CIE (1931) is a RGB color space, based on monochromatic primary colors with wavelengths of 700 nm (red), 546.1 nm (green) and 435.8 nm (blue).

However the entire color space of CIE RGB (1931) includes points where some of the RGB components are negative.

Because positive components are sometimes desirable (e.g. because one can make light filters whose outputs are those components), the alternative XYZ representation is derived by computation from the original CIE RGB, which had been obtained from experiments with human subjects.

Any RGB model can capture the entire color space, even sRGB. The limitation is not at capturing, but only at reproducing colors when using RGB emitters, because the emitters cannot reproduce components with negative values.

There are no RGB models that can capture the entire color space without having points where some components are negative. This is caused mostly by defects of the human color vision, e.g. by the fact that the red receptors are also sensitive to violet light, not only to red light, and by the fact that the selectivity curves of the photoreceptors do not have ideal shapes.

Since you seem to know, and I am curious, doesn't CIE[1] effectively use RGB to describe its space, too? Eg: the r̅(λ) g̅(λ) b̅(λ) color matching functions? Or is there something else in CIE you're referring to?

[1] https://en.wikipedia.org/wiki/CIE_1931_color_space