Multiple bond counts

This planar structure is the one expected for three pairs of electrons around a central atom, which means that a double bond should be counted as one effective pair in using the VSEPR model. This makes sense because the two pairs of electrons involved in the double bond are not independent pairs. Both of the electron pairs must be in the space between the nuclei of the two atoms to form the double bond. In other words, the double bond acts as one center of electron density to repel the other pairs of electrons. The same holds true for triple bonds. This leads us to another general rule For the VSEPR model multiple bonds count as one effective electron pair. 

The simplest descriptors of the degree of unsaturation of a molecule are -> count descriptors based on the presence of double bonds, triple bonds, and aromatic bonds they are the double-bond count, the triple-bond count n= and the aromatic-bond count The MCB index was proposed as the number of multiple C-C bonds in the molecule accounting for double, triple, and aromatic bonds [Bakken and Jurs, 1999b]. Another simple global multiple bond count is defined as ... 

A---- (a multiple bond counts as a single group of electrons)... 

The unsaturation index UN can be expressed by a more general form, called multiple bond count, as... 

Nc, Nh, Nx, Nn, Np, and C are the number of carbon atoms, hydrogen, halogen, nitrogen, phosphorous, and independent cycles, respectively. Nq-s and Nsos are the number of oxygen atoms bonded to sulfur and the number of SO3 groups, respectively. When no sulfur atoms are present, this index can be easily calculated from the chemical formula otherwise, it is coincident with the index calculated replacing b with b, the multiple bond count. 

Use VSEPR theory to predict the geometries. Remember that multiple bonds count as one pair of electrons for the purpose of VSEPR theory. 

Coluit the number of electron pairs around the central atom. Since there are two electron pairs around N, the electron airangement that minimizes electron-pair repulsion is linear (AB2). Remember, for VSEPR piuposes a multiple bond counts the same as a single bond. 

A Multiple Bond Counts as a Single Space Unit... 

Multiple bonds count as single bonds. It is the number of terminal atoms (X) that are counted, not the munber of bonds. 

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