Si−E (E = N, O, F) Bonding in a Hexacoordinated Silicon Complex:  New Facts from Experimental and Theoretical Charge Density Studies

Abstract

The concept of hypervalency in molecules, which hold more than eight valence electrons at the central atom, still is a topic of constant debate. There is general interest in silicon compounds with more than four substituents at the central silicon atom. The dispute, whether this silicon is hypervalent or highly coordinated, is enlightened by the first experimental charge density determination and subsequent topological analysis of three different highly polar Si-E (E = N, O, F) bonds in a hexacoordinated compound. The experiment reveals predominantly ionic bonding and much less covalent contribution than commonly anticipated. For comparison gas-phase ab initio calculations were performed on this compound. The results of the theoretical calculations underline the findings of the experiment.

N,N‐Dimethylaminopropylsilane: A Case Study on the Nature of Weak Intramolecular Si⋅⋅⋅N Interactions

“…[15] ] and ellipticities e at the bcp. In this respect the electron-density topology is similar to that of other hypervalent silicon compounds such as the hexacoordinate difluorobis[N-(dimethylamino)phenylacetimidato-N,O]silicon with an SiO 2 F 2 core plus two nitrogen donor substituents, recently investigated by Stalke et al [20] This compound has much shorter and stronger Si···N interactions than 3 a, and consequently we find a much smaller electron density at the bcp of the Si···N bonding interaction for 3 a of 0.15 e À3 compared with difluorobis[N-(dimethylamino)phenylacetimidato-N,O]silicon (0.50 e À3 ). The description of the Si···N interaction in 3 a as a closed-shell interaction follows from the positive Laplacian value, but its magnitude is much less than those of the other bonds about the silicon atom and also less than in the compound investigated by Stalke et al The relatively high ellipticity of 0.55 at the bcp is noteworthy, but care is suggested in not over-interpreting this value due to the low electron density at this bcp.…”

Ylenes in the MII→SiIV (M=Si, Ge, Sn) Coordination Mode

“…[22] Compound 1, however, reveals some distinct differences to the others (2, 3, and 3 a). Whereas for 1 similar NCs were calculated, for Si and Sn (the one of Sn being slightly lower (0.31 e)), a clear divergence is found for 2 (0.95 e), 3 (0.92 e), and 3 a (1.39 e), in favor of the S 4 -coordinated Group-14-element atom having the notably lower charge.…”

Meaningful Structural Descriptors from Charge Density

“…In the hexacoordinate silicon complex difluoro-bis[N-(dimethylamino)-phenylacetimidato-N,O] silicon, the silicon atom is coordinated by two nitrogen, oxygen, and fluorine atoms, respectively. [54] Thus, this compound contains three different sets of highly polar silicon-element bonds (Si À E, E = N, O, F) in While the spatial distribution of the Laplacian is almost perfectly spherical around the silicon atom, it still shows a rather ionic-like behavior around the oxygen and fluorine atoms. This indicates a distinct electronic depletion around the silicon atom and predominantly ionic Si À E bonding with differing-but always small-covalent bonding contributions.…”