PBS Space Time: What If Alien Life Were Silicon-Based?https://youtu.be/469chceiiUQ

There are very few atoms that allow for the complex scaffolding for shapes. Many atoms are too large (and thus bond too weakly). Silicon is interesting, but it has some difficulties.

https://en.wikipedia.org/wiki/Silicon–oxygen_bond

> Silicon–oxygen single bonds are longer (1.6 vs 1.4 Å) but stronger (452 vs. about 360 kJ mol−1) than carbon–oxygen single bonds.

It is much easier to lock up oxygen and silicon in SiO than in CO compounds and in turn makes them less available for more complex structures of life.

My organic chemistry professor always liked to point out that while CO2 is a gas that is easily dealt with following metabolism, SiO2 - silica/quartz - is most decidedly not a gas. Add that to the list of challenges for silicon-based lifeforms. Not to say that it isn't possible, but it does constrain the solution space somewhat.

Anything else would be so exotic that it’s not worth comparing to Earth.