The synthesis of the (7)5-C6Ph5)2Mo2(CO)6 complex is described. In solution, the dimer is in equilibrium with 2 17-electron (jj5-C5Ph5)Mo(CO)3 monomers.The equilibrium constant for the dimer-monomer equilibrium, as determined by electronic absorption spectroscopy, is 8.7 (±5.1) X 10™5 at 23 °C. (A 1 X 10"4 M solution of the dimer is thus 40% ± 15% dissociated.) (775-C5Ph6)2Mo2(CO)6 reacts with L2 (L2 is the chelating phosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride) to form the 19-electron ("18 + 5") (7j5-C5Ph5)Mo(CO)2(L2-P,P/) and the 17-electron (Tj5-CgPhs)Mo(CO)2(L2-P) complexes. (L2-P,P' indicates two P atoms are coordinated, and L2-P indicates one P atom is coordinated.) Variable-temperature electron spin resonance (ESR) showed a dynamic equilibrium between these 19-and 17-electron complexes with lower temperature favoring the 19-electron complex, and higher temperature the 17-electron complex. The (7)5-C5Ph5)Mo(CO)2(L2-P,P') complex has two magnetically equivalent P atoms, and the room-temperature ESR spectrum is a 1:2:1 triplet. This spectrum and its temperature-dependent behavior is compared and contrasted with the spectrum of the analogous ()?5-C5Ph4H)Mo(CO)2(L2-P,P') complex. Infrared, ESR, and electronic spectroscopic data are reported for all of the complexes generated in this study.
Measurements of water 'H spin-lattice relaxation rates as a function of magnetic field strength are reported for aqueous solutions of iron(II1) and manganese(I1) and X-band EPR measurements are reported for manganese(II), iron@), and gadolinium(II1) solutions that provide an improved understanding of what controls nuclear-and electron-spin relaxation in these electronically symmetric paramagnetic centers. The electronspin relaxation rates of the iron(II1) and gadolinium(II1) aquo ions are higher than in other symmetrical complexes of these metal centers, which appears to be caused by intramolecular motions of the coordinated water molecules. The water proton relaxation rate at low magnetic field strengths in aqueous manganese(I1) and iron(II1) solutions increases with increasing perchloric acid concentration, which results in part from an increase in the metal-proton hyperfine coupling constant. In the iron(II1) case, the additional relaxation efficiency is interpreted in terms of a change in the orientation of the coordinated water molecule that brings the protons closer to the metal center. The gadolinium(II1) electron relaxation rate decreases with increasing perchloric acid and glycerol concentration, which is interpreted in terms of a change in the number of coordinated water molecules changing from 9 to 8. Electron-spin relaxation rates estimated from electronspin resonance line widths of iron(II1) and Gd(II1) ions in symmetric complexes such as [FeF6I3-or [FeClJ are long and demonstrate that construction of complex species with long electron-spin relaxation times that will be efficient water proton spin relaxation agents should be possible using iron(II1) centers.Nuclear magnetic relaxation of water protons is crucial to contrast control and interpretation of magnetic resonance images.' The nuclear-spin relaxation process and its regulation by paramagnetic metal ions is of general interest in its own right and raises several interesting structural and dynamical questions. Of particular interest are the metal complexes of half-filled d or f orbitals that have S electronic ground-state configurations because the electron relaxation times in these complexes are relatively long and permit the development of high nuclear-spin relaxation rates in the solvent water protons. Thus, complexes of gadolinium(II1) are used as contrast agents for medical magnetic imaging. Manganese(I1) complexes are nearly as efficient but suffer from lower chemical stability. However, iron(III), which is isoelectronic with manganese(I1) and may be of lower toxicity, is generally thought to be a poor relaxation agent because the iron(II1) ion changes the water proton spin-lattice relaxation rate relatively little compared with manganese(II) ion. These observations are generally understood in terms of the Solomon, Bloembergen, and Morgan equation^,^-^ which are thoroughly discussed elsewhere5-' but reproduced here for convenience. The paramagnetic contribution from fistcoordination-sphere water molecules to the water proton spinlattice r...
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