Cyclobutadiene is very unstable. But, some sources claim that this instability can be attributed to other factors such as ring and angle strain rather than antiaromaticity.

According to some, cyclobutadiene is simply non-aromatic (as opposed to antiaromatic) because it doesn't even have a fully conjugated pi system.

What is/are the real reason(s) for its instability?

• I always stumble at the words anti-aromatic. Is this a well defined concept? I believe not. From my point of view, your whole question is a little bit philosophical, as we dive in the realm of interpretation only. Also one should not confuse instability with high reactivity... – Martin - マーチン Oct 18 '14 at 11:20

If we go back to your earlier question on Frost diagrams (I've reproduced the key figure below), we see why simple molecular orbital theory or the "$$4n+2$$" rule suggests that benzene is aromatic while cyclobutadiene is antiaromatic.

Forming a planar, conjugated, 6-membered ring and placing 6 π-electrons in it creates a π-system that is energetically more stable than 3 conjugated C=C bonds (e.g. in hexa-1,3,5-triene). On the other hand, forming a planar, conjugated, 4-membered ring and placing 4 π-electrons in it creates a π-system that is energetically less stable than 2 conjugated C=C bonds (e.g. in butadiene). For these reasons we say that the first system, benzene, is "aromatic", while the second system, cyclobutadiene, is "antiaromatic". Other measurements and physical phenomenon such as reactivity, bond lengths, ring currents, etc. support these conclusions.

Free cyclobutadiene has been observed as a transient intermediate. Further theoretical analysis suggests that due to its lack of aromaticity it will distort from a square structure to a rectangular one with alternating single and double bonds (Jahn–Teller effect).

sources claim that this instability can be attributed to other factors such as ring and angle strain rather than antiaromaticity

It turns out that other systems involving the cyclobutadiene skeleton have been prepared and studied. Derivatives of both the cyclobutadiene dication[1] ($$4n+2$$, $$n=0$$) and the cyclobutadiene dianion[2] ($$4n+2$$, $$n=1$$) have been prepared and studied. Their stability is much less than that of benzene, but much more than that of cyclobutadiene. Placing two charges in these small rings results in extremely high coulombic repulsions in both the dianion and dication, and this may be a significant part of the explanation as to why they are less stable than say, benzene. The fact that they exist as planar structures, with appropriate ring currents and are more stable than cyclobutadiene suggests that the instability observed with cyclobutadiene is not due to ring strain and angle strain alone. Rather the aromatic stabilization - antiaromatic destabilization suggested by simple MO theory seems to be involved.

References

1. Olah, G. A.; Staral, J. S. Novel aromatic systems. 4. cyclobutadiene dications. J. Am. Chem. Soc. 1976, 98 (20), 6290–6304. DOI: 10.1021/ja00436a037.

2. Takanashi, K.; Inatomi, A.; Lee, V. Y.; Nakamoto, M.; Ichinohe, M.; Sekiguchi, A. Tetrakis(trimethylsilyl)cyclobutadiene dianion alkaline earth metal salts: new members of the 6π-electron aromatics family. Eur. J. Inorg. Chem. 2008, 2008 (11), 1752–1755. DOI: 10.1002/ejic.200800066.

• I saw mention that dianion is non-planar according to one study. – Mithoron Jul 13 '17 at 14:06
• It was computational paper for unsubstituted dianion, not derivtives though. – Mithoron Jul 13 '17 at 14:17
• Strictly cyclobutadiene is non aromatic. Still it answers. – Alchimista Feb 16 '20 at 10:17