Mononuclear and binuclear manganese carbonyl hydrides: the preference for bridging hydrogens over bridging carbonyls

Abstract

The structures of the mononuclear derivatives HMn(CO)n (n=4 and 3) are shown by density functional theory (B3LYP and PB86) to derive from octahedral HMn(CO)5 by losses of various combinations of carbonyl groups with relatively little change in the C-Mn-C angles involving the remaining carbonyl groups. The binuclear H2Mn2(CO)n structures are predicted to have bridging hydrogen atoms in preference to bridging carbonyl groups. Thus, two structures are found for the binuclear H2Mn2(CO)9, isoelectronic with Fe2(CO)9, in which all nine of the carbonyl groups are terminal carbonyl groups. The lowest lying H2Mn2(CO)9 structure is the dihydrogen complex (OC)5Mn-Mn(CO)4(eta2-H2), in which one of the equatorial CO groups of Mn2(CO)10 is replaced by a dihydrogen ligand. A slightly higher energy H2Mn2(CO)9 structure by approximately 6 kcal mol(-1) has an Mn-Mn bond bridged by a single hydrogen and all terminal carbonyl groups as well as a single terminal hydrogen. The H2Mn2(CO)8 molecule is predicted to have a structure with a central Mn(micro-H)2Mn core related to diborane. In this structure the manganese-manganese bond of length 2.703 A (BP86) can be considered the diprotonated formal double bond required to give both manganese atoms the favoured 18-electron configuration. The more highly unsaturated H2Mn2(CO)n (n=7, 6) derivatives have similar structures derived from the H2Mn2(CO)8 structure by loss of one or two carbonyl groups. In many cases the Mn[triple bond, length as m-dash]Mn distance in the central Mn(micro-H)2Mn unit shortens to approximately 2.4 A suggesting a diprotonated triple bond.

Diverse Roles of Hydrogen in Rhenium Carbonyl Chemistry: Hydrides, Dihydrogen Complexes, and a Formyl Derivative

“…In the H 2 Re 2 (CO) 9 structure 29-2S the terminal hydrogen is predicted to exhibit a ν(ReH) frequency of 1826 cm -1 and the bridging hydrogen is predicted to exhibit ν(ReH) frequencies at 1077 and 1604 cm -1 . 8 . Two singlet equilibrium structures were found for H 2 Re 2 (CO) 8 .…”

“…8 . Two singlet equilibrium structures were found for H 2 Re 2 (CO) 8 . The lowest lying triplet H 2 Re 2 (CO) 8 structure is predicted to lie >50 kcal/mol above the H 2 Re 2 (CO) 8 global minimum.…”

“…For HRe(CO) 5 , HRe(CO) 4 , and HRe(CO) 3 , there are 191, 161, and 131 contracted Gaussian basis functions, respectively. For the binuclear rhenium carbonyl hydride derivatives H 2 Re 2 (CO) 9 , H 2 Re 2 (CO) 8 , H 2 Re 2 (CO) 7 , and H 2 Re 2 (CO) 6 , there are 352, 322, 292, and 262 contracted Gaussian basis functions, respectively.…”

“…However, a detailed 13 C and 1 H NMR study on the HRe 2 (CO) 9 -acidification product suggests an unusual HRe 2 (µ-H)(CO) 9 structure with one terminal and one bridging hydrogen atom (Figure 1). A more stable binuclear neutral rhenium carbonyl hydride is Re 2 (µ-H) 2 (CO) 8 , which can be obtained by photochemical hydrogenation of Re 2 (CO) 10 . 4 The central Re 2 (µ-H) 2 unit in Re 2 (CO) 8 (µ-H) 2 is closely related to the central B 2 (µ-H) 2 unit in diborane.…”

“…A more stable binuclear neutral rhenium carbonyl hydride is Re 2 (µ-H) 2 (CO) 8 , which can be obtained by photochemical hydrogenation of Re 2 (CO) 10 . 4 The central Re 2 (µ-H) 2 unit in Re 2 (CO) 8 (µ-H) 2 is closely related to the central B 2 (µ-H) 2 unit in diborane. 5 Furthermore, Re 2 (µ-H) 2 (CO) 8 is the first member of a series of [Re(CO) 4 H] n oligomers (n ) 2, 3,4,5,6), of which the square 6 Re 4 (CO) 16 (µ-H) 4 and the hexagon 7 Re 6 (CO) 24 (µ-H) 6 have been characterized structurally.…”