Unprecedented diamagnetic, four-coordinate, formally 14-electron (Cy-PSiP)RuX (Cy-PSiP = [K-3-(2-R2PC6H4)(2)SiMe](-); X = amido, alkoxo) complexes that do not require agostic stabilization and that adopt a highly unusual trigonal pyramidal coordination geometry are reported. The tertiary silane [(2-Cy2PC6H4)(2)SiMe]H ((Cy-PSiP)H) reacted with 0.5 [(p-cymene)RuCl2](2) in the presence of Et3N and PCy3 to afford [(Cy-PSiP)RuCl](2) (1) in 74% yield. Treatment of 1 with (KOBu)-Bu-t led to the formation of (Cy-PSiP)(RuOBu)-Bu-t (2,97% yield), which was crystallographically characterized and shown to adopt a trigonal pyramidal coordination geometry in the solid state. Treatment of 1 with NaN(SiMe3)(2) led to the formation of (Cy-PSiP)RuN(SiMe3)(2) (3, 70% yield), which was also found to adopt a trigonal pyramidal coordination geometry in the solid state. The related anilido complexes (Cy-PSiP)RuNH(2,6-R2C6H3) (4, R = H; 5, R = Me) were also prepared in >90% yields by treating 1 with LiNH(2,6-R2C6H3) (R = H, Me) reagents. The solid state structure of 5 indicates a monomeric trigonal pyramidal complex that features a C-H agostic interaction. Complexes 2 and 3 were found to react readily with 1 equiv of H2O to form the dimeric hydroxo-bridged complex [(Cy-PSiP)RuOH](2) (6, 94% yield), which was crystallographically characterized. Complexes 2 and 3 also reacted with 1 equiv of PhOH to form the new 18-electron eta(5)-oxocyclohexadienyl complex (Cy-PSiP)Ru(eta(5)-C6H5O) (7, 84% yield). Both amido and alkoxo (Cy-PSiP)RuX complexes reacted with H3B center dot NHRR' reagents to form bis(sigma-B-H) complexes of the type (Cy-PSiP)RuH(eta(2):eta(2)-H2BNRR') (8, R = R' = H; 9, R = R' = Me; 10, R = H, = Bu-t), which illustrates that such four-coordinate (Cy-PSiP)RuX (X = amido, alkoxo) complexes are able to undergo multiple E-H (E = main group element) bond activation steps. Computational methods were used to investigate structurally related PCP, PPP, PNP, and PSiP four-coordinate Ru complexes and confirmed the key role of the strongly sigma-donating silyl group of the PSiP ligand set in enforcing the unusual trigonal pyramidal coordination geometry featured in complexes 2-5, thus substantiating a new strategy for the synthesis of low-coordinate Ru species. The mechanism of the activation of ammonia-borane by such low-coordinate (R-PSiP)RuX (X = amido, alkoxo) species was also studied computationally and was determined to proceed most likely in a stepwise fashion via intramolecular deprotonation of ammonia and subsequent borane B-H bond oxidative addition steps.