I've been trying to develop a theory about why the eneasílabo, 9 syllable verse, is not more common in Spanish poetry. It seems to fall awkwardly in the middle, between the octosílabo and the endecasílabo, both of which have four main rhythmic variants. Darío, Neruda, and Hierro are the poets who have written most successfully in verses of 9 syllables. This line is also frequent in free verse, which without it would basically be a combination of sevens and elevens. I find my ear doesn't really "hear" 9-syllable lines very well. This is from José Hierro:

Bajo la falda, vientre, muslos,
cintura, sexo: desnudez
impura, sed de aniquilarse,
de apoderarse, de morir
salvajemente. Y un sabor
a animal que destruye el tiempo
uniéndose al tropel oscuro.

The accentual scheme is the following:

4,6,8,
2,4,8
2,4,8
4,8
2,4,8
2,5,8
3,6,8
2,6,8

It would appear that 4 is the other main rhythmic accent in this passage. This would mean that the cadence is quite similar to that of 11 syllable line, which usually ends with a group of 7 syllables (with accents on the 2nd and 6th syllables of the group). Yet the 9 syllables don't offer as much room for variation in the initial part of the line. 11 syllables provide a nice balance between an initial, more varied group of 4 syllables and a regular cadence at the end. Hierro's enjambments and "agudo" lines (lines ending with the 8th, accented syllable) make it even more difficult for me to hear this rhythm. It is definitely a tour de force.

If a poet does not maintain the accent on 4, then we basically have six syllables of freedom combined with shorter fixed pattern at the end. Maybe there are too many possibilities: most popular meters in Spanish have four major variants. I'm going to have to look at what Neruda does. I can't seem to find my copy of Extravagario.