trans-(DMPP)(2)Rh(CO)Cl (DMPP = 1-phenyl-3,4-dimethylphosphole) reacts with AgBF4 to form [Rh(eta(1)-DMPP)-(mu-sigma, eta(4)-DMPP)(CO)](2) (BF4)(2), 1. [(COE)(2)RhCl](2) (COE = cyclooctene) reacts with DMPP to form [Rh(eta(1)-DMPP)(mu-sigma, eta(4)-DMPP) Cl](2), 2. In both 1 and 2 the phospholes are bound to rhodium(I) as 2- and [4 + 2]-electron donors. The 2-electron donor is bound through phosphorus only and the [4 + 2] donor is bound through its diene system to one rhodium and through phosphorus to a second rhodium such that each rhodium atom is pentacoordinate. Substitution of the chloride in 2 by N-3(-), OH-, and DMPP (the latter in the presence of AgBF4) produces 3, 4, and 5. Reaction of 2 with diphenylvinylphosphine and 2-vinylpyridine in the presence of AgBF4 produces the [4 + 2] Diels-Alder adducts 6 and 7, respectively. New complexes were characterized by elemental analyses, infrared and H-1,C-13{H-1}, P-31{H-1}, P-31{H-1,P-31(sel)} and in some cases by H-1{P-31} nuclear magnetic resonance spectroscopy. The structure of 7, [Rh(2-pyridyl-5,6-dimethyl-7-phosphabicyclo[2.2.1]hept-5-ene)(mu-sigma, eta(4)-1-phenyl-3,4-dimethylphosphole](2)(BF4) was confirmed by X-ray crystallography. It crystallized in the monoclinic P2(1)/c space group in a unit cell with the following dimensions : a = 11.460(3) Angstrom A, b = 20.088(6) Angstrom A, c = 16.002(4) Angstrom A, beta = 109.46(2)degrees, rho(calcd) = 1.519 g cm(-3), Z = 2. Refinement converged to R(F) = 0.033 for 4907 independent observed (I greater than or equal to 3 sigma(I)) reflections. The dirhodium cation is centrosymmetric with considerable differences in the three Rh-P distances (RhP1 = 2.8336 (9) Angstrom A, RhP1’ = 2.3173 (9) Angstrom A, and RhP2 = 2.2690 (9) Angstrom A. These distances do not correlate with the relative magnitudes of the rhodium-phosphorus coupling constants (J(RhP1) = -13.86 Hz, (1)J(RhP1’) = 172.87 Hz, and (1)J(RhP2) = 119.96 Hz. The short P1...P1’ separation (3.352(9) Angstrom A) gives rise to a large through-space P...P coupling of 243.93 Hz.