Noel J. Fitzpatrick

Conformational behaviour of hydroxamic acids: ab initio and structural studies

The conformational behaviour of a series of monohydroxamic acids, p-RC6H4CONR′OH (R = Me, R′= H, Me; R = MeO, R′= H, Me; R = NO2, R′= H), and a series of dihydroxamic acids, (CH2)n(CONR′OH)2(n= 3–8, 10, R′= H and n= 7, R′= Me), in methanol, DMSO and chloroform and in the solid state has been examined using IR and NMR spectroscopy. X-Ray crystal structure determinations of p-MeC6H4CONMeOH and the monohydrate of glutarodihydroxamic acid (n= 3) together with ab initio molecular orbital calculations for several hydrated and unhydrated hydroxamic acids have been performed. Hydrogen bonding effects are shown to be important in both the solid state and solution. The cis(Z) conformation of the hydroxamate group(s)(CONHOH) is preferentially stabilized by hydrogen bonding with water molecules.

Extended CNDO/2 studies of the isoelectronic series AM(CO)3 (AM η6-C6H6Cr,η5-C5H5Mn,η4-C4H4Fe,η3-C3H5Co,η2-C2H4Ni) and of (η6-C6H6)2Cr

Abstract The electronic structure of isoelectronic AM(C0)3 (AM  η6-C6H6Cr,η5-C5-H5Mn,η4-C4H44Fe,η3-C3H5Co,η2-C2H4Ni) and of (η6-C6H6)2Cr is investigated within an extended CNDO/2 formalism using both experimental and standard geometries. The computed trends for bond strengths and stretching frequencies (as measured by Wiberg indices or bicentric energy terms) and for reactivity (charge distributions) correlate with the experimental ones. For chromium complexes, the extended CNDO/2 results seem more reasonable, for example the charge and the orbital populations on chromium, than are the ab initio single > results. A general agreement is observed between CNDO/2 and SCCC electronic structures.

Double zeta d radial wave functions for transition elements

Abstract A consistent double zeta set of exponents and coefficients for neutral and +1 charged transition metals is presented. The results were calculated by fitting analytical functions to numerical Herman-Skillman calculations.

Consistent approximate wavefunctions for all elements of the periodic table

Abstract A consistent set of exponents for all elements of the periodic table, suitable for semiempirical molecular orbital and other calculations is presented. These exponents were calculated by fitting analytical functions to numerical Herman-Skillman results. The new exponents compare well with existing exponents.

Reactions of some Group 15 ligands with [(η5-indenyl)Fe(CO)3]BF4

Abstract [(η 5 -Indenyl)Fe(CO) 3 ]BF 4 ( I ) undergoes facile monocarbonyl substitution at room temperature in acetone by monophosphines, monophosphites and MPh 3 (M =1cr; As, Sb, Bi) ligands (L) to form [(η 5 -Indenyl)Fe(CO) 2 L]BF 4 . Ditertiary phosphines, PPh 2 (CH 2 ) n PPh 2 ( n = 1, 2, 4, 6, 8) and both the arsines, AsPh 2 (CH 2 ) 2 AsPh 2 and AsMe 2 (CH 2 ) 5 AsMe 2 react similarly to form monosubstituted complexes e.g. [(η 5 -Indenyl)Fe(CO) 2 (η 1 - PPh 2 (CH 2 ) n PPh 2 )]BF 4 and dimeric complexes, e.g. [{(η 5 -Indenyl) Fe(CO) 2 } 2 -μ-PPh 2 (CH 2 ) n PPh 2 )] [BF 4 ] 2 Prolonged refluxing of I with these ligands gives the chelates, e.g. [(η 5 -Indenyl)Fe(CO)(η 2 - PPh 2 (CH 2 ) n PPh 2 )]BF 4 . The enhanced reactivity of the [(η 5 -Indenyl)Fe(CO) 3 ] + cation over that of [(η 5 ,-C 5 H 5 )Fe(CO) 3 ] + in solvents such as acetone may be attributed to the “indenyl” effect, i.e. ring slippage from η 5 to η 3 . However, no evidence was obtained for intermediates such as [(η 3 -C 9 H 7 )Fe(CO) 3 (acetone)] + , and so the effect must operate solely in the transition state of the reaction.

A THEORETICAL-STUDY OF THE [HP2]+ CATION AND [H2P2]2+ DICATIONS - STABLE BRIDGED STRUCTURES

Abstract Structures and stabilities of the [HP2]+ and [H2P2]2+ ions are studied in their singlet ground state employing ab initio calculations. Geometries of stationary points were determined at the MP2/6-31G(d,p) level while relative energies were estimated using MPn calculations with various basis sets (up to 6-31 + +G(2df,p)). The bridged structure (C2v) is found to be the only minimum on the [HP2]+ surface, lying about 8-9] kcal/mol below the linear structure. The proton affinity of P2 is estimated to be 158±3 kcal/mol. Analogous to [H2Si2], the non-planar doubly bridged structure represents the global minimum lying a few kcal/mol below the planar [H2P-P]2+ species. The two dication isomers are separated by a barrier of 10–16 kcal/mol. The planar-bridged structure is a transition structure for autoconversion of the bent-bridged form while the linear structure is a second-order saddle point. The fragmention [H2P2]2+→[HP2]++ H+ is exothermic (ΔHf= −9 kcal/mol). Coupled with the large barriers for deprotonation, this suggests that the bridged [H2P2]2+ dication might be observed in mass spectroscopic experiments. The unusual structural feature of the ions may be understood from a simple orbital picture.

Molecular orbital study of the complexation of P5 and P6 rings with arenemetal fragments

Extended Huckel calculations on the model complexes (C6H6)2Cr,(C6H6)CrP6, (C5H5)2Cr2−, (C5H5)CrP52−, (C5H5)2Fe and (C5H5)FeP5 showed that the complexes containing P5 and P6 ring systems may show stabilities comparable to, or even stronger than, their carbon analogues, despite large differences between the free cyclic fragments. Although the HOMO-LUMO energy gaps in both P and C complexes are often similar, the pattern of MO interactions differ significantly for P and C rings and there is less distinct MO binding in P complexes.

Molecular orbital theory of organometallic compounds. Part XII. Nature of metal–metal bonding in some binuclear metal carbonyls

The nature of the metal–metal bond in a series of binuclear metal carbonyls M2(CO)10(M2= Mn2, Tc2, Re2, and MnRe) is described by application of the self-consistent charge and configuration (SCCC) molecular orbital method. Good correlation between calculated quantities such as orbital energies and overlap populations and experimental quantities such as photoionization spectra and bond dissociation energies are obtained. It is suggested that the term ‘unsupported’ metal–metal bond, especially for atoms of the first transition series requires modification because of the important contribution to the metal–metal bond energy of the cross-interaction between a metal atom and ligands directly bonded to the other metal atom.

The prediction of nucleophilic attacking sites via determinants of ‘active’ frontier and near-frontier orbitals☆

Abstract A method for computing the most favourable initial attacking site for a nucleophile on a transition metal complex is described, and applied to the nucleophilic substitution and addition reactions of fluoromethane, and the cationic complexes [BFe(CO)3]+ (B = C6H7 and C7H9). The reactions considered are classified according to whether they are frontier or non-frontier orbitally or charge controlled. It is found that initial attack on the (polyenyl)M(CO)3 cations is always predicted to occur at the M(CO)3 moiety, in agreement with the experimental observation of intermediates in several such reactions and suggesting that the existence of these species is a general phenomenon.

Vibrational spectra of σ-allylmanganese pentacarbonyl

Abstract The vibrational spectra of σ-(C 3 H 5 )Mn(CO) 5 are reported. Assignment of bands is made and carbonyl force constants are calculated. The results indicate that the Mn(CO) 5 moiety has C 4ν symmetry. The calculated angle between the axial and equatorial carbonyl groups is approximately 95°. The bonding in this compound is very similar to that in (CH 3 )Mn(CO) 5 . In the far-infrared region, seven bands are expected in C 4ν symmetry (3 A 1 + 4 E ), and all are observed.