2942 Inorg. Chem. 1985, 24, zyxwvu 2942-2946 the solvent; it even diffuses in rare-gas matrices at 10 K. The question remains as to what the mechanism is for the release of CO. In the case of the d6 M(CO),(a-diimine) com- plexes, CO is released for complexes in which the ,MLCT state is delocalized over the carbonyls in the cis position with respect to the a-diimine ligand.,, As a result of this delocalization the metal-CO zyxwvutsrqp A back-bonding is weakened, and a CO ligand is re- leased. This delocalization of the excited state over the cis car- bonyls is reflected in a high relative intensity of V,(CO),~~ in the RR spectra. In the case of the d8 M(CO),(a-diimine) complexes the release of CO could not be explained with a delocalization of the 3MLCT state over one or more carbonyl ligands. Instead a strong coupling model36 was proposed in which the primary step in solution is the breaking of a metal-nitrogen bond with formation of an intermediate in which the a-diimine ligand is u-monodentate bound.23 A nucleophilic ligand then attacks the open site, CO is released and the up-coordination of the a-diimine ligand is restored again. Although the intermediate could not be identified in the case of the M(CO),(a-diimine) complexes, it has been observed for the zyxwvutsrq (CO)SMnRe(CO)3(a-diimine) complexes. We propose the same mechanism for these complexes as for the d8 M(CO),(a-diimine) ones, and this will also be the case for the corresponding (CO),MMn(CO),(a-diimine) (M zyxwvutsr = Mn, Re) complexes in which CO is photosubstituted by 2-MeTHF. The difference in behavior between the two types of complexes is certainly due to the fact that the Re-CO bond is much stronger than the Mn-CO bond. As a result the reaction stops before CO is released in the case of the (CO),MnRe(CO),(a-diimine) complexes. When these latter complexes are irradiated with higher energy (A = 350 nm), disproportionation into the ions is observed just as in the case of the (CO),MMn(CO),(a-diimine) complexes (Figure 11). This wavelength dependence of the reaction is also in agreement with the results for the dB M(CO)3(a-diimine) complexe~.~~,~~ (36) Johnson, C. E.; Trogler, zyxwvutsrqpo W. C. J. Am. Chem. SOC. 1981, zyxwvutsrq 103, 6352. There is a close relationship between this photochemistry and the mechanisms proposed by McCullen and Brown6 and Stiegman and Tyler’ for the disproportionation reaction of Mn2(CO),oin pyridine. McCullen and Brown proposed a mechanism in which electron transfer takes place from a 17-electron .Mn(CO),N, radical to Mn2(CO) whereas Stiegman and Tyler proposed electron transfer from a 19-electron radical with three basic groups, .Mn(CO),N,. Our experiments show that heterolysis takes place for the (CO),MMn( CO) *( a-diimine) (2-MeTHF) complexes with three basic donor atoms at one metal fragment and not for the corresponding parent compounds (CO),MMn(CO),(a-diimine). This result is in agreement with the mechanism proposed by Stiegman and Tyler. Acknowledgment. We thank Anja M. F. Brouwers for per- forming the first experiments leading to this study, Wim de Lange and Henk Gijben for preparing the complexes, Gerard Schoemaker for assistance during the IR experiments, Henk Luiten for making a low-temperature IR cell, Andries Terpstra for assistance during UV experiments, and Dr. Johan Lub for assistance during the ESR experiments. Registry No. (CO),MnMn(CO),(bpy’), 