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

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.