In "Complementary Letters" in the November 2001 Word Ways, Anil defines complementary letters as those whose ordinal places in the alphabet add to 27, e.g., A and Z, or L and O. He asks for words and phrases composed solely of such pairs, giving EVOLVE as an example, and asking whether LOVERISH is a word (I cannot find it). Such words must be even length, say 2n, and have n pairs of letters. My findings are presented below. As can be seen, such words are scarce. An asterisk denotes words that have this property in extremis: every one of the (n-1) pairs of letters 1 and 2, 2 and 3, etc., is complementary. That clearly requires the same two letters to alternate.
"Complementary Letters and Words,"
Word Ways: Vol. 35
, Article 21.
Available at: https://digitalcommons.butler.edu/wordways/vol35/iss1/21