Reactions of trans-Fe(CO)(3)(P((CH2)(m) CH=CH2)(3))(2) (m = a/4; b/5, c/6, e/8) and Grubbs' catalyst (12-24 mol %, CH2Cl2, reflux) give the cage-like trienes trans-Fe(CO)(3)(P((CH2)(m)CH=CH(CH2)(m))(3)P) (3a-c,e, 60-81%). Hydrogenations (ClRh(PPh3)(3), 60-80 degrees C) yield the title compounds trans-Fe(CO)(3)(P((CH2))(3)P) (4a-c,e, 74-86%; n = 2m + 2), which have idealized D-3h symmetry. A crystal structure of 4c suggests enough van der Waals clearance for the Fe(CO)(3) moiety to rotate within the three P(CH2)(14)P linkages; structures of E,E,E-3a show rotation to be blocked by the shorter P(CH2)(4)CH=CH(CH2)(4)P linkages. Additions of NO+BF4- give the isoelectronic and isosteric cations [Fe(CO)(2)(NO)(P((CH2))(3)P)](+)rBF(4)(-) (5a-c(+)3F(4)(-); 81-98%). Additions of [H(OEt2)(2)]+BArf- (BArf = B(3,5-C6H3(CF3)(2)))(4), afford the metal hydride complexes mer,trans-[Fe(CO)(3)(H)(P((CH2)(n))(3)P]+TBArf- (6a-c,e(+)BAr(f)-(;) 98-99%). The behavior of the rotators in the preceding complexes is probed by VT NMR. At ambient temperature in solution, 5a,b(+)BF(4) and 6a(+)BArf(-) show two sets of P(CH2)(n/2) C-13 NMR signals (2:1), whereas 5c(+)BF(4)(-) and 6b,c(+)BArf(-) show only one. At higher temperatures, the signals of 5b(+)BF(4)(-) coalesce; at lower temperatures, those of 5c(+)BF(4)(-) and 6b(+)BA(f)(-) decoalesce. These data give Delta H double dagger/Delta S double dagger values (kcal/mol and eu) of 8.3/-28.4 and 9.5/-6.5 for Fe(CO)(2)(NO)(+) rotation (5b,c(+)) and 6.1/-23.5 for Fe(CO)(3)(H)(+) rotation (6b(+)). C-13 CP/MAS NMR spectra show that the Fe(CO)(3) moiety in polycrystalline 4c (but not 4a) undergoes rapid rotation between -60 and 95 degrees C. Approaches to minimizing these barriers and developing molecular gyroscopes are discussed.