl group. Formation of acetoxy-carbene complexes in the reaction between the salts [(CO)5Cr{C(O–NMe4+)Y}](Y = CH2SiMe3 or C3H5) and acetyl chloride is inferred from their further reaction with hydrogen azide which gives rise to [(YNC)Cr(CO)5] and [(YCN)Cr(CO)5]. The mechanism of the reaction is discussed and it is suggested that migration of the group Y from the carbon to the nitrogen atom in a common intermediate [(CO)5MC(N)Y] will occur most readily when there is little or no stabilisation of the carbocation, through a vertical process, by the group Y. The complex [(cyclopropyl isocyanide)Cr(CO)5] has been fully characterised. The preparation and characterisation of [(acetyl isocyanide)Cr(CO)5] is described and the presence of extensive conjugation between the isocyano- and the carbonyl group is established. The acetyl isocyanide ligand is rapidly solvolysed by methanol giving [(HNC)Cr(CO)5] and methyl acetate. When the complex [(Me3CNC)Cr(CO)5] is treated with n-butyl-lithium and subsequently alkylated the sole product is the complex cis-[(CO)4(Me3CNC)Cr{C(OEt)Bu}] resulting from nucleophilic attack at a carbonyl rather than the isocyano-ligand.