ISOTROPIC P.M.R.SHIFTSIN a-BONDING LIGANDS
Aug. 5 , 1964
order of thermal energies. Thus a Boltzmann distribution could exist between the two. In this way, one might explain the reported existencez4of both singlet and triplet binuclear J ! f O ( v ) species in hydrochloric acid
[ COSTRIBUTION
FROM THE
3031
solutions of 1Llo(V). Both would have the general constitution [Cl&fo(O)-O-Mo( O)C1,14-. (24)
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R H a r e , I Bernal, a n d H B G r a y ,
DEPARTMENT O F CHEMISTRY, PRIXCETON UNIVERSITY,
rnorg
Chem , I , 831 (1962)
PRISCETON, N E W JERSEY]
Isotropic Proton Magnetic Resonance Shifts in a-Bonding Ligands Coordinated to Paramagnetic Nickel(I1) and Cobalt(I1) Acetylacetonates BY \VILLIAMD. HORROCKS, J R . , R.CRAIG TAYLOR,
AND
GERDi\: LAMAR
RECEIVED APRIL 15, 1964 Isotropic proton magnetic resonance shifts due to contact and pseudo-contact interactions have been observed for protons in certain triarylphosphines and isonitrile molecules when these are placed in solution in CDCIa with the paramagnetic S i ( I 1 ) and Co(I1) acetylacetonates. The alternation in sign of the observed shifts for adjacent protons on the phenyl rings is evidence for delocalization of spin density into the rr-orbitals of these ligands when coordinated t o both Co(1Ij and Ki(I1) acetylacetonates. Evidence for 3 large upfield pseudo-contact shift in the cobalt systems is presented Proton spin-spin coupling constants for the arylphosphines and isonitriles are given
Introduction This paper reports a high resolution proton magnetic resonance (p.m.r.) study of interactions in solution between a-bonding ligands and the paramagnetic chelates, cobalt(I1) and nickel(I1) acetylacetonate [bis(2,4-pentanediono)cobalt(II)and -nickel(II) 1, hereafter referred to as C O ( X ~and ~ ) ~Tu’i(A4A4)2. Large, copcentration-dependent chemical shifts from the values in the diamagnetic ligands are observed for the proton resonances of certain triarylphosphines and isonitrile molecules when these are placed in chloroform solution with the paramagnetic complexes. A great deal of extremely detailed and fundamental knowledge about the electronic structure of certain paramagnetic systems can be obtained from their p.m.r. spectra. The elegant work of Phillips and his c o - ~ o r k e r s l - ~on the Si(I1)aminotroponeimineates and related systems illustrates the potential of this technique. Most of the recent work in this field has been confined to systems containing Si(I1) ; however, recently several studies of systems containing Co(I1) have a ~ p e a r e d . ~ -Of~ particular interest to this work is the investigation by Happe and Ward5 of the p.m.r. spectra of pyridine-type bases complexed with Xi(AA)* and CO(X.I)~. The theory of isotropic nuclear resonance shifts has been discussed a t length by McConnell and Robertson.’” The conditions necessary for the observation of proton resonances in paramagnetic systems are by now well established.*lo l 1 Either the electronic (1) D. K . E a t o n , A . 13. Josey, W. D . Phillips, and R. E. Benson, J . Chem. P h y s . , 37, 317 (1962). (2) D. R. E a t o n . A . D. Josey, R . E Benson, W. D. Phillips, and T. I
