Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rate of C-H activation of alkanes by Cp'Rh(CO) {Cp' = η(5)-C(5)H(5) or η(5)-C(5)Me(5); alkane = cyclopentane, cyclohexane and neopentane (Cp only)} in solution at room temperature and allowed the determination of how the change in rate of oxidative cleavage varies between complexes and alkanes. Density functional theory calculations on these complexes, transition states, and intermediates provide insight into the mechanism and barriers observed in the experimental results. Unlike our previous study of the linear alkanes, where activation occurred at the primary C-H bonds with a rate governed by a balance between these activations and hopping along the chain, the rate of C-H activation in cyclic alkanes is controlled mainly by the strength of the alkane binding. Although the reaction of CpRh(CO)(neopentane) to form CpRh(CO)(neopentyl)H clearly occurs at a primary C-H bond, the rate is much slower than the corresponding reactions with cyclic alkanes because of steric factors with this bulky alkane.

The mechanism of a Ni-centered C−Br bond-forming reaction has been investigated with density functional theory calculations. Unlike the typical behavior of heavier group 10 metals that make use of higher oxidation states, Ni undergoes a change in multiplicity to provide a low-energy path. The calculated pathway begins with the singlet state complex Ni(Ar)(Br)(pic) (Ar = 2-phenylpyridine, pic = 2-picoline) and remains on the singlet state reaction surface for the axial addition of Br 2 . The Br 2 forms a three-center, four-electron bond with Ni that stabilizes the singlet state without increasing the oxidation state of the Ni center. The singlet state is then destabilized by the loss of the three-center, four-electron interaction as the Br−Br bond is broken and the second Br from the Br 2 (the Br previously unbound to Ni) binds to the Ni center. At this point in the mechanism, the triplet state offers a lower energy pathway, as the triplet state is stabilized by a Br 2 − /Ni III interaction where one unpaired electron is localized on the Ni center and the second unpaired electron is localized in the Br 2 σ* orbital. The reaction continues on the triplet reaction surface because the Br 2 − /Ni III interaction can be maintained until the final reductive elimination takes place. The mechanism concludes with the formation of the reductively eliminated C Ar −Br product and the addition of dppe to stabilize the Ni byproducts as a singlet-state Ni II (dppe)(Br) 2 complex.

A series of new bridging phosphane and phosphonite structures forming three‐ and six‐membered rings with the metal centers were synthesized and characterized. The resulting compounds of phenylbis(pyrid‐2‐ylmethyl)phosphane (1) with the silver(I) salts of trifluoroacetate (tfa–), tetrafluoroborate (BF4–), and trifluoromethanesulfonate (OTf–), and copper tetrakis(acetonitrile) hexafluorophosphate (PF6–) shows the flexibility of the ligand by displaying different coordination modes associated with the electronic and structural characteristics of the corresponding anion. Accordingly, ligand 1 in these complexes displays two different binding modes. With Agtfa and AgBF4 compounds 3 and 4 are obtained where the ligand chelates to two silver atoms that exhibit normalAg–Ag contacts in the range of 2.9 Å. When AgOTf or Cu(NCCH3)4PF6 are used, one molecule of 1 bridges the metal centers through a phosphorus atom while another is terminally bound. This induces short M–M distances of 2.6871 and 2.568 Å for 5 and 6, respectively. Similarly, the coordination behavior of the heterofunctional bis(pyrid‐2‐ylmethyl) phenylphosphonite ligand (2) is reported with Cu(NCCH3)4PF6 (7) and AgBF4 (8) to form two novel discrete molecules. In these complexes 2 coordinates through the P and N atoms, with the difference that in 7 the O atom of one of the carbinol arms is also bound to the Cu. Elemental analysis, variable‐temperature multinuclear NMR spectroscopy, single‐crystal X‐ray diffraction, and low‐temperature luminescence studies were carried out to fully characterize the compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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