Phosphorus-carbon bonding orbital

In summary, independently of the second carbene substituent, phosphinocarbenes have a singlet ground state, with a small singlet-triplet gap. They have a planar geometry at phosphorus and a short phosphorus-carbon bond, indicating an interaction between the phosphorus lone pair and the carbene vacant orbital. In the case of silyl- and phosphoniophosphinocar-benes, there is an additional interaction between the carbene lone pair and lowlying [Pg.179]

While 4bp/-type structures can be considered as resonance structures, 4cp/is also an alternative. The importance of structure 4cp/is supported by the fact that the negative charge at the carbon atoms in the 2 and 5 positions is indeed larger than at the 3 and 4 positions. ° The common feature in all 4ap/-, 4bp7-, and 4cp/-type structures is that the lone pair of the planar tricoordinate phosphorus is fully delocalized, resulting in an enhanced bond order about the tricoordinate planar phosphorus. The d-orbital participation at phosphorus is insignificant in 4, either in the planar or nonplanar form. °... 

Diphosphaallene derivatives ArP=C=PAr are peculiar compounds because of the presence of the two orthogonal carbon-phosphorus double bonds. The compounds were transformed into cation-radicals on electrochemical or chemical one-electron oxidation. As found, the unpaired electron is located on an MO constituted mainly by a p orbital of each phosphorus atom and a p orbital of the carbon atom (Chentit et al. 1997, Alberti et al. 1999). 

The overlap of carbon p orbitals with arsenic d orbitals is less effective than with the d orbitals of phosphorus, and so the covalent canonical structure is expected to make less of a contribution to the hybrid structure. This has been confirmed in an X-ray study of 2-acetyl-3,4,5-triphenylcyclopentadienetriphenylarsorane.6 Yamamoto and Schmidbaur7 found (13CNMR) that the bonding in arsenic ylides was probably sp3 (cf. phosphorus, which changes from sp3—>sp2), resulting in arsenic pseudotetrahedral geometry (cf. phosphorus ylides, which are planar). 

Other theoretical studies discussed above include investigations of the potential energy profiles of 18 gas-phase identity S 2 reactions of methyl substrates using G2 quantum-chemical calculations," the transition structures, and secondary a-deuterium and solvent KIEs for the S 2 reaction between microsolvated fluoride ion and methyl halides,66 the S 2 reaction between ethylene oxide and guanine,37 the complexes formed between BF3 and MeOH, HOAc, dimethyl ether, diethyl ether, and ethylene oxide,38 the testing of a new nucleophilicity scale,98 the potential energy surfaces for the Sn2 reactions at carbon, silicon, and phosphorus,74 and a natural bond orbital-based CI/MP through-space/bond interaction analysis of the S 2 reaction between allyl bromide and ammonia.17... 

The CoNMR spectra of the tetrahedral [Co(PF3)4] and [Co(CO)4] anions have been recorded (191) and values obtained for the V( Co- P) (1222 25 Hz) and V( Co- C) (287 13 Hz) coupling constants. Approximate values for the cr-bond order were calculated and together with shift data suggested that bonding to the ligand tt orbitals is more important in the carbonylate ion than bonding to the phosphorus 3d orbitals of the tetrakis(trifluorophosphine)cobaltate, and that the cobalt-phosphorus u-bond order apparently is greater than the cobalt-carbon c7-bond order (191). 

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