Combined solution calorimetric and quantum mechanics studies of reactions involving saturated and unsaturated N-heterocyclic carbene (NHC) ligands show that the difference in their relative bond dissociation energies is very small (1 kcal·mol-1). Structural and computational studies reveal small metric parameter differences. These observations in conjunction with relative reactivity profiles of NHC-modified ruthenium-based olefin metathesis catalysts suggest that very small changes in the donor properties of the NHC ligands can translate into significant differences in catalytic properties.

The reactions of 1,5,9-triethyl-1,5,9-triphosphacyclododecane, [12]aneP 3 Et 3 , with first-row transition metal halides MCl 3 [M = Ti (1), V (2), and Cr (3)] or their THF adducts are reported. The oxidation of 2 gives the complex ([12]aneP 3 Et 3 )-V(O)Cl 2 (4). This is the first example of a phosphane ligand that is trans to a vanadyl moiety. Also reported are V III (5) and Cr III (6) complexes of the pendant ether macrocycles [12]aneP 3 (C 2 H 4 OEt) 3 and 12[ane]P 3 (C 3 H 6 OMe) 3 respectively. The complexes 1−6 have been characterised crystallographically and represent a rare class of phosphane com-

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