Himmel, Krossing and Schnepf on dative bonds

11 January 2014 - Coordination chemistry

A dative bond takes place between an atom with an available lone pair and an atom that is an electron acceptor. A good example is the adduct of ammonia and borane or ammonia borane with the dative bond depicted as H3N->BH3 with an arrow indicating that nitrogen is donating both electrons to create the chemical bond. The classic Lewis structure H3N+-B-H3 would have a positive change on nitrogen even though nitrogen is more electronegative than boron and creates confusion. Dative bonds are considered weak, are longer than a regular bond, charge transfer is small and in the case the bond is severed the mode is heterolytic and not homolytic. Application of the dative bond is troublesome. The problems starts with amine oxides that are for some unknown reason are also often depicted with a dative N-O bond.

In a recent essay Himmel, Krossing and Schnepf (HK&S) see more issues with the dative bond. They see an increased popularity of them but without justification. They examined some recent invocations and assign most of them to the scrapheap. The so-called carbones, a class of carbon(0) compounds of the type L->C<-L (example bis(triphenylphosphoranylidene)methane) as advocated by Frenkel are generally depicted with two dative bonds. According to HK&S however a class Lewis structure is more in agreement with the calculated high partial charge on carbon. Same case with the PPN cation, depicted as PPh3->N+<-PPh3. The positive charge on nitrogen is an eyesore given that its partial charge is -1.55.

In a similar vein a N3+ cation with three dative ligands also has been reported. Quote of the day: "Is there any intellectual profit in describing a negatively polarized nitrogen atom through the use of arrows as cationic N3+?"