Metathesis of n and r in Old Spanish was conditioned by vowel syncope in the future and conditional formation of the verb which resulted in the contiguity of /nr/.

n/r metathesis

Acoustic and Auditory Similarity (Hume 1997): The observation that /n/ and /r/ undergo metathesis can be attributed to the acoustic and auditory similarity between the two consonants. That poor syntagmatic contrast is at issue is supported by the observation that the insertion of an obstruent stop between the two sonorants, e.g. pondré, etc., also emerged as a strategy to resolve the otherwise ill-formed sequence. The two alternatives co-existed in all forms of the future and conditional. The variant with the intrusive consonant has survived in Modern Spanish while the forms with metathesis have not (Martínez-Gil, p.c., Wanner 1989). Similarity between /n, r/ relates to the fact that both are sonorants and thus have formant structure, both have vowel transitions for coronal (apicoalveolar) place of articulation, and both are noncontinuants ([r] was realized as an apicoalveolar tap).

Perceptual Enhancement (Hume 1997, 1998): In terms of a segment’s inherent properties, rhotics have traditionally been viewed as more salient, or sonorous, than nasals. This may be attributed to clear formant structure throughout the duration of the liquid, which provides strong cues to both place and manner of articulation. Nasals, on the other hand, are characterized by formants as well as antiformants, the latter having the affect, among other things, of lowering the amplitude of all higher formants (Fant 1960, Johnson 1996). A nasal’s internal cues provide information concerning manner of articulation and more weakly, place of articulation. These facts suggest that the nasal has less robust internal cues than the liquid. Positioning the nasal in prevocalic position can therefore be viewed as a means of enhancing the salience of the consonant, further strengthened by the presence of final stress.

metathesis of an alveolar stop and a lateral/alveolar nasal

Perceptual Enhancement (Hume 1997, 1998):

Prevocalic position is a particularly favorable position for the realization of stop consonants, given the presence of both a vowel transition and release burst in this context. In preconsonantal position, on the other hand, no audible release of the stop is present. Thus, shifting the alveolar stop with vulnerable cues to prevocalic position strengthens the syntagmatic contrast between the stop and contiguous lateral/nasal. In the case of nasals, nasal murmur and weak formant structure provides strong cues to both place and manner of articulation. Also, anticipatory coarticulation in the form of vowel nasalization on a preceding vowel provides strong cues to the nasal manner. In case of a lateral, weak formant structure provides cues to both place and manner of articulation.