Well, like I said I think carbon is the only suitable basis of life. Silicon is a far, far second. Si-Si bonds are very difficult to make, and Si=Si double bonds are even more difficult. The likelihood of catalytic silicon-based macromolecules developing spontaneously would be far lower than for carbon. Even in a highly exotic environment. Further reasons for the necessity of carbon (and water) are given in the first few pages of Voet and Voet’s Biochemistry. (This means I don’t feel like looking for it, but I’ll quote some if anyone really wants it.) Silicon-based life might be possible, but I don’t think it could be adequately hypothesized with our (read: my) current understanding of Si chemistry. Maybe in the future we’ll discover that silicon can do things we don’t now know it can do.
If C is vastly better than Si, then Si is even more vastly than the next-best alternative. I honestly can’t see chalcogens, pnictogens or halogens forming the basis of life. They simply cannot concatenate to the degree necessary to form catalysts. I suppose it might be vaguely possible to use a different catalytic paradigm for life that would emphasize inorganic catalysis – this would take some of the emphasis off carbon, but it would be much less likely to arise spontaneously and would lack much of the specificity of organic catalysts.
**Solvents:**I can see how sulfur dioxide could be a decent solvent. I disagree with ammonia because of its reactivity. In the real world, ammonia-water oceans would be suitable for life, as has been mentioned, but the OP excludes that.
I hadn’t thought of liquid CO, but that’s very intriguing. Unfortunately I can’t find a phase diagram, so I don’t know what conditions would be required. But CO is an excellent feedstock for organic synthesis. CO oceans – especially ones with abundant dissolved metals – would give rise to all kinds of organic compounds. The compounds would be different than terrestrial biological molecules, but they could easily be catalytic. Assuming the right atmospheric conditions, any organisms that happened to evolve would likely be able to sustain themselves by oxidizing CO to CO[sub]2[/sub].
The only problem is that these organisms would be constantly surrounded by a fairly reactive substance. Keeping CO out would be a priority, and they would have to find a way to protect themselves against it. One idea: with all that CO around, perhaps they could use polymers instead of lipid membranes. A polyalkane or polyketone ‘shell’ could be used to keep out CO and sustain an internal environment of some other solvent.
It’s not inconceivable that that solvent could be water. If there were hydrogen in the atmosphere, molecular hydrogen rather than water could be used as the hydrogen and proton source. And it’s possible that some water could be produced as the byproduct of CO-based organic synthesis. This water could be used inside the organisms as a less reactive solvent – and then a very different biochemistry would begin to appear. I don’t think it violates the OP to introduce water after the beginning of life. Remember, most of the oxygen in the Earth’s atmosphere is the result of life, so it’s not inconceivable that an initially waterless planet could have water in abundance after a couple billion years of life.