Covalent vs. Dative Bonding in Carbon Monoxide and Other 10-Valence-Electron Diatomics

Abstract

Valence bond theory (VB) was used to determine the extent and driving forces for covalent vs. dative bonding in 10-valence-electron diatomic molecules N₂, CO, NO⁺, CN^-, P₂, SiS, PS⁺, and SiP^-. VBSCF calculations were performed at the CCSD(T)/cc-pVDZ optimized geometries. The full triply bonded system included 20 VB structures. A separation of the σ and π space allowed for a subdivision of the full 20 structure set into sets of 8 and 3 for the π and σ systems, respectively. The smaller structure sets allowed for a more focused look at each type of bond. In situ bond energies for σ bonds, individual π bonds, the π system, and triple bonds follow expected trends. Our data shows that N₂ and P₂ have three covalent bonds whereas CO and SiS contain two covalent and one dative bond, and charged species NO⁺, CN^-, PS⁺, and SiP^- are a mixture of N₂ and CO type electronic arrangements, resulting in a nearly equal charge distribution. Dative bonds prefer to be in the π position due to enhanced σ covalency and π resonance. Both σ and π resonance energies depend on a balance of ionic strength, orbital compactness, σ constraints, and bond directionality. Resonance energy is a major contributor to bond strength, making up more than 50% of the π bonds in SiS and PS⁺ (charge-shift bonds), and is greater than charge transfer in dative bonds.

Keywords: charge-shift bonding; dative bonds; resonance energy; valence bond theory.

Figure 1

Lewis-type structural representations of carbon monoxide. In (a) octet rule is followed but negative formal charge resides on the more electronegative O atom. Structure (b) reduces formal charge but violate octet rule. Structure (c) uses arrow notation to indicate a dative bond.

Figure 2

A subset of the 20 structures used in the present work. For mixed diatomics, B is the more electronegative atom. Structure 1 is the covalent (spin-paired) structure for both σ and π bonds, structures 2 and 3 are the σ ionic structures, and structures 4 and 5 are the ionic structures for the π_x bond. The π_y bond has structures similar to 4 and 5 in the zy-plane.

Figure 3

Non-bonded determinants with dative electrons in the σ (a) and π (b) positions.

Figure 4

Lewis-type structural representation of CO indicating the resonance of dative bonds in the π position.

Figure 5

Valence bond descriptions of (a) covalent and (b) dative bonds. In the covalent case (a), the spin-paired state, f_cov(A−B), is low in energy and electronically neutral. In this case, A is more electronegative than B, resulting in ϕ_{ion 1} (A:⁻ B⁺) being lower in energy than ϕ_{ion 2} (A⁺ :B⁻). In the dative case (b), species B donates both electrons to the bond, resulting in ϕ_dat being the low energy/electronically neutral structure while the spin-paired structure, ϕ_{ion 1}, carries charge.

Figure 6

Quasiclassical determination of in situ energy of a 2-electron bond.