How are there pi bonds in B_2 molecule without sigma bonds?

There shouldn't be, and that's why "B"_2 is very unstable; it has two orthogonally-localized half-pi bonds, which is quite a weak bond.

Imagine that each "B" takes the place of a "CH" on "HC"-="CH".

Then, take away the sigma bond, the two "H" atoms, and one electron from each of the two full pi bonds, and you have a structure similar to "B"_2.

Che dire "Li"_2 "molecule"? Is that stable? (Its bond length is "267.3 pm", over twice the length of an average bond.)

The first chemical bond made in a molecule is preferenzialmente a bbsigma legame.

sigma bonds are formed from a direct atomic orbital overlap. In comparison, pi bonds are sidelong overlaps and thus, sigma overlaps are made preferentially because they form the stronger bond.

"B"_2 contains two boron atoms, which each use a basis of a 1s, un 2sE tre 2p orbitali atomici.

Per qualificarti per il "Li"_2 attraverso "N"_2, there exists an orbital mixing effect che rende il sigma molecular orbital for a 2p_z-2p_z overlap (sigma_(g(2p))) superiore in energia rispetto al pi molecular orbitals for a 2p_(x//y)-2p_(x//y) overlaps (pi_(u(2p))).

So, this particular orbital energy ordering takes place in the diagramma orbitale molecolare, simile al "C"_2 molecola:

where |E_(sigma_(2p_z))| > {|E_(pi_(2p_x))| = |E_(pi_(2p_y))|}.

Dal "C" has one more electron, "B"_2 would have a similar MO diagram, EXCEPT for two meno electrons. This, at first glance, seems to suggest that "B"_2 has two half-pi bonds, with a molecular configurazione elettronica di:

color(green)((sigma_(1s))^2(sigma_(1s^"*"))^2(sigma_(2s))^2(sigma_(2s^"*"))^2(pi_(2p_x))^1(pi_(2p_y))^1)

Dal "B"_2 is paramagnetica with two electrons, in order to make that bond, there are indeed two half-bbpi obbligazioni, which form what we represent improperly in line notation as a sigma bond... and it isn't actually a sigma legame.

:"B"-"B":

That isn't a sigma bond, but two half-pi bonds. We would then expect "B"_2 to be very unstable, which it is. (The lone pairs are from the sigma_(2s) e sigma_(2s)^"*", since bonding + antibonding filled = nonbonding.)