vsepr theory - Why is disilyne bent?

Why is disilyne $\ce{Si2H2}$ bent when the steric number of silicon is 2?

Answer

The following disilyne has been prepared and found to be stable to ~100 C. An X-ray crystal structure found that the two silicon atoms in the triple bond, Si(1) and Si(1'), along with the two attached silicon atoms, Si(2) and Si(2') are coplanar and the Si(1)-Si(1')-Si(2') angle is 137.44 degrees ("trans-bent").

The full text of the article describing the preparation, characterization and reactivity of the compound can be found here. See figure 4 in the paper for a MO explanation as to why the trans-bent geometry occurs. Basically, the authors suggest that, "the bending is thought to be the result of the mixing of an in-plane $\ce{\pi}$-orbital with a $\ce{\sigma^{\ast}}$ orbital whose energies are close enough to cause the interaction."