97570-64-4; (CO),MnMn- (CO)3(phen), 60166- 19-0; (CO),MnMn(CO),(i-Pr-DAB), 7 1603-98-0; (CO)5MnMn(CO)3@-Tol-DAB), 71604-00-7; (CO),MnMn(CO),@- Tol-PyCa), 97591-94-1; (CO),ReMn(CO),(phen), 61993-44-0; (CO)5ReMn(CO)3(i-Pr-DAB), 97570-65-5;(CO),ReMn(CO),@-Tol- DAB), 97570-66-6; (CO),MnMn(CO),(bpy’)(2-MeTHF), 97570-67-7; (CO),MnMn(CO),(phen)(2-MeTHF), 97570-68-8; (CO),MnMn- (CO),(phen)(P-Bu),), 97570-69-9; (CO),MnMn(CO),(i-Pr-DAB)(2- MeTHF), 97570-70-2; (CO),M~MII(CO)~@-TO~-P~C~)(~-M~THF) 9759 1-95-2; (CO),ReMn(CO),(phen)(2-MeTHF), 97570-7 1-3; (CO),ReMn(CO),(phen)(P(n-Bu),), 97570-72-4; (CO),ReMn(CO),(i- Pr-DAB)(2-MeTHF),97570-73-5; (CO),ReMn(CO),(i-Pr-DAB)(P(n- Bu),), 97570-74-6; (CO),ReMn(CO),(p-Tol-DAB)(2-MeTHF), 97570- 75-7; Mn,(CO),(bpy’),, 97570-76-8; Mn2(C0),(phen),, 97570-77-9; Mn2(CO)6(i-Pr-DAB)2, 90885-36-2; MII,(C~),(~-P~-DAB)(~-M~THF)~, 97570-78-0; Mn2(C0)6(p-Tol-PyCa)(2-MeTHF)2, 97570-79-1; Re2- (CO),(~-TOI-DAB)(~-M~THF)~, 97570-80-4; Mn,(CO),,,, 10170-69- 1. Contribution from the Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13901 Characterization and Reaction Kinetics of Intermediates Produced in the Photolysis of M(C0)6 (M = Cr, Mo, W) Solutions Containing a 1,4-Diazabutadiene Ligand MARK J. SCHADT, NANCY J. GRESALFI, and ALISTAIR J. LEES* Received June 29, I984 Electronic absorption data have been recorded on a microprocessor-controlled diode-array spectrophotometer at short time intervals following the photolysis of M(C0)6(M = Cr, Mo, W) in benzene containing 1,4-di-tert-butyl-l,4-diazabutadiene (1,4-dab). These spectra illustrate rapid formation of a primary reaction intermediate that is assigned to be a solvent impurity species, M- (CO)s(impurity). This species is then scavenged by ligand to form monodentate M(CO),(1,4-dab). UV-visible difference spectra of these reaction intermediates are reported. Rates of formation of M(CO)5(1,4-dab)have been measured as a function of temperature and ligand concentration. The data imply that M(CO),(impurity) is converted to M(CO),( 1,4-dab) via a dissociative mechanism. Monodentate M(CO),( 1,4-dab) subsequently extrudes CO by a relatively slow first-order kinetic process to form M(C0),(1,4-dab). Rates of chelation for L (ligand) = 1,4-dabare compared with literature values for L = l,lO-phenanthroline and 2,2‘-bipyridineand are discussed in terms of the stereochemistry of L when it is coordinated to a metal center in a monodentate fashion. Introduction synthesis and in the design of homogeneous catalytic processes. In many of these processes unsaturated species are thought to be involved as reaction intermediates.’ However, direct spectral evidence for these reaction intermediates and quantitative mea- Although a great deal is now known about ligand substitution of low-valent transition-metal centers, our knowledge of the identity, structure, and reactivity of their reaction inter- mediates is still very limited. This knowledge is important to a (1) (a) Henrici-Olive, G.; Olive, S. ”Coordination and Catalysis”; Verlag Chemie: New York, 1977. (b) Geoffroy, zyxw G. L.; Wrighton, M. S. “Organometallic Photochemistry”; Academic Press: New York, 1979. further development of organometallic chemistry. Information about the nature of ligand substitution events at low-valent transition-metal centers is useful in systematic organometallic 0020-1669/85/1324-2942$01.50/0 0 1985 American Chemical Society