John R. Wilkinson

A 13C NMR study of isonitrile transition metal complexes

Abstract The 13 C NMR spectra of a variety of isonitrile complexes have been studied. The isonitrile carbon shielding trend is consistent with the hypothesis that some π-back bonding occurs in zero oxidation state complexes but is not important in positive oxidation state complexes. A metal triad shielding trend is observed with the isonitrile complexes as has been found previously in carbonyl-, carbene-, and alkylmetal compounds.

Oxygen-17 nuclear magnetic resonancé spectra of transition metal carbonyl compounds

Abstract The natural abundance 17 O NMR shielding values observed by FTNMR in the present study on a representative variety of terminal transition metal carbonyl complexes are shown to range 400–300 ppm downfield from 17 OH 2 . The carbonyl 17 O chemical shift trends are generally opposite to those for the carbonyl 13 C chemical shifts and this is explained by metal π-backbonding to the carbonyl π ★ orbitals. A metal triad effect is observed for 17 O chemical shifts. There is an upfield shift in the carbonyl oxygen shielding values on descending a given group in the periodic table. This is the same effect as observed previously for the 13 C shieldings of carbonyl groups.

Application of 13C NMR to the determination of metal—carbon σ bond formation in cyclometallation reactions with nitrogen donor ligands

A series of cyclometallated complexes of the nitrogen donor ligands, azobenzene, N,N-dimethylbenzylamine, 8-methylquinoline, and benzo[h]quinoline have been examined by 13C NMR. The total number of expected aromatic quaternary and CH carbon atom resonances were determined by comparison of the noise decoupled and single frequency off resonance decoupled spectra of a given complex. In this manner it can be readily determined that cyclometallation may have occurred. In those cases where metal-13C coupling is observed an unambiguous determination of metal—carbon σ bond formation is achieved.

A carbon-13 NMR study of some metal carbonyl compounds containing one-electron ligands

Carbon-13 NMR spectral data for complexes having the general formula CpM(CO)nX (Cp = η5-C5H5; M = Mo or W, n = 3; M = Fe, n = 2; X = halogen, methyl or acetyl) and their phosphine and isocyanide substitution products are reported. For CpM(CO)3X complexes two carbonyl resonances (1 : 2 ratio) are observed in all cases, consistent with the retention of the “piano-stool” geometries observed in the solid state. Substituted complexes CpM(CO)2(L)X (M = Mo or W; L = PR3 or cyclohexyl isocyanide) are unequivocally assigned cis or trans geometries on the basis of the number of observed carbonyl resonances and values of 2J(PC) for the phosphine substituted derivatives. Spectral data for [M(CO)5X]− (M = Cr, Mo or W; X = Cl, Br or I) and η7-C7H7Mo(CO)2X and the halide derivatives above generally show an increase in the shielding for carbonyls adjacent to the halide ligand in the order Cl < Br < I. Carbonyl resonances are more shielded in isostructural complexes in the order Cr < Mo < W (triad effect).

Dynamics and structure of some tricarbonylferrole-iron tricarbonyl derivatives

Abstract A series of compounds of the formula Fe2(CO)6-x(PR3)x(R′C2R″)2 (x = 0, R′ and R″ = Ph, R′ and R″ = H, R′ = Ph and R″ = H; x = 1, K = Ph or n-Bu, and R′ and R″ = Ph) were studied by 13C NMR to observe their solution properties. The tricarbonylferrole unit was found to be static from −125 to +95° C, while the π-Fe(CO)3 group appeared to be fluxional over the same temperature range. Definite assignments of the carbonyl carbon and ferrole ring carbon resonances have been made. A low temperature single crystal X-ray study of Fe2(CO)5PPh3(PhC2Ph)2 demonstrated that the phosphine ligand was attached to the ferrole iron contrary to previous belief based on chemical evidence.

Solution properties of the HFe3(CO)11−ion

Abstract A variable temperature 13 C NMR study of HFe 3 (CO) 11 − has revealed that at least two dynamic processes are occurring over the range −107 to +30°C. Infrared and 13 C NMR studies of the HFe 3 (CO) 11 − ion in various solution environments show that the bridging carbonyl unit is very basic and forms acidbase complexes with BF 3 as well as HN(C 2 H 5 ) 3 + . These acid-base interactions apparently cause the fluxional processes of the HFe 3 (CO) 11 − ion to stop at a higher temperature.

Dynamics and structure of some rhodium-acetylene derivatives

Abstract The acetylene ligand of (C 5 H 5 ) 3 Rh 3 (CO)(Ph 13 CCPh) is fluxional in solution at room-temperature but static at −87°C. The acetylene ligand of (C 5 H 5 ) 3 - RH 3 (Ph 13 CCPh) is fluxional even at −127°C. The acetylene carbon signal is very deshielded suggesting the possibility of carbenoid-type bonding for this ligand According to C NMR evidence the derivative [(C 5 H 5 )Rh(PhC 2 Ph)] 2 does not have a metallocyclopentadiene structure but an unsymmetrical four-carbon ligand. The complex [(C 5 H 5 )Rh(CO)] 2 CF 3 C 2 CF 3 possesses a static bridging acetylene ligand. However the carbonyl groups appear to be scrambling at room temperature and in a static terminal position at −60°C.

The chemistry of some metalloborane derivatives having iron—boron single bonds

Abstract Reaction of [(η 5 -C 5 H 5 )Fe(CO) 2 (cyclohexene)]PF 6 with B 10 H 13 − and 7,8-B 9 C 2 H 12 − forms 6-[(η 5 -C 5 H 5 )Fe(CO) 2 ]B 10 H 13 and (η 5 -C 5 H 5 )Fe(CO) 2 (7,8-B 9 C 2 H 12 ) respectively which have iron—boron single bonds. The reactions of these compounds with bromine and with Lewis bases such as trialkylamines, CH 3 CN and Ph 3 P are described.

A 13C NMR of metal carbonyl derivative having quadrupolar nuclei (Mn, Co, Re and Ir)

The room and low temperature 13C NMR spectra of (PhCCPh))Co2(CO)6, C6H5CH2Mn(CO)6, Re2(CO)10, Re(CO)5 Br and (η-C5H5)Ir(CO)2 have been studied. The application of “thermal coupling” in the case of the first row metal carbonyl derivatives was found to be useful for clarification of the spectra.

A 13C NMR study of some derivatives obtained from iron carbonyls and diphenylacetylene

Abstract The molecule Fe2(CO)7(PhC2Ph)2 has been studied by 13C NMR and is static at −40°C with a spectrum which is consistent with the known structure observed in the crystals. In the range −40 to +41°C the three carbonyl resonance coalesce possibly by a single carbonyl averaging process. At room temperature violet Fe3(CO)8(PhC2Ph)2 is found to be static by 13C NMR having a spectrum consistent with the known solid state structure. Dark green Fe3(CO)8(PhC2Ph)2 is static via 13C NMR at −62°C with a spectrum consistent with the known solid state structure. Between −62 and +96°C a fluxional process occurs which scrambles the bridge carbonyls and four of the six terminal carbonyl groups. In this process the bridging carbonyls appear to be preferentially moving toward the iron atoms with which they have the shortest iron-carbon bond as observed in the solid state structure